diff --git a/src/doc/unstable-book/src/language-features/negative-impls.md b/src/doc/unstable-book/src/language-features/negative-impls.md
new file mode 100644
index 0000000000000..151520f0e4abc
--- /dev/null
+++ b/src/doc/unstable-book/src/language-features/negative-impls.md
@@ -0,0 +1,57 @@
+# `negative_impls`
+
+The tracking issue for this feature is [#68318].
+
+[#68318]: https://github.com/rust-lang/rust/issues/68318
+
+----
+
+With the feature gate `negative_impls`, you can write negative impls as well as positive ones:
+
+```rust
+#![feature(negative_impls)]
+trait DerefMut { }
+impl<T: ?Sized> !DerefMut for &T { }
+```
+
+Negative impls indicate a semver guarantee that the given trait will not be implemented for the given types. Negative impls play an additional purpose for auto traits, described below.
+
+Negative impls have the following characteristics:
+
+* They do not have any items.
+* They must obey the orphan rules as if they were a positive impl.
+* They cannot "overlap" with any positive impls.
+
+## Semver interaction
+
+It is a breaking change to remove a negative impl. Negative impls are a commitment not to implement the given trait for the named types.
+
+## Orphan and overlap rules
+
+Negative impls must obey the same orphan rules as a positive impl. This implies you cannot add a negative impl for types defined in upstream crates and so forth.
+
+Similarly, negative impls cannot overlap with positive impls, again using the same "overlap" check that we ordinarily use to determine if two impls overlap. (Note that positive impls typically cannot overlap with one another either, except as permitted by specialization.)
+
+## Interaction with auto traits
+
+Declaring a negative impl `impl !SomeAutoTrait for SomeType` for an
+auto-trait serves two purposes:
+
+* as with any trait, it declares that `SomeType` will never implement `SomeAutoTrait`;
+* it disables the automatic `SomeType: SomeAutoTrait` impl that would otherwise have been generated.
+
+Note that, at present, there is no way to indicate that a given type
+does not implement an auto trait *but that it may do so in the
+future*. For ordinary types, this is done by simply not declaring any
+impl at all, but that is not an option for auto traits. A workaround
+is that one could embed a marker type as one of the fields, where the
+marker type is `!AutoTrait`.
+
+## Immediate uses
+
+Negative impls are used to declare that `&T: !DerefMut`  and `&mut T: !Clone`, as required to fix the soundness of `Pin` described in [#66544](https://github.com/rust-lang/rust/issues/66544).
+
+This serves two purposes:
+
+* For proving the correctness of unsafe code, we can use that impl as evidence that no `DerefMut` or `Clone` impl exists.
+* It prevents downstream crates from creating such impls.
diff --git a/src/doc/unstable-book/src/language-features/optin-builtin-traits.md b/src/doc/unstable-book/src/language-features/optin-builtin-traits.md
index 5c8124c9c6b7d..673836c0b9029 100644
--- a/src/doc/unstable-book/src/language-features/optin-builtin-traits.md
+++ b/src/doc/unstable-book/src/language-features/optin-builtin-traits.md
@@ -10,7 +10,8 @@ The `optin_builtin_traits` feature gate allows you to define auto traits.
 
 Auto traits, like [`Send`] or [`Sync`] in the standard library, are marker traits
 that are automatically implemented for every type, unless the type, or a type it contains, 
-has explicitly opted out via a negative impl. 
+has explicitly opted out via a negative impl. (Negative impls are separately controlled
+by the `negative_impls` feature.)
 
 [`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html
 [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
@@ -22,6 +23,7 @@ impl !Type for Trait
 Example:
 
 ```rust
+#![feature(negative_impls)]
 #![feature(optin_builtin_traits)]
 
 auto trait Valid {}
@@ -43,3 +45,63 @@ fn main() {
     // must_be_valid( MaybeValid(False) );
 }
 ```
+
+## Automatic trait implementations
+
+When a type is declared as an `auto trait`, we will automatically
+create impls for every struct/enum/union, unless an explicit impl is
+provided. These automatic impls contain a where clause for each field
+of the form `T: AutoTrait`, where `T` is the type of the field and
+`AutoTrait` is the auto trait in question. As an example, consider the
+struct `List` and the auto trait `Send`:
+
+```rust
+struct List<T> {
+  data: T,
+  next: Option<Box<List<T>>>,
+}
+```
+
+Presuming that there is no explicit impl of `Send` for `List`, the
+compiler will supply an automatic impl of the form:
+
+```rust
+struct List<T> {
+  data: T,
+  next: Option<Box<List<T>>>,
+}
+
+unsafe impl<T> Send for List<T>
+where
+  T: Send, // from the field `data`
+  Option<Box<List<T>>>: Send, // from the field `next`
+{ }
+```
+
+Explicit impls may be either positive or negative. They take the form:
+
+```rust,ignore
+impl<...> AutoTrait for StructName<..> { }
+impl<...> !AutoTrait for StructName<..> { }
+```
+
+## Coinduction: Auto traits permit cyclic matching
+
+Unlike ordinary trait matching, auto traits are **coinductive**. This
+means, in short, that cycles which occur in trait matching are
+considered ok. As an example, consider the recursive struct `List`
+introduced in the previous section. In attempting to determine whether
+`List: Send`, we would wind up in a cycle: to apply the impl, we must
+show that `Option<Box<List>>: Send`, which will in turn require
+`Box<List>: Send` and then finally `List: Send` again. Under ordinary
+trait matching, this cycle would be an error, but for an auto trait it
+is considered a successful match.
+
+## Items
+
+Auto traits cannot have any trait items, such as methods or associated types. This ensures that we can generate default implementations.
+
+## Supertraits
+
+Auto traits cannot have supertraits. This is for soundness reasons, as the interaction of coinduction with implied bounds is difficult to reconcile.
+
diff --git a/src/etc/gdb_rust_pretty_printing.py b/src/etc/gdb_rust_pretty_printing.py
index 0914c22eb13f0..cae64ef6665bb 100755
--- a/src/etc/gdb_rust_pretty_printing.py
+++ b/src/etc/gdb_rust_pretty_printing.py
@@ -370,12 +370,17 @@ def to_string(self):
                 ("(len: %i)" % self.__val.get_wrapped_value()['map']['length']))
 
     def children(self):
-        root = self.__val.get_wrapped_value()['map']['root']
-        node_ptr = root['node']
-        i = 0
-        for child in children_of_node(node_ptr, root['height'], False):
-            yield (str(i), child)
-            i = i + 1
+        prev_idx = None
+        innermap = GdbValue(self.__val.get_wrapped_value()['map'])
+        if innermap.get_wrapped_value()['length'] > 0:
+            root = GdbValue(innermap.get_wrapped_value()['root'])
+            type_name = str(root.type.ty.name).replace('core::option::Option<', '')[:-1]
+            root = root.get_wrapped_value().cast(gdb.lookup_type(type_name))
+            node_ptr = root['node']
+            i = 0
+            for child in children_of_node(node_ptr, root['height'], False):
+                yield (str(i), child)
+                i = i + 1
 
 
 class RustStdBTreeMapPrinter(object):
@@ -391,13 +396,16 @@ def to_string(self):
                 ("(len: %i)" % self.__val.get_wrapped_value()['length']))
 
     def children(self):
-        root = self.__val.get_wrapped_value()['root']
-        node_ptr = root['node']
-        i = 0
-        for child in children_of_node(node_ptr, root['height'], True):
-            yield (str(i), child[0])
-            yield (str(i), child[1])
-            i = i + 1
+        if self.__val.get_wrapped_value()['length'] > 0:
+            root = GdbValue(self.__val.get_wrapped_value()['root'])
+            type_name = str(root.type.ty.name).replace('core::option::Option<', '')[:-1]
+            root = root.get_wrapped_value().cast(gdb.lookup_type(type_name))
+            node_ptr = root['node']
+            i = 0
+            for child in children_of_node(node_ptr, root['height'], True):
+                yield (str(i), child[0])
+                yield (str(i), child[1])
+                i = i + 1
 
 
 class RustStdStringPrinter(object):
diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
index 9da324ba2d4f1..3ba7befc04609 100644
--- a/src/liballoc/collections/btree/map.rs
+++ b/src/liballoc/collections/btree/map.rs
@@ -122,7 +122,7 @@ use UnderflowResult::*;
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct BTreeMap<K, V> {
-    root: node::Root<K, V>,
+    root: Option<node::Root<K, V>>,
     length: usize,
 }
 
@@ -147,10 +147,11 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
         {
             match node.force() {
                 Leaf(leaf) => {
-                    let mut out_tree = BTreeMap { root: node::Root::new_leaf(), length: 0 };
+                    let mut out_tree = BTreeMap { root: Some(node::Root::new_leaf()), length: 0 };
 
                     {
-                        let mut out_node = match out_tree.root.as_mut().force() {
+                        let root = out_tree.root.as_mut().unwrap();
+                        let mut out_node = match root.as_mut().force() {
                             Leaf(leaf) => leaf,
                             Internal(_) => unreachable!(),
                         };
@@ -169,9 +170,14 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
                 }
                 Internal(internal) => {
                     let mut out_tree = clone_subtree(internal.first_edge().descend());
+                    out_tree.ensure_root_is_owned();
 
                     {
-                        let mut out_node = out_tree.root.push_level();
+                        // Ideally we'd use the return of ensure_root_is_owned
+                        // instead of re-unwrapping here but unfortunately that
+                        // borrows all of out_tree and we need access to the
+                        // length below.
+                        let mut out_node = out_tree.root.as_mut().unwrap().push_level();
                         let mut in_edge = internal.first_edge();
                         while let Ok(kv) = in_edge.right_kv() {
                             let (k, v) = kv.into_kv();
@@ -190,7 +196,7 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
                                 (root, length)
                             };
 
-                            out_node.push(k, v, subroot);
+                            out_node.push(k, v, subroot.unwrap_or_else(|| node::Root::new_leaf()));
                             out_tree.length += 1 + sublength;
                         }
                     }
@@ -203,9 +209,9 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
         if self.is_empty() {
             // Ideally we'd call `BTreeMap::new` here, but that has the `K:
             // Ord` constraint, which this method lacks.
-            BTreeMap { root: node::Root::shared_empty_root(), length: 0 }
+            BTreeMap { root: None, length: 0 }
         } else {
-            clone_subtree(self.root.as_ref())
+            clone_subtree(self.root.as_ref().unwrap().as_ref())
         }
     }
 
@@ -271,14 +277,14 @@ where
     type Key = K;
 
     fn get(&self, key: &Q) -> Option<&K> {
-        match search::search_tree(self.root.as_ref(), key) {
+        match search::search_tree(self.root.as_ref()?.as_ref(), key) {
             Found(handle) => Some(handle.into_kv().0),
             GoDown(_) => None,
         }
     }
 
     fn take(&mut self, key: &Q) -> Option<K> {
-        match search::search_tree(self.root.as_mut(), key) {
+        match search::search_tree(self.root.as_mut()?.as_mut(), key) {
             Found(handle) => Some(
                 OccupiedEntry { handle, length: &mut self.length, _marker: PhantomData }
                     .remove_kv()
@@ -290,7 +296,7 @@ where
 
     fn replace(&mut self, key: K) -> Option<K> {
         self.ensure_root_is_owned();
-        match search::search_tree::<marker::Mut<'_>, K, (), K>(self.root.as_mut(), &key) {
+        match search::search_tree::<marker::Mut<'_>, K, (), K>(self.root.as_mut()?.as_mut(), &key) {
             Found(handle) => Some(mem::replace(handle.into_kv_mut().0, key)),
             GoDown(handle) => {
                 VacantEntry { key, handle, length: &mut self.length, _marker: PhantomData }
@@ -344,15 +350,18 @@ pub struct IterMut<'a, K: 'a, V: 'a> {
 /// [`BTreeMap`]: struct.BTreeMap.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct IntoIter<K, V> {
-    front: Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>,
-    back: Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>,
+    front: Option<Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>>,
+    back: Option<Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>>,
     length: usize,
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IntoIter<K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let range = Range { front: self.front.reborrow(), back: self.back.reborrow() };
+        let range = Range {
+            front: self.front.as_ref().map(|f| f.reborrow()),
+            back: self.back.as_ref().map(|b| b.reborrow()),
+        };
         f.debug_list().entries(range).finish()
     }
 }
@@ -417,8 +426,8 @@ pub struct ValuesMut<'a, K: 'a, V: 'a> {
 /// [`BTreeMap`]: struct.BTreeMap.html
 #[stable(feature = "btree_range", since = "1.17.0")]
 pub struct Range<'a, K: 'a, V: 'a> {
-    front: Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>,
-    back: Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>,
+    front: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>>,
+    back: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>>,
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
@@ -437,8 +446,8 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Range<'_, K, V> {
 /// [`BTreeMap`]: struct.BTreeMap.html
 #[stable(feature = "btree_range", since = "1.17.0")]
 pub struct RangeMut<'a, K: 'a, V: 'a> {
-    front: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
-    back: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
+    front: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
+    back: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
 
     // Be invariant in `K` and `V`
     _marker: PhantomData<&'a mut (K, V)>,
@@ -447,7 +456,10 @@ pub struct RangeMut<'a, K: 'a, V: 'a> {
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for RangeMut<'_, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let range = Range { front: self.front.reborrow(), back: self.back.reborrow() };
+        let range = Range {
+            front: self.front.as_ref().map(|f| f.reborrow()),
+            back: self.back.as_ref().map(|b| b.reborrow()),
+        };
         f.debug_list().entries(range).finish()
     }
 }
@@ -544,7 +556,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new() -> BTreeMap<K, V> {
-        BTreeMap { root: node::Root::shared_empty_root(), length: 0 }
+        BTreeMap { root: None, length: 0 }
     }
 
     /// Clears the map, removing all elements.
@@ -589,7 +601,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_ref(), key) {
+        match search::search_tree(self.root.as_ref()?.as_ref(), key) {
             Found(handle) => Some(handle.into_kv().1),
             GoDown(_) => None,
         }
@@ -616,7 +628,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_ref(), k) {
+        match search::search_tree(self.root.as_ref()?.as_ref(), k) {
             Found(handle) => Some(handle.into_kv()),
             GoDown(_) => None,
         }
@@ -645,7 +657,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let front = self.root.as_ref().first_leaf_edge();
+        let front = self.root.as_ref()?.as_ref().first_leaf_edge();
         front.right_kv().ok().map(Handle::into_kv)
     }
 
@@ -674,7 +686,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let front = self.root.as_mut().first_leaf_edge();
+        let front = self.root.as_mut()?.as_mut().first_leaf_edge();
         if let Ok(kv) = front.right_kv() {
             Some(OccupiedEntry {
                 handle: kv.forget_node_type(),
@@ -708,7 +720,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let back = self.root.as_ref().last_leaf_edge();
+        let back = self.root.as_ref()?.as_ref().last_leaf_edge();
         back.left_kv().ok().map(Handle::into_kv)
     }
 
@@ -737,7 +749,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let back = self.root.as_mut().last_leaf_edge();
+        let back = self.root.as_mut()?.as_mut().last_leaf_edge();
         if let Ok(kv) = back.left_kv() {
             Some(OccupiedEntry {
                 handle: kv.forget_node_type(),
@@ -801,7 +813,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_mut(), key) {
+        match search::search_tree(self.root.as_mut()?.as_mut(), key) {
             Found(handle) => Some(handle.into_kv_mut().1),
             GoDown(_) => None,
         }
@@ -896,7 +908,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_mut(), key) {
+        match search::search_tree(self.root.as_mut()?.as_mut(), key) {
             Found(handle) => Some(
                 OccupiedEntry { handle, length: &mut self.length, _marker: PhantomData }
                     .remove_entry(),
@@ -992,11 +1004,15 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<T>,
         R: RangeBounds<T>,
     {
-        let root1 = self.root.as_ref();
-        let root2 = self.root.as_ref();
-        let (f, b) = range_search(root1, root2, range);
+        if let Some(root) = &self.root {
+            let root1 = root.as_ref();
+            let root2 = root.as_ref();
+            let (f, b) = range_search(root1, root2, range);
 
-        Range { front: f, back: b }
+            Range { front: Some(f), back: Some(b) }
+        } else {
+            Range { front: None, back: None }
+        }
     }
 
     /// Constructs a mutable double-ended iterator over a sub-range of elements in the map.
@@ -1036,11 +1052,15 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<T>,
         R: RangeBounds<T>,
     {
-        let root1 = self.root.as_mut();
-        let root2 = unsafe { ptr::read(&root1) };
-        let (f, b) = range_search(root1, root2, range);
+        if let Some(root) = &mut self.root {
+            let root1 = root.as_mut();
+            let root2 = unsafe { ptr::read(&root1) };
+            let (f, b) = range_search(root1, root2, range);
 
-        RangeMut { front: f, back: b, _marker: PhantomData }
+            RangeMut { front: Some(f), back: Some(b), _marker: PhantomData }
+        } else {
+            RangeMut { front: None, back: None, _marker: PhantomData }
+        }
     }
 
     /// Gets the given key's corresponding entry in the map for in-place manipulation.
@@ -1065,7 +1085,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
         // FIXME(@porglezomp) Avoid allocating if we don't insert
         self.ensure_root_is_owned();
-        match search::search_tree(self.root.as_mut(), &key) {
+        match search::search_tree(self.root.as_mut().unwrap().as_mut(), &key) {
             Found(handle) => {
                 Occupied(OccupiedEntry { handle, length: &mut self.length, _marker: PhantomData })
             }
@@ -1077,7 +1097,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
     fn from_sorted_iter<I: Iterator<Item = (K, V)>>(&mut self, iter: I) {
         self.ensure_root_is_owned();
-        let mut cur_node = self.root.as_mut().last_leaf_edge().into_node();
+        let mut cur_node = self.root.as_mut().unwrap().as_mut().last_leaf_edge().into_node();
         // Iterate through all key-value pairs, pushing them into nodes at the right level.
         for (key, value) in iter {
             // Try to push key-value pair into the current leaf node.
@@ -1126,7 +1146,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
     fn fix_right_edge(&mut self) {
         // Handle underfull nodes, start from the top.
-        let mut cur_node = self.root.as_mut();
+        let mut cur_node = self.root.as_mut().unwrap().as_mut();
         while let Internal(internal) = cur_node.force() {
             // Check if right-most child is underfull.
             let mut last_edge = internal.last_edge();
@@ -1187,14 +1207,14 @@ impl<K: Ord, V> BTreeMap<K, V> {
         let total_num = self.len();
 
         let mut right = Self::new();
-        right.root = node::Root::new_leaf();
-        for _ in 0..(self.root.as_ref().height()) {
-            right.root.push_level();
+        let right_root = right.ensure_root_is_owned();
+        for _ in 0..(self.root.as_ref().unwrap().as_ref().height()) {
+            right_root.push_level();
         }
 
         {
-            let mut left_node = self.root.as_mut();
-            let mut right_node = right.root.as_mut();
+            let mut left_node = self.root.as_mut().unwrap().as_mut();
+            let mut right_node = right.root.as_mut().unwrap().as_mut();
 
             loop {
                 let mut split_edge = match search::search_node(left_node, key) {
@@ -1223,7 +1243,9 @@ impl<K: Ord, V> BTreeMap<K, V> {
         self.fix_right_border();
         right.fix_left_border();
 
-        if self.root.as_ref().height() < right.root.as_ref().height() {
+        if self.root.as_ref().unwrap().as_ref().height()
+            < right.root.as_ref().unwrap().as_ref().height()
+        {
             self.recalc_length();
             right.length = total_num - self.len();
         } else {
@@ -1261,19 +1283,19 @@ impl<K: Ord, V> BTreeMap<K, V> {
             res
         }
 
-        self.length = dfs(self.root.as_ref());
+        self.length = dfs(self.root.as_ref().unwrap().as_ref());
     }
 
     /// Removes empty levels on the top.
     fn fix_top(&mut self) {
         loop {
             {
-                let node = self.root.as_ref();
+                let node = self.root.as_ref().unwrap().as_ref();
                 if node.height() == 0 || node.len() > 0 {
                     break;
                 }
             }
-            self.root.pop_level();
+            self.root.as_mut().unwrap().pop_level();
         }
     }
 
@@ -1281,7 +1303,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         self.fix_top();
 
         {
-            let mut cur_node = self.root.as_mut();
+            let mut cur_node = self.root.as_mut().unwrap().as_mut();
 
             while let Internal(node) = cur_node.force() {
                 let mut last_kv = node.last_kv();
@@ -1307,7 +1329,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         self.fix_top();
 
         {
-            let mut cur_node = self.root.as_mut();
+            let mut cur_node = self.root.as_mut().unwrap().as_mut();
 
             while let Internal(node) = cur_node.force() {
                 let mut first_kv = node.first_kv();
@@ -1326,13 +1348,6 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
         self.fix_top();
     }
-
-    /// If the root node is the shared root node, allocate our own node.
-    fn ensure_root_is_owned(&mut self) {
-        if self.root.is_shared_root() {
-            self.root = node::Root::new_leaf();
-        }
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1458,12 +1473,21 @@ impl<K, V> IntoIterator for BTreeMap<K, V> {
     type IntoIter = IntoIter<K, V>;
 
     fn into_iter(self) -> IntoIter<K, V> {
-        let root1 = unsafe { ptr::read(&self.root).into_ref() };
-        let root2 = unsafe { ptr::read(&self.root).into_ref() };
+        if self.root.is_none() {
+            mem::forget(self);
+            return IntoIter { front: None, back: None, length: 0 };
+        }
+
+        let root1 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
+        let root2 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
         let len = self.length;
         mem::forget(self);
 
-        IntoIter { front: root1.first_leaf_edge(), back: root2.last_leaf_edge(), length: len }
+        IntoIter {
+            front: Some(root1.first_leaf_edge()),
+            back: Some(root2.last_leaf_edge()),
+            length: len,
+        }
     }
 }
 
@@ -1478,9 +1502,9 @@ impl<K, V> Drop for IntoIter<K, V> {
                 // don't have to care about panics this time (they'll abort).
                 while let Some(_) = self.0.next() {}
 
-                // No need to avoid the shared root, because the tree was definitely not empty.
                 unsafe {
-                    let mut node = ptr::read(&self.0.front).into_node().forget_type();
+                    let mut node =
+                        unwrap_unchecked(ptr::read(&self.0.front)).into_node().forget_type();
                     while let Some(parent) = node.deallocate_and_ascend() {
                         node = parent.into_node().forget_type();
                     }
@@ -1495,14 +1519,13 @@ impl<K, V> Drop for IntoIter<K, V> {
         }
 
         unsafe {
-            let mut node = ptr::read(&self.front).into_node().forget_type();
-            if node.is_shared_root() {
-                return;
-            }
-            // Most of the nodes have been deallocated while traversing
-            // but one pile from a leaf up to the root is left standing.
-            while let Some(parent) = node.deallocate_and_ascend() {
-                node = parent.into_node().forget_type();
+            if let Some(front) = ptr::read(&self.front) {
+                let mut node = front.into_node().forget_type();
+                // Most of the nodes have been deallocated while traversing
+                // but one pile from a leaf up to the root is left standing.
+                while let Some(parent) = node.deallocate_and_ascend() {
+                    node = parent.into_node().forget_type();
+                }
             }
         }
     }
@@ -1517,7 +1540,7 @@ impl<K, V> Iterator for IntoIter<K, V> {
             None
         } else {
             self.length -= 1;
-            Some(unsafe { self.front.next_unchecked() })
+            Some(unsafe { self.front.as_mut().unwrap().next_unchecked() })
         }
     }
 
@@ -1533,7 +1556,7 @@ impl<K, V> DoubleEndedIterator for IntoIter<K, V> {
             None
         } else {
             self.length -= 1;
-            Some(unsafe { self.back.next_back_unchecked() })
+            Some(unsafe { self.back.as_mut().unwrap().next_back_unchecked() })
         }
     }
 }
@@ -1683,7 +1706,7 @@ impl<'a, K, V> Range<'a, K, V> {
     }
 
     unsafe fn next_unchecked(&mut self) -> (&'a K, &'a V) {
-        self.front.next_unchecked()
+        unwrap_unchecked(self.front.as_mut()).next_unchecked()
     }
 }
 
@@ -1696,7 +1719,7 @@ impl<'a, K, V> DoubleEndedIterator for Range<'a, K, V> {
 
 impl<'a, K, V> Range<'a, K, V> {
     unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a V) {
-        self.back.next_back_unchecked()
+        unwrap_unchecked(self.back.as_mut()).next_back_unchecked()
     }
 }
 
@@ -1734,7 +1757,7 @@ impl<'a, K, V> RangeMut<'a, K, V> {
     }
 
     unsafe fn next_unchecked(&mut self) -> (&'a mut K, &'a mut V) {
-        self.front.next_unchecked()
+        unwrap_unchecked(self.front.as_mut()).next_unchecked()
     }
 }
 
@@ -1755,7 +1778,7 @@ impl<K, V> FusedIterator for RangeMut<'_, K, V> {}
 
 impl<'a, K, V> RangeMut<'a, K, V> {
     unsafe fn next_back_unchecked(&mut self) -> (&'a mut K, &'a mut V) {
-        self.back.next_back_unchecked()
+        unwrap_unchecked(self.back.as_mut()).next_back_unchecked()
     }
 }
 
@@ -1969,8 +1992,8 @@ impl<K, V> BTreeMap<K, V> {
     pub fn iter(&self) -> Iter<'_, K, V> {
         Iter {
             range: Range {
-                front: self.root.as_ref().first_leaf_edge(),
-                back: self.root.as_ref().last_leaf_edge(),
+                front: self.root.as_ref().map(|r| r.as_ref().first_leaf_edge()),
+                back: self.root.as_ref().map(|r| r.as_ref().last_leaf_edge()),
             },
             length: self.length,
         }
@@ -1999,13 +2022,17 @@ impl<K, V> BTreeMap<K, V> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
-        let root1 = self.root.as_mut();
-        let root2 = unsafe { ptr::read(&root1) };
         IterMut {
-            range: RangeMut {
-                front: root1.first_leaf_edge(),
-                back: root2.last_leaf_edge(),
-                _marker: PhantomData,
+            range: if let Some(root) = &mut self.root {
+                let root1 = root.as_mut();
+                let root2 = unsafe { ptr::read(&root1) };
+                RangeMut {
+                    front: Some(root1.first_leaf_edge()),
+                    back: Some(root2.last_leaf_edge()),
+                    _marker: PhantomData,
+                }
+            } else {
+                RangeMut { front: None, back: None, _marker: PhantomData }
             },
             length: self.length,
         }
@@ -2116,6 +2143,12 @@ impl<K, V> BTreeMap<K, V> {
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
+
+    /// If the root node is the empty (non-allocated) root node, allocate our
+    /// own node.
+    fn ensure_root_is_owned(&mut self) -> &mut node::Root<K, V> {
+        self.root.get_or_insert_with(|| node::Root::new_leaf())
+    }
 }
 
 impl<'a, K: Ord, V> Entry<'a, K, V> {
diff --git a/src/liballoc/collections/btree/node.rs b/src/liballoc/collections/btree/node.rs
index 1132ffdaf8005..6ebb98c42cd4f 100644
--- a/src/liballoc/collections/btree/node.rs
+++ b/src/liballoc/collections/btree/node.rs
@@ -44,34 +44,7 @@ const B: usize = 6;
 pub const MIN_LEN: usize = B - 1;
 pub const CAPACITY: usize = 2 * B - 1;
 
-/// The underlying representation of leaf nodes. Note that it is often unsafe to actually store
-/// these, since only the first `len` keys and values are assumed to be initialized. As such,
-/// these should always be put behind pointers, and specifically behind `BoxedNode` in the owned
-/// case.
-///
-/// We have a separate type for the header and rely on it matching the prefix of `LeafNode`, in
-/// order to statically allocate a single dummy node to avoid allocations. This struct is
-/// `repr(C)` to prevent them from being reordered. `LeafNode` does not just contain a
-/// `NodeHeader` because we do not want unnecessary padding between `len` and the keys.
-/// Crucially, `NodeHeader` can be safely transmuted to different K and V. (This is exploited
-/// by `as_header`.)
-#[repr(C)]
-struct NodeHeader<K, V> {
-    /// We use `*const` as opposed to `*mut` so as to be covariant in `K` and `V`.
-    /// This either points to an actual node or is null.
-    parent: *const InternalNode<K, V>,
-
-    /// This node's index into the parent node's `edges` array.
-    /// `*node.parent.edges[node.parent_idx]` should be the same thing as `node`.
-    /// This is only guaranteed to be initialized when `parent` is non-null.
-    parent_idx: MaybeUninit<u16>,
-
-    /// The number of keys and values this node stores.
-    ///
-    /// This next to `parent_idx` to encourage the compiler to join `len` and
-    /// `parent_idx` into the same 32-bit word, reducing space overhead.
-    len: u16,
-}
+/// The underlying representation of leaf nodes.
 #[repr(C)]
 struct LeafNode<K, V> {
     /// We use `*const` as opposed to `*mut` so as to be covariant in `K` and `V`.
@@ -111,21 +84,6 @@ impl<K, V> LeafNode<K, V> {
     }
 }
 
-impl<K, V> NodeHeader<K, V> {
-    fn is_shared_root(&self) -> bool {
-        ptr::eq(self, &EMPTY_ROOT_NODE as *const _ as *const _)
-    }
-}
-
-// We need to implement Sync here in order to make a static instance.
-unsafe impl Sync for NodeHeader<(), ()> {}
-
-// An empty node used as a placeholder for the root node, to avoid allocations.
-// We use just a header in order to save space, since no operation on an empty tree will
-// ever take a pointer past the first key.
-static EMPTY_ROOT_NODE: NodeHeader<(), ()> =
-    NodeHeader { parent: ptr::null(), parent_idx: MaybeUninit::uninit(), len: 0 };
-
 /// The underlying representation of internal nodes. As with `LeafNode`s, these should be hidden
 /// behind `BoxedNode`s to prevent dropping uninitialized keys and values. Any pointer to an
 /// `InternalNode` can be directly casted to a pointer to the underlying `LeafNode` portion of the
@@ -154,12 +112,9 @@ impl<K, V> InternalNode<K, V> {
 }
 
 /// A managed, non-null pointer to a node. This is either an owned pointer to
-/// `LeafNode<K, V>`, an owned pointer to `InternalNode<K, V>`, or a (not owned)
-/// pointer to `NodeHeader<(), ()` (more specifically, the pointer to EMPTY_ROOT_NODE).
-/// All of these types have a `NodeHeader<K, V>` prefix, meaning that they have at
-/// least the same size as `NodeHeader<K, V>` and store the same kinds of data at the same
-/// offsets; and they have a pointer alignment at least as large as `NodeHeader<K, V>`'s.
-/// However, `BoxedNode` contains no information as to which of the three types
+/// `LeafNode<K, V>` or an owned pointer to `InternalNode<K, V>`.
+///
+/// However, `BoxedNode` contains no information as to which of the two types
 /// of nodes it actually contains, and, partially due to this lack of information,
 /// has no destructor.
 struct BoxedNode<K, V> {
@@ -184,8 +139,9 @@ impl<K, V> BoxedNode<K, V> {
     }
 }
 
-/// Either an owned tree or a shared, empty tree.  Note that this does not have a destructor,
-/// and must be cleaned up manually if it is an owned tree.
+/// An owned tree.
+///
+/// Note that this does not have a destructor, and must be cleaned up manually.
 pub struct Root<K, V> {
     node: BoxedNode<K, V>,
     /// The number of levels below the root node.
@@ -196,20 +152,6 @@ unsafe impl<K: Sync, V: Sync> Sync for Root<K, V> {}
 unsafe impl<K: Send, V: Send> Send for Root<K, V> {}
 
 impl<K, V> Root<K, V> {
-    /// Whether the instance of `Root` wraps a shared, empty root node. If not,
-    /// the entire tree is uniquely owned by the owner of the `Root` instance.
-    pub fn is_shared_root(&self) -> bool {
-        self.as_ref().is_shared_root()
-    }
-
-    /// Returns a shared tree, wrapping a shared root node that is eternally empty.
-    pub fn shared_empty_root() -> Self {
-        Root {
-            node: unsafe { BoxedNode::from_ptr(NonNull::from(&EMPTY_ROOT_NODE).cast()) },
-            height: 0,
-        }
-    }
-
     /// Returns a new owned tree, with its own root node that is initially empty.
     pub fn new_leaf() -> Self {
         Root { node: BoxedNode::from_leaf(Box::new(unsafe { LeafNode::new() })), height: 0 }
@@ -245,7 +187,6 @@ impl<K, V> Root<K, V> {
     /// Adds a new internal node with a single edge, pointing to the previous root, and make that
     /// new node the root. This increases the height by 1 and is the opposite of `pop_level`.
     pub fn push_level(&mut self) -> NodeRef<marker::Mut<'_>, K, V, marker::Internal> {
-        debug_assert!(!self.is_shared_root());
         let mut new_node = Box::new(unsafe { InternalNode::new() });
         new_node.edges[0].write(unsafe { BoxedNode::from_ptr(self.node.as_ptr()) });
 
@@ -308,11 +249,6 @@ impl<K, V> Root<K, V> {
 ///   `Leaf`, the `NodeRef` points to a leaf node, when this is `Internal` the
 ///   `NodeRef` points to an internal node, and when this is `LeafOrInternal` the
 ///   `NodeRef` could be pointing to either type of node.
-///   Note that in case of a leaf node, this might still be the shared root!
-///   Only turn this into a `LeafNode` reference if you know it is not the shared root!
-///   Shared references must be dereferenceable *for the entire size of their pointee*,
-///   so '&LeafNode` or `&InternalNode` pointing to the shared root is undefined behavior.
-///   Turning this into a `NodeHeader` reference is always safe.
 pub struct NodeRef<BorrowType, K, V, Type> {
     /// The number of levels below the node.
     height: usize,
@@ -354,7 +290,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     /// Note that, despite being safe, calling this function can have the side effect
     /// of invalidating mutable references that unsafe code has created.
     pub fn len(&self) -> usize {
-        self.as_header().len as usize
+        self.as_leaf().len as usize
     }
 
     /// Returns the height of this node in the whole tree. Zero height denotes the
@@ -374,35 +310,24 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
         NodeRef { height: self.height, node: self.node, root: self.root, _marker: PhantomData }
     }
 
-    /// Exposes the leaf "portion" of any leaf or internal node that is not the shared root.
+    /// Exposes the leaf "portion" of any leaf or internal node.
     /// If the node is a leaf, this function simply opens up its data.
     /// If the node is an internal node, so not a leaf, it does have all the data a leaf has
     /// (header, keys and values), and this function exposes that.
-    /// Unsafe because the node must not be the shared root. For more information,
-    /// see the `NodeRef` comments.
-    unsafe fn as_leaf(&self) -> &LeafNode<K, V> {
-        debug_assert!(!self.is_shared_root());
-        self.node.as_ref()
-    }
-
-    fn as_header(&self) -> &NodeHeader<K, V> {
-        unsafe { &*(self.node.as_ptr() as *const NodeHeader<K, V>) }
-    }
-
-    /// Returns whether the node is the shared, empty root.
-    pub fn is_shared_root(&self) -> bool {
-        self.as_header().is_shared_root()
+    fn as_leaf(&self) -> &LeafNode<K, V> {
+        // The node must be valid for at least the LeafNode portion.
+        // This is not a reference in the NodeRef type because we don't know if
+        // it should be unique or shared.
+        unsafe { self.node.as_ref() }
     }
 
     /// Borrows a view into the keys stored in the node.
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    pub unsafe fn keys(&self) -> &[K] {
+    pub fn keys(&self) -> &[K] {
         self.reborrow().into_key_slice()
     }
 
     /// Borrows a view into the values stored in the node.
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn vals(&self) -> &[V] {
+    fn vals(&self) -> &[V] {
         self.reborrow().into_val_slice()
     }
 
@@ -416,7 +341,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     pub fn ascend(
         self,
     ) -> Result<Handle<NodeRef<BorrowType, K, V, marker::Internal>, marker::Edge>, Self> {
-        let parent_as_leaf = self.as_header().parent as *const LeafNode<K, V>;
+        let parent_as_leaf = self.as_leaf().parent as *const LeafNode<K, V>;
         if let Some(non_zero) = NonNull::new(parent_as_leaf as *mut _) {
             Ok(Handle {
                 node: NodeRef {
@@ -425,7 +350,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
                     root: self.root,
                     _marker: PhantomData,
                 },
-                idx: unsafe { usize::from(*self.as_header().parent_idx.as_ptr()) },
+                idx: unsafe { usize::from(*self.as_leaf().parent_idx.as_ptr()) },
                 _marker: PhantomData,
             })
         } else {
@@ -464,7 +389,6 @@ impl<K, V> NodeRef<marker::Owned, K, V, marker::LeafOrInternal> {
     pub unsafe fn deallocate_and_ascend(
         self,
     ) -> Option<Handle<NodeRef<marker::Owned, K, V, marker::Internal>, marker::Edge>> {
-        assert!(!self.is_shared_root());
         let height = self.height;
         let node = self.node;
         let ret = self.ascend().ok();
@@ -507,41 +431,37 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
     /// (header, keys and values), and this function exposes that.
     ///
     /// Returns a raw ptr to avoid asserting exclusive access to the entire node.
-    /// This also implies you can invoke this member on the shared root, but the resulting pointer
-    /// might not be properly aligned and definitely would not allow accessing keys and values.
     fn as_leaf_mut(&mut self) -> *mut LeafNode<K, V> {
         self.node.as_ptr()
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn keys_mut(&mut self) -> &mut [K] {
-        self.reborrow_mut().into_key_slice_mut()
+    fn keys_mut(&mut self) -> &mut [K] {
+        // SAFETY: the caller will not be able to call further methods on self
+        // until the key slice reference is dropped, as we have unique access
+        // for the lifetime of the borrow.
+        unsafe { self.reborrow_mut().into_key_slice_mut() }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn vals_mut(&mut self) -> &mut [V] {
-        self.reborrow_mut().into_val_slice_mut()
+    fn vals_mut(&mut self) -> &mut [V] {
+        // SAFETY: the caller will not be able to call further methods on self
+        // until the value slice reference is dropped, as we have unique access
+        // for the lifetime of the borrow.
+        unsafe { self.reborrow_mut().into_val_slice_mut() }
     }
 }
 
 impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_key_slice(self) -> &'a [K] {
-        debug_assert!(!self.is_shared_root());
-        // We cannot be the shared root, so `as_leaf` is okay.
-        slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().keys), self.len())
+    fn into_key_slice(self) -> &'a [K] {
+        unsafe { slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().keys), self.len()) }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_val_slice(self) -> &'a [V] {
-        debug_assert!(!self.is_shared_root());
-        // We cannot be the shared root, so `as_leaf` is okay.
-        slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().vals), self.len())
+    fn into_val_slice(self) -> &'a [V] {
+        unsafe { slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().vals), self.len()) }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_slices(self) -> (&'a [K], &'a [V]) {
-        let k = ptr::read(&self);
+    fn into_slices(self) -> (&'a [K], &'a [V]) {
+        // SAFETY: equivalent to reborrow() except not requiring Type: 'a
+        let k = unsafe { ptr::read(&self) };
         (k.into_key_slice(), self.into_val_slice())
     }
 }
@@ -553,28 +473,27 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         unsafe { &mut *(self.root as *mut Root<K, V>) }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_key_slice_mut(mut self) -> &'a mut [K] {
-        debug_assert!(!self.is_shared_root());
-        // We cannot be the shared root, so `as_leaf_mut` is okay.
-        slice::from_raw_parts_mut(
-            MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
-            self.len(),
-        )
+    fn into_key_slice_mut(mut self) -> &'a mut [K] {
+        // SAFETY: The keys of a node must always be initialized up to length.
+        unsafe {
+            slice::from_raw_parts_mut(
+                MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
+                self.len(),
+            )
+        }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_val_slice_mut(mut self) -> &'a mut [V] {
-        debug_assert!(!self.is_shared_root());
-        slice::from_raw_parts_mut(
-            MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).vals),
-            self.len(),
-        )
+    fn into_val_slice_mut(mut self) -> &'a mut [V] {
+        // SAFETY: The values of a node must always be initialized up to length.
+        unsafe {
+            slice::from_raw_parts_mut(
+                MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).vals),
+                self.len(),
+            )
+        }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_slices_mut(mut self) -> (&'a mut [K], &'a mut [V]) {
-        debug_assert!(!self.is_shared_root());
+    fn into_slices_mut(mut self) -> (&'a mut [K], &'a mut [V]) {
         // We cannot use the getters here, because calling the second one
         // invalidates the reference returned by the first.
         // More precisely, it is the call to `len` that is the culprit,
@@ -582,8 +501,13 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         // overlap with the keys (and even the values, for ZST keys).
         let len = self.len();
         let leaf = self.as_leaf_mut();
-        let keys = slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).keys), len);
-        let vals = slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).vals), len);
+        // SAFETY: The keys and values of a node must always be initialized up to length.
+        let keys = unsafe {
+            slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).keys), len)
+        };
+        let vals = unsafe {
+            slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).vals), len)
+        };
         (keys, vals)
     }
 }
@@ -592,7 +516,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Leaf> {
     /// Adds a key/value pair the end of the node.
     pub fn push(&mut self, key: K, val: V) {
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         let idx = self.len();
 
@@ -607,7 +530,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Leaf> {
     /// Adds a key/value pair to the beginning of the node.
     pub fn push_front(&mut self, key: K, val: V) {
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         unsafe {
             slice_insert(self.keys_mut(), 0, key);
@@ -624,7 +546,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
     pub fn push(&mut self, key: K, val: V, edge: Root<K, V>) {
         assert!(edge.height == self.height - 1);
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         let idx = self.len();
 
@@ -658,7 +579,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
     pub fn push_front(&mut self, key: K, val: V, edge: Root<K, V>) {
         assert!(edge.height == self.height - 1);
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         unsafe {
             slice_insert(self.keys_mut(), 0, key);
@@ -744,8 +664,7 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
         }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_kv_pointers_mut(mut self) -> (*mut K, *mut V) {
+    fn into_kv_pointers_mut(mut self) -> (*mut K, *mut V) {
         (self.keys_mut().as_mut_ptr(), self.vals_mut().as_mut_ptr())
     }
 }
@@ -904,7 +823,6 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge
     fn insert_fit(&mut self, key: K, val: V) -> *mut V {
         // Necessary for correctness, but in a private module
         debug_assert!(self.node.len() < CAPACITY);
-        debug_assert!(!self.node.is_shared_root());
 
         unsafe {
             slice_insert(self.node.keys_mut(), self.idx, key);
@@ -1081,7 +999,6 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV>
     /// - All the key/value pairs to the right of this handle are put into a newly
     ///   allocated node.
     pub fn split(mut self) -> (NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, K, V, Root<K, V>) {
-        assert!(!self.node.is_shared_root());
         unsafe {
             let mut new_node = Box::new(LeafNode::new());
 
@@ -1113,7 +1030,6 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV>
     pub fn remove(
         mut self,
     ) -> (Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>, K, V) {
-        assert!(!self.node.is_shared_root());
         unsafe {
             let k = slice_remove(self.node.keys_mut(), self.idx);
             let v = slice_remove(self.node.vals_mut(), self.idx);
diff --git a/src/liballoc/collections/btree/search.rs b/src/liballoc/collections/btree/search.rs
index 2ba5cebbdee74..4e80f7f21ebff 100644
--- a/src/liballoc/collections/btree/search.rs
+++ b/src/liballoc/collections/btree/search.rs
@@ -67,19 +67,16 @@ where
     Q: Ord,
     K: Borrow<Q>,
 {
-    // This function is defined over all borrow types (immutable, mutable, owned),
-    // and may be called on the shared root in each case.
+    // This function is defined over all borrow types (immutable, mutable, owned).
     // Using `keys()` is fine here even if BorrowType is mutable, as all we return
     // is an index -- not a reference.
     let len = node.len();
-    if len > 0 {
-        let keys = unsafe { node.keys() }; // safe because a non-empty node cannot be the shared root
-        for (i, k) in keys.iter().enumerate() {
-            match key.cmp(k.borrow()) {
-                Ordering::Greater => {}
-                Ordering::Equal => return (i, true),
-                Ordering::Less => return (i, false),
-            }
+    let keys = node.keys();
+    for (i, k) in keys.iter().enumerate() {
+        match key.cmp(k.borrow()) {
+            Ordering::Greater => {}
+            Ordering::Equal => return (i, true),
+            Ordering::Less => return (i, false),
         }
     }
     (len, false)
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index ffa4176cc7969..c3875d1621ad8 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -80,6 +80,7 @@
 #![feature(box_into_raw_non_null)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
+#![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
 #![feature(const_generic_impls_guard)]
@@ -98,6 +99,7 @@
 #![feature(internal_uninit_const)]
 #![feature(lang_items)]
 #![feature(libc)]
+#![cfg_attr(not(bootstrap), feature(negative_impls))]
 #![feature(nll)]
 #![feature(optin_builtin_traits)]
 #![feature(pattern)]
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 4a0cf2984edd9..d9b54fb0b177a 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -40,6 +40,23 @@ mod tests;
 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 
+#[cfg(not(sanitize = "thread"))]
+macro_rules! acquire {
+    ($x:expr) => {
+        atomic::fence(Acquire)
+    };
+}
+
+// ThreadSanitizer does not support memory fences. To avoid false positive
+// reports in Arc / Weak implementation use atomic loads for synchronization
+// instead.
+#[cfg(sanitize = "thread")]
+macro_rules! acquire {
+    ($x:expr) => {
+        $x.load(Acquire)
+    };
+}
+
 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
 /// Reference Counted'.
 ///
@@ -402,7 +419,7 @@ impl<T> Arc<T> {
             return Err(this);
         }
 
-        atomic::fence(Acquire);
+        acquire!(this.inner().strong);
 
         unsafe {
             let elem = ptr::read(&this.ptr.as_ref().data);
@@ -739,7 +756,7 @@ impl<T: ?Sized> Arc<T> {
         ptr::drop_in_place(&mut self.ptr.as_mut().data);
 
         if self.inner().weak.fetch_sub(1, Release) == 1 {
-            atomic::fence(Acquire);
+            acquire!(self.inner().weak);
             Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
         }
     }
@@ -1243,7 +1260,7 @@ unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
         //
         // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
         // [2]: (https://github.com/rust-lang/rust/pull/41714)
-        atomic::fence(Acquire);
+        acquire!(self.inner().strong);
 
         unsafe {
             self.drop_slow();
@@ -1701,7 +1718,7 @@ impl<T: ?Sized> Drop for Weak<T> {
         let inner = if let Some(inner) = self.inner() { inner } else { return };
 
         if inner.weak.fetch_sub(1, Release) == 1 {
-            atomic::fence(Acquire);
+            acquire!(inner.weak);
             unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
         }
     }
diff --git a/src/liballoc/tests/btree/map.rs b/src/liballoc/tests/btree/map.rs
index d05eec19346de..3a3462d546f7a 100644
--- a/src/liballoc/tests/btree/map.rs
+++ b/src/liballoc/tests/btree/map.rs
@@ -67,7 +67,7 @@ fn test_basic_large() {
 #[test]
 fn test_basic_small() {
     let mut map = BTreeMap::new();
-    // Empty, shared root:
+    // Empty, root is absent (None):
     assert_eq!(map.remove(&1), None);
     assert_eq!(map.len(), 0);
     assert_eq!(map.get(&1), None);
@@ -123,7 +123,7 @@ fn test_basic_small() {
     assert_eq!(map.values().collect::<Vec<_>>(), vec![&4]);
     assert_eq!(map.remove(&2), Some(4));
 
-    // Empty but private root:
+    // Empty but root is owned (Some(...)):
     assert_eq!(map.len(), 0);
     assert_eq!(map.get(&1), None);
     assert_eq!(map.get_mut(&1), None);
@@ -263,13 +263,6 @@ fn test_iter_mut_mutation() {
     do_test_iter_mut_mutation::<Align32>(144);
 }
 
-#[test]
-fn test_into_key_slice_with_shared_root_past_bounds() {
-    let mut map: BTreeMap<Align32, ()> = BTreeMap::new();
-    assert_eq!(map.get(&Align32(1)), None);
-    assert_eq!(map.get_mut(&Align32(1)), None);
-}
-
 #[test]
 fn test_values_mut() {
     let mut a = BTreeMap::new();
diff --git a/src/libcore/clone.rs b/src/libcore/clone.rs
index 9a412e5729427..eb101fc72fd9e 100644
--- a/src/libcore/clone.rs
+++ b/src/libcore/clone.rs
@@ -219,7 +219,7 @@ mod impls {
         }
     }
 
-    // Shared references can be cloned, but mutable references *cannot*!
+    /// Shared references can be cloned, but mutable references *cannot*!
     #[stable(feature = "rust1", since = "1.0.0")]
     impl<T: ?Sized> Clone for &T {
         #[inline]
@@ -227,4 +227,9 @@ mod impls {
             *self
         }
     }
+
+    /// Shared references can be cloned, but mutable references *cannot*!
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg(not(bootstrap))]
+    impl<T: ?Sized> !Clone for &mut T {}
 }
diff --git a/src/libcore/hint.rs b/src/libcore/hint.rs
index f4fb9ab1757cd..6cbd26a78de72 100644
--- a/src/libcore/hint.rs
+++ b/src/libcore/hint.rs
@@ -114,6 +114,6 @@ pub fn black_box<T>(dummy: T) -> T {
     // more than we want, but it's so far good enough.
     unsafe {
         asm!("" : : "r"(&dummy));
-        return dummy;
+        dummy
     }
 }
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 5a731766054bd..0bee16f98bd5e 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -98,6 +98,7 @@
 #![feature(is_sorted)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
+#![cfg_attr(not(bootstrap), feature(negative_impls))]
 #![feature(never_type)]
 #![feature(nll)]
 #![feature(exhaustive_patterns)]
@@ -140,6 +141,7 @@
 #![feature(associated_type_bounds)]
 #![feature(const_type_id)]
 #![feature(const_caller_location)]
+#![feature(option_zip)]
 #![feature(no_niche)] // rust-lang/rust#68303
 
 #[prelude_import]
diff --git a/src/libcore/marker.rs b/src/libcore/marker.rs
index 2b908f07af8da..549f80de36aa5 100644
--- a/src/libcore/marker.rs
+++ b/src/libcore/marker.rs
@@ -87,7 +87,7 @@ impl<T: ?Sized> !Send for *mut T {}
     message = "the size for values of type `{Self}` cannot be known at compilation time",
     label = "doesn't have a size known at compile-time",
     note = "to learn more, visit <https://doc.rust-lang.org/book/\
-          ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>"
+            ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>"
 )]
 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
 #[cfg_attr(not(bootstrap), rustc_specialization_trait)]
@@ -790,7 +790,7 @@ mod copy_impls {
     #[stable(feature = "rust1", since = "1.0.0")]
     impl<T: ?Sized> Copy for *mut T {}
 
-    // Shared references can be copied, but mutable references *cannot*!
+    /// Shared references can be copied, but mutable references *cannot*!
     #[stable(feature = "rust1", since = "1.0.0")]
     impl<T: ?Sized> Copy for &T {}
 }
diff --git a/src/libcore/ops/deref.rs b/src/libcore/ops/deref.rs
index 68244fdb38114..e2deeb556610c 100644
--- a/src/libcore/ops/deref.rs
+++ b/src/libcore/ops/deref.rs
@@ -81,6 +81,10 @@ impl<T: ?Sized> Deref for &T {
     }
 }
 
+#[cfg(not(bootstrap))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> !DerefMut for &T {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Deref for &mut T {
     type Target = T;
diff --git a/src/libcore/ops/range.rs b/src/libcore/ops/range.rs
index 8ffad82b69d7c..adee8cea442b4 100644
--- a/src/libcore/ops/range.rs
+++ b/src/libcore/ops/range.rs
@@ -1,5 +1,5 @@
 use crate::fmt;
-use crate::hash::{Hash, Hasher};
+use crate::hash::Hash;
 
 /// An unbounded range (`..`).
 ///
@@ -330,7 +330,7 @@ impl<Idx: PartialOrd<Idx>> RangeTo<Idx> {
 /// assert_eq!(arr[1..=3], [  1,2,3  ]);  // RangeInclusive
 /// ```
 #[doc(alias = "..=")]
-#[derive(Clone)] // not Copy -- see #27186
+#[derive(Clone, PartialEq, Eq, Hash)] // not Copy -- see #27186
 #[stable(feature = "inclusive_range", since = "1.26.0")]
 pub struct RangeInclusive<Idx> {
     // Note that the fields here are not public to allow changing the
@@ -350,26 +350,6 @@ pub struct RangeInclusive<Idx> {
     pub(crate) exhausted: bool,
 }
 
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl<Idx: PartialEq> PartialEq for RangeInclusive<Idx> {
-    #[inline]
-    fn eq(&self, other: &Self) -> bool {
-        self.start == other.start && self.end == other.end && self.exhausted == other.exhausted
-    }
-}
-
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl<Idx: Eq> Eq for RangeInclusive<Idx> {}
-
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl<Idx: Hash> Hash for RangeInclusive<Idx> {
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        self.start.hash(state);
-        self.end.hash(state);
-        self.exhausted.hash(state);
-    }
-}
-
 impl<Idx> RangeInclusive<Idx> {
     /// Creates a new inclusive range. Equivalent to writing `start..=end`.
     ///
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index 9b32442371c37..3aab8b1b3337c 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -913,6 +913,63 @@ impl<T> Option<T> {
     pub fn replace(&mut self, value: T) -> Option<T> {
         mem::replace(self, Some(value))
     }
+
+    /// Zips `self` with another `Option`.
+    ///
+    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
+    /// Otherwise, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_zip)]
+    /// let x = Some(1);
+    /// let y = Some("hi");
+    /// let z = None::<u8>;
+    ///
+    /// assert_eq!(x.zip(y), Some((1, "hi")));
+    /// assert_eq!(x.zip(z), None);
+    /// ```
+    #[unstable(feature = "option_zip", issue = "70086")]
+    pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
+        self.zip_with(other, |a, b| (a, b))
+    }
+
+    /// Zips `self` and another `Option` with function `f`.
+    ///
+    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
+    /// Otherwise, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_zip)]
+    ///
+    /// #[derive(Debug, PartialEq)]
+    /// struct Point {
+    ///     x: f64,
+    ///     y: f64,
+    /// }
+    ///
+    /// impl Point {
+    ///     fn new(x: f64, y: f64) -> Self {
+    ///         Self { x, y }
+    ///     }
+    /// }
+    ///
+    /// let x = Some(17.5);
+    /// let y = Some(42.7);
+    ///
+    /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
+    /// assert_eq!(x.zip_with(None, Point::new), None);
+    /// ```
+    #[unstable(feature = "option_zip", issue = "70086")]
+    pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
+    where
+        F: FnOnce(T, U) -> R,
+    {
+        Some(f(self?, other?))
+    }
 }
 
 impl<T: Copy> Option<&T> {
diff --git a/src/libcore/tests/mem.rs b/src/libcore/tests/mem.rs
index 8337ab103419f..59588d97787b7 100644
--- a/src/libcore/tests/mem.rs
+++ b/src/libcore/tests/mem.rs
@@ -129,21 +129,3 @@ fn test_discriminant_send_sync() {
     is_send_sync::<Discriminant<Regular>>();
     is_send_sync::<Discriminant<NotSendSync>>();
 }
-
-#[test]
-fn test_const_forget() {
-    const _: () = forget(0i32);
-    const _: () = forget(Vec::<Vec<Box<i32>>>::new());
-
-    // Writing this function signature without const-forget
-    // triggers compiler errors:
-    // 1) That we use a non-const fn inside a const fn
-    // 2) without the forget, it complains about the destructor of Box
-    const fn const_forget_box<T>(x: Box<T>) {
-        forget(x);
-    }
-
-    // Call the forget_box at runtime,
-    // as we can't const-construct a box yet.
-    const_forget_box(Box::new(0i32));
-}
diff --git a/src/libproc_macro/lib.rs b/src/libproc_macro/lib.rs
index 8f16e3c08ec5b..a975ce93bb1a3 100644
--- a/src/libproc_macro/lib.rs
+++ b/src/libproc_macro/lib.rs
@@ -24,6 +24,7 @@
 #![feature(decl_macro)]
 #![feature(extern_types)]
 #![feature(in_band_lifetimes)]
+#![cfg_attr(not(bootstrap), feature(negative_impls))]
 #![feature(optin_builtin_traits)]
 #![feature(rustc_attrs)]
 #![cfg_attr(bootstrap, feature(specialization))]
diff --git a/src/librustc/lint.rs b/src/librustc/lint.rs
index d4d01a716db97..4dd276d2e032c 100644
--- a/src/librustc/lint.rs
+++ b/src/librustc/lint.rs
@@ -85,7 +85,7 @@ impl LintLevelSets {
             level = cmp::min(*driver_level, level);
         }
 
-        return (level, src);
+        (level, src)
     }
 
     pub fn get_lint_id_level(
diff --git a/src/librustc/middle/region.rs b/src/librustc/middle/region.rs
index 2735c4afca2c8..1a63dc9dcf977 100644
--- a/src/librustc/middle/region.rs
+++ b/src/librustc/middle/region.rs
@@ -467,7 +467,7 @@ impl<'tcx> ScopeTree {
         }
 
         debug!("temporary_scope({:?}) = None", expr_id);
-        return None;
+        None
     }
 
     /// Returns the lifetime of the variable `id`.
@@ -498,7 +498,7 @@ impl<'tcx> ScopeTree {
 
         debug!("is_subscope_of({:?}, {:?})=true", subscope, superscope);
 
-        return true;
+        true
     }
 
     /// Returns the ID of the innermost containing body.
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 742d57fb58a51..0e3776f32e0e8 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -1447,11 +1447,11 @@ impl<'tcx> TyCtxt<'tcx> {
             _ => return None,
         };
 
-        return Some(FreeRegionInfo {
+        Some(FreeRegionInfo {
             def_id: suitable_region_binding_scope,
             boundregion: bound_region,
             is_impl_item,
-        });
+        })
     }
 
     pub fn return_type_impl_trait(&self, scope_def_id: DefId) -> Option<(Ty<'tcx>, Span)> {
diff --git a/src/librustc/ty/relate.rs b/src/librustc/ty/relate.rs
index fb4184a9fb347..872e06e1176dc 100644
--- a/src/librustc/ty/relate.rs
+++ b/src/librustc/ty/relate.rs
@@ -440,7 +440,7 @@ pub fn super_relate_tys<R: TypeRelation<'tcx>>(
                         (Some(sz_a_val), Some(sz_b_val)) => Err(TypeError::FixedArraySize(
                             expected_found(relation, &sz_a_val, &sz_b_val),
                         )),
-                        _ => return Err(err),
+                        _ => Err(err),
                     }
                 }
             }
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index e265a2f8257fb..42cd2f52cb3ad 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -1612,7 +1612,7 @@ impl<'tcx> PolyExistentialProjection<'tcx> {
     }
 
     pub fn item_def_id(&self) -> DefId {
-        return self.skip_binder().item_def_id;
+        self.skip_binder().item_def_id
     }
 }
 
@@ -2000,8 +2000,8 @@ impl<'tcx> TyS<'tcx> {
     #[inline]
     pub fn is_unsafe_ptr(&self) -> bool {
         match self.kind {
-            RawPtr(_) => return true,
-            _ => return false,
+            RawPtr(_) => true,
+            _ => false,
         }
     }
 
diff --git a/src/librustc/ty/subst.rs b/src/librustc/ty/subst.rs
index a005581283550..a3acc14856e1f 100644
--- a/src/librustc/ty/subst.rs
+++ b/src/librustc/ty/subst.rs
@@ -524,7 +524,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
             self.root_ty = None;
         }
 
-        return t1;
+        t1
     }
 
     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
diff --git a/src/librustc_ast_passes/feature_gate.rs b/src/librustc_ast_passes/feature_gate.rs
index 364c86bd99b4e..55e1be5b472e2 100644
--- a/src/librustc_ast_passes/feature_gate.rs
+++ b/src/librustc_ast_passes/feature_gate.rs
@@ -286,8 +286,8 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                         start,
                         i.span,
                         "`#[start]` functions are experimental \
-                                       and their signature may change \
-                                       over time"
+                         and their signature may change \
+                         over time"
                     );
                 }
                 if attr::contains_name(&i.attrs[..], sym::main) {
@@ -296,8 +296,8 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                         main,
                         i.span,
                         "declaration of a non-standard `#[main]` \
-                                        function may change over time, for now \
-                                        a top-level `fn main()` is required"
+                         function may change over time, for now \
+                         a top-level `fn main()` is required"
                     );
                 }
             }
@@ -341,7 +341,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 if let ast::ImplPolarity::Negative(span) = polarity {
                     gate_feature_post!(
                         &self,
-                        optin_builtin_traits,
+                        negative_impls,
                         span.to(of_trait.as_ref().map(|t| t.path.span).unwrap_or(span)),
                         "negative trait bounds are not yet fully implemented; \
                          use marker types for now"
diff --git a/src/librustc_builtin_macros/deriving/decodable.rs b/src/librustc_builtin_macros/deriving/decodable.rs
index ac5d08ba62d0d..64a810bdcf687 100644
--- a/src/librustc_builtin_macros/deriving/decodable.rs
+++ b/src/librustc_builtin_macros/deriving/decodable.rs
@@ -87,7 +87,7 @@ fn decodable_substructure(
     let blkarg = cx.ident_of("_d", trait_span);
     let blkdecoder = cx.expr_ident(trait_span, blkarg);
 
-    return match *substr.fields {
+    match *substr.fields {
         StaticStruct(_, ref summary) => {
             let nfields = match *summary {
                 Unnamed(ref fields, _) => fields.len(),
@@ -178,7 +178,7 @@ fn decodable_substructure(
             )
         }
         _ => cx.bug("expected StaticEnum or StaticStruct in derive(Decodable)"),
-    };
+    }
 }
 
 /// Creates a decoder for a single enum variant/struct:
diff --git a/src/librustc_builtin_macros/deriving/default.rs b/src/librustc_builtin_macros/deriving/default.rs
index cb85a0b1a10cc..27d5263320041 100644
--- a/src/librustc_builtin_macros/deriving/default.rs
+++ b/src/librustc_builtin_macros/deriving/default.rs
@@ -53,7 +53,7 @@ fn default_substructure(
     let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
     let default_call = |span| cx.expr_call_global(span, default_ident.clone(), Vec::new());
 
-    return match *substr.fields {
+    match *substr.fields {
         StaticStruct(_, ref summary) => match *summary {
             Unnamed(ref fields, is_tuple) => {
                 if !is_tuple {
@@ -83,5 +83,5 @@ fn default_substructure(
             DummyResult::raw_expr(trait_span, true)
         }
         _ => cx.span_bug(trait_span, "method in `derive(Default)`"),
-    };
+    }
 }
diff --git a/src/librustc_builtin_macros/deriving/encodable.rs b/src/librustc_builtin_macros/deriving/encodable.rs
index 9073085381ac1..54926ec3fd502 100644
--- a/src/librustc_builtin_macros/deriving/encodable.rs
+++ b/src/librustc_builtin_macros/deriving/encodable.rs
@@ -173,7 +173,7 @@ fn encodable_substructure(
         ],
     ));
 
-    return match *substr.fields {
+    match *substr.fields {
         Struct(_, ref fields) => {
             let emit_struct_field = cx.ident_of("emit_struct_field", trait_span);
             let mut stmts = Vec::new();
@@ -283,5 +283,5 @@ fn encodable_substructure(
         }
 
         _ => cx.bug("expected Struct or EnumMatching in derive(Encodable)"),
-    };
+    }
 }
diff --git a/src/librustc_builtin_macros/deriving/generic/mod.rs b/src/librustc_builtin_macros/deriving/generic/mod.rs
index 84ed6e96aafc8..ee32e914acba4 100644
--- a/src/librustc_builtin_macros/deriving/generic/mod.rs
+++ b/src/librustc_builtin_macros/deriving/generic/mod.rs
@@ -489,7 +489,6 @@ impl<'a> TraitDef<'a> {
                 // set earlier; see
                 // librustc_expand/expand.rs:MacroExpander::fully_expand_fragment()
                 // librustc_expand/base.rs:Annotatable::derive_allowed()
-                return;
             }
         }
     }
diff --git a/src/librustc_builtin_macros/format_foreign.rs b/src/librustc_builtin_macros/format_foreign.rs
index cc3c403450e04..e6a87e4d82586 100644
--- a/src/librustc_builtin_macros/format_foreign.rs
+++ b/src/librustc_builtin_macros/format_foreign.rs
@@ -359,7 +359,7 @@ pub mod printf {
         //
         // Note: `move` used to capture copies of the cursors as they are *now*.
         let fallback = move || {
-            return Some((
+            Some((
                 Substitution::Format(Format {
                     span: start.slice_between(next).unwrap(),
                     parameter: None,
@@ -371,7 +371,7 @@ pub mod printf {
                     position: InnerSpan::new(start.at, next.at),
                 }),
                 next.slice_after(),
-            ));
+            ))
         };
 
         // Next parsing state.
diff --git a/src/librustc_builtin_macros/source_util.rs b/src/librustc_builtin_macros/source_util.rs
index 662bbe6a287a3..718498f04b94e 100644
--- a/src/librustc_builtin_macros/source_util.rs
+++ b/src/librustc_builtin_macros/source_util.rs
@@ -4,6 +4,7 @@ use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast_pretty::pprust;
 use rustc_expand::base::{self, *};
+use rustc_expand::module::DirectoryOwnership;
 use rustc_expand::panictry;
 use rustc_parse::{self, new_sub_parser_from_file, parser::Parser};
 use rustc_session::lint::builtin::INCOMPLETE_INCLUDE;
@@ -11,6 +12,7 @@ use rustc_span::symbol::Symbol;
 use rustc_span::{self, Pos, Span};
 
 use smallvec::SmallVec;
+use std::rc::Rc;
 
 use rustc_data_structures::sync::Lrc;
 
@@ -101,7 +103,7 @@ pub fn expand_include<'cx>(
         None => return DummyResult::any(sp),
     };
     // The file will be added to the code map by the parser
-    let file = match cx.resolve_path(file, sp) {
+    let mut file = match cx.resolve_path(file, sp) {
         Ok(f) => f,
         Err(mut err) => {
             err.emit();
@@ -110,6 +112,15 @@ pub fn expand_include<'cx>(
     };
     let p = new_sub_parser_from_file(cx.parse_sess(), &file, None, sp);
 
+    // If in the included file we have e.g., `mod bar;`,
+    // then the path of `bar.rs` should be relative to the directory of `file`.
+    // See https://github.com/rust-lang/rust/pull/69838/files#r395217057 for a discussion.
+    // `MacroExpander::fully_expand_fragment` later restores, so "stack discipline" is maintained.
+    file.pop();
+    cx.current_expansion.directory_ownership = DirectoryOwnership::Owned { relative: None };
+    let mod_path = cx.current_expansion.module.mod_path.clone();
+    cx.current_expansion.module = Rc::new(ModuleData { mod_path, directory: file });
+
     struct ExpandResult<'a> {
         p: Parser<'a>,
     }
diff --git a/src/librustc_codegen_llvm/back/archive.rs b/src/librustc_codegen_llvm/back/archive.rs
index 239ca57ba4143..f1fe40d919eeb 100644
--- a/src/librustc_codegen_llvm/back/archive.rs
+++ b/src/librustc_codegen_llvm/back/archive.rs
@@ -146,7 +146,7 @@ impl<'a> ArchiveBuilder<'a> for LlvmArchiveBuilder<'a> {
             }
 
             // ok, don't skip this
-            return false;
+            false
         })
     }
 
diff --git a/src/librustc_codegen_llvm/back/bytecode.rs b/src/librustc_codegen_llvm/back/bytecode.rs
index db29556e70ccc..0c8ce39132abb 100644
--- a/src/librustc_codegen_llvm/back/bytecode.rs
+++ b/src/librustc_codegen_llvm/back/bytecode.rs
@@ -83,7 +83,7 @@ pub fn encode(identifier: &str, bytecode: &[u8]) -> Vec<u8> {
         encoded.push(0);
     }
 
-    return encoded;
+    encoded
 }
 
 pub struct DecodedBytecode<'a> {
@@ -132,7 +132,7 @@ impl<'a> DecodedBytecode<'a> {
     pub fn bytecode(&self) -> Vec<u8> {
         let mut data = Vec::new();
         DeflateDecoder::new(self.encoded_bytecode).read_to_end(&mut data).unwrap();
-        return data;
+        data
     }
 
     pub fn identifier(&self) -> &'a str {
diff --git a/src/librustc_codegen_llvm/common.rs b/src/librustc_codegen_llvm/common.rs
index 609ddfc1d3a80..f72060868128c 100644
--- a/src/librustc_codegen_llvm/common.rs
+++ b/src/librustc_codegen_llvm/common.rs
@@ -96,15 +96,11 @@ impl BackendTypes for CodegenCx<'ll, 'tcx> {
 
 impl CodegenCx<'ll, 'tcx> {
     pub fn const_array(&self, ty: &'ll Type, elts: &[&'ll Value]) -> &'ll Value {
-        unsafe {
-            return llvm::LLVMConstArray(ty, elts.as_ptr(), elts.len() as c_uint);
-        }
+        unsafe { llvm::LLVMConstArray(ty, elts.as_ptr(), elts.len() as c_uint) }
     }
 
     pub fn const_vector(&self, elts: &[&'ll Value]) -> &'ll Value {
-        unsafe {
-            return llvm::LLVMConstVector(elts.as_ptr(), elts.len() as c_uint);
-        }
+        unsafe { llvm::LLVMConstVector(elts.as_ptr(), elts.len() as c_uint) }
     }
 
     pub fn const_bytes(&self, bytes: &[u8]) -> &'ll Value {
@@ -330,7 +326,7 @@ pub fn val_ty(v: &Value) -> &Type {
 pub fn bytes_in_context(llcx: &'ll llvm::Context, bytes: &[u8]) -> &'ll Value {
     unsafe {
         let ptr = bytes.as_ptr() as *const c_char;
-        return llvm::LLVMConstStringInContext(llcx, ptr, bytes.len() as c_uint, True);
+        llvm::LLVMConstStringInContext(llcx, ptr, bytes.len() as c_uint, True)
     }
 }
 
diff --git a/src/librustc_codegen_llvm/context.rs b/src/librustc_codegen_llvm/context.rs
index 7b1526e9da154..4427997c2732d 100644
--- a/src/librustc_codegen_llvm/context.rs
+++ b/src/librustc_codegen_llvm/context.rs
@@ -800,7 +800,7 @@ impl CodegenCx<'b, 'tcx> {
             ifn!("llvm.dbg.declare", fn(self.type_metadata(), self.type_metadata()) -> void);
             ifn!("llvm.dbg.value", fn(self.type_metadata(), t_i64, self.type_metadata()) -> void);
         }
-        return None;
+        None
     }
 }
 
diff --git a/src/librustc_codegen_llvm/debuginfo/metadata.rs b/src/librustc_codegen_llvm/debuginfo/metadata.rs
index 6a7ed4e1dc384..f35220cc6666a 100644
--- a/src/librustc_codegen_llvm/debuginfo/metadata.rs
+++ b/src/librustc_codegen_llvm/debuginfo/metadata.rs
@@ -203,7 +203,7 @@ impl TypeMap<'ll, 'tcx> {
         let key = self.unique_id_interner.intern(&unique_type_id);
         self.type_to_unique_id.insert(type_, UniqueTypeId(key));
 
-        return UniqueTypeId(key);
+        UniqueTypeId(key)
     }
 
     /// Gets the `UniqueTypeId` for an enum variant. Enum variants are not really
@@ -314,7 +314,7 @@ impl RecursiveTypeDescription<'ll, 'tcx> {
                     member_holding_stub,
                     member_descriptions,
                 );
-                return MetadataCreationResult::new(metadata_stub, true);
+                MetadataCreationResult::new(metadata_stub, true)
             }
         }
     }
@@ -364,7 +364,7 @@ fn fixed_vec_metadata(
         )
     };
 
-    return MetadataCreationResult::new(metadata, false);
+    MetadataCreationResult::new(metadata, false)
 }
 
 fn vec_slice_metadata(
@@ -445,7 +445,7 @@ fn subroutine_type_metadata(
 
     return_if_metadata_created_in_meantime!(cx, unique_type_id);
 
-    return MetadataCreationResult::new(
+    MetadataCreationResult::new(
         unsafe {
             llvm::LLVMRustDIBuilderCreateSubroutineType(
                 DIB(cx),
@@ -454,7 +454,7 @@ fn subroutine_type_metadata(
             )
         },
         false,
-    );
+    )
 }
 
 // FIXME(1563): This is all a bit of a hack because 'trait pointer' is an ill-
@@ -781,7 +781,7 @@ fn file_metadata_raw(
     let key = (file_name, directory);
 
     match debug_context(cx).created_files.borrow_mut().entry(key) {
-        Entry::Occupied(o) => return o.get(),
+        Entry::Occupied(o) => o.get(),
         Entry::Vacant(v) => {
             let (file_name, directory) = v.key();
             debug!("file_metadata: file_name: {:?}, directory: {:?}", file_name, directory);
@@ -831,7 +831,7 @@ fn basic_type_metadata(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'ll DIType {
         )
     };
 
-    return ty_metadata;
+    ty_metadata
 }
 
 fn foreign_type_metadata(
@@ -1273,11 +1273,11 @@ fn prepare_union_metadata(
 fn use_enum_fallback(cx: &CodegenCx<'_, '_>) -> bool {
     // On MSVC we have to use the fallback mode, because LLVM doesn't
     // lower variant parts to PDB.
-    return cx.sess().target.target.options.is_like_msvc
+    cx.sess().target.target.options.is_like_msvc
         // LLVM version 7 did not release with an important bug fix;
         // but the required patch is in the LLVM 8.  Rust LLVM reports
         // 8 as well.
-        || llvm_util::get_major_version() < 8;
+        || llvm_util::get_major_version() < 8
 }
 
 // FIXME(eddyb) maybe precompute this? Right now it's computed once
@@ -2075,7 +2075,7 @@ fn prepare_enum_metadata(
         }
     };
 
-    return create_and_register_recursive_type_forward_declaration(
+    create_and_register_recursive_type_forward_declaration(
         cx,
         enum_type,
         unique_type_id,
@@ -2088,7 +2088,7 @@ fn prepare_enum_metadata(
             containing_scope,
             span,
         }),
-    );
+    )
 }
 
 /// Creates debug information for a composite type, that is, anything that
diff --git a/src/librustc_codegen_llvm/debuginfo/mod.rs b/src/librustc_codegen_llvm/debuginfo/mod.rs
index 85decff35b9e0..41829d4ee4256 100644
--- a/src/librustc_codegen_llvm/debuginfo/mod.rs
+++ b/src/librustc_codegen_llvm/debuginfo/mod.rs
@@ -444,7 +444,7 @@ impl DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
                 vec![]
             };
 
-            return create_DIArray(DIB(cx), &template_params[..]);
+            create_DIArray(DIB(cx), &template_params[..])
         }
 
         fn get_parameter_names(cx: &CodegenCx<'_, '_>, generics: &ty::Generics) -> Vec<Symbol> {
diff --git a/src/librustc_codegen_llvm/debuginfo/utils.rs b/src/librustc_codegen_llvm/debuginfo/utils.rs
index bef40decdf3ab..b42d760a77345 100644
--- a/src/librustc_codegen_llvm/debuginfo/utils.rs
+++ b/src/librustc_codegen_llvm/debuginfo/utils.rs
@@ -24,9 +24,7 @@ pub fn is_node_local_to_unit(cx: &CodegenCx<'_, '_>, def_id: DefId) -> bool {
 
 #[allow(non_snake_case)]
 pub fn create_DIArray(builder: &DIBuilder<'ll>, arr: &[Option<&'ll DIDescriptor>]) -> &'ll DIArray {
-    return unsafe {
-        llvm::LLVMRustDIBuilderGetOrCreateArray(builder, arr.as_ptr(), arr.len() as u32)
-    };
+    unsafe { llvm::LLVMRustDIBuilderGetOrCreateArray(builder, arr.as_ptr(), arr.len() as u32) }
 }
 
 #[inline]
diff --git a/src/librustc_codegen_llvm/llvm/archive_ro.rs b/src/librustc_codegen_llvm/llvm/archive_ro.rs
index ab9df4162472c..64db4f7462df8 100644
--- a/src/librustc_codegen_llvm/llvm/archive_ro.rs
+++ b/src/librustc_codegen_llvm/llvm/archive_ro.rs
@@ -27,13 +27,13 @@ impl ArchiveRO {
     /// If this archive is used with a mutable method, then an error will be
     /// raised.
     pub fn open(dst: &Path) -> Result<ArchiveRO, String> {
-        return unsafe {
+        unsafe {
             let s = path_to_c_string(dst);
             let ar = super::LLVMRustOpenArchive(s.as_ptr()).ok_or_else(|| {
                 super::last_error().unwrap_or_else(|| "failed to open archive".to_owned())
             })?;
             Ok(ArchiveRO { raw: ar })
-        };
+        }
     }
 
     pub fn iter(&self) -> Iter<'_> {
diff --git a/src/librustc_codegen_ssa/back/command.rs b/src/librustc_codegen_ssa/back/command.rs
index 30b055b313149..0208bb73abdbe 100644
--- a/src/librustc_codegen_ssa/back/command.rs
+++ b/src/librustc_codegen_ssa/back/command.rs
@@ -119,7 +119,7 @@ impl Command {
         for k in &self.env_remove {
             ret.env_remove(k);
         }
-        return ret;
+        ret
     }
 
     // extensions
diff --git a/src/librustc_codegen_ssa/base.rs b/src/librustc_codegen_ssa/base.rs
index e57cae30b7795..5fd16cb121fda 100644
--- a/src/librustc_codegen_ssa/base.rs
+++ b/src/librustc_codegen_ssa/base.rs
@@ -852,7 +852,7 @@ impl CrateInfo {
             info.missing_lang_items.insert(cnum, missing);
         }
 
-        return info;
+        info
     }
 }
 
@@ -887,7 +887,7 @@ pub fn provide_both(providers: &mut Providers<'_>) {
                 }
             }
         }
-        return tcx.sess.opts.optimize;
+        tcx.sess.opts.optimize
     };
 
     providers.dllimport_foreign_items = |tcx, krate| {
diff --git a/src/librustc_data_structures/graph/dominators/mod.rs b/src/librustc_data_structures/graph/dominators/mod.rs
index 5283bd78a3029..a7f9340dead88 100644
--- a/src/librustc_data_structures/graph/dominators/mod.rs
+++ b/src/librustc_data_structures/graph/dominators/mod.rs
@@ -125,9 +125,9 @@ impl<'dom, Node: Idx> Iterator for Iter<'dom, Node> {
             } else {
                 self.node = Some(dom);
             }
-            return Some(node);
+            Some(node)
         } else {
-            return None;
+            None
         }
     }
 }
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 34f0c182499db..e3e076e769f5d 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -752,7 +752,7 @@ impl RustcDefaultCalls {
                 PrintRequest::NativeStaticLibs => {}
             }
         }
-        return Compilation::Stop;
+        Compilation::Stop
     }
 }
 
diff --git a/src/librustc_error_codes/error_codes.rs b/src/librustc_error_codes/error_codes.rs
index 85339ab9aa15b..836e722946dd2 100644
--- a/src/librustc_error_codes/error_codes.rs
+++ b/src/librustc_error_codes/error_codes.rs
@@ -97,7 +97,6 @@ E0184: include_str!("./error_codes/E0184.md"),
 E0185: include_str!("./error_codes/E0185.md"),
 E0186: include_str!("./error_codes/E0186.md"),
 E0191: include_str!("./error_codes/E0191.md"),
-E0192: include_str!("./error_codes/E0192.md"),
 E0193: include_str!("./error_codes/E0193.md"),
 E0195: include_str!("./error_codes/E0195.md"),
 E0197: include_str!("./error_codes/E0197.md"),
@@ -424,6 +423,9 @@ E0745: include_str!("./error_codes/E0745.md"),
 E0746: include_str!("./error_codes/E0746.md"),
 E0747: include_str!("./error_codes/E0747.md"),
 E0748: include_str!("./error_codes/E0748.md"),
+E0749: include_str!("./error_codes/E0749.md"),
+E0750: include_str!("./error_codes/E0750.md"),
+E0751: include_str!("./error_codes/E0751.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
@@ -458,6 +460,7 @@ E0748: include_str!("./error_codes/E0748.md"),
 //  E0188, // can not cast an immutable reference to a mutable pointer
 //  E0189, // deprecated: can only cast a boxed pointer to a boxed object
 //  E0190, // deprecated: can only cast a &-pointer to an &-object
+//  E0192, // negative impl only applicable to auto traits
 //  E0194, // merged into E0403
 //  E0196, // cannot determine a type for this closure
     E0208,
diff --git a/src/librustc_error_codes/error_codes/E0439.md b/src/librustc_error_codes/error_codes/E0439.md
index e6da2117ac5f2..3e663df866caa 100644
--- a/src/librustc_error_codes/error_codes/E0439.md
+++ b/src/librustc_error_codes/error_codes/E0439.md
@@ -1,5 +1,6 @@
-The length of the platform-intrinsic function `simd_shuffle`
-wasn't specified. Erroneous code example:
+The length of the platform-intrinsic function `simd_shuffle` wasn't specified.
+
+Erroneous code example:
 
 ```compile_fail,E0439
 #![feature(platform_intrinsics)]
diff --git a/src/librustc_error_codes/error_codes/E0749.md b/src/librustc_error_codes/error_codes/E0749.md
new file mode 100644
index 0000000000000..9eb8ee4e3fdf7
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0749.md
@@ -0,0 +1,4 @@
+Negative impls are not allowed to have any items. Negative impls
+declare that a trait is **not** implemented (and never will be) and
+hence there is no need to specify the values for trait methods or
+other items.
diff --git a/src/librustc_error_codes/error_codes/E0750.md b/src/librustc_error_codes/error_codes/E0750.md
new file mode 100644
index 0000000000000..e0cf56f716f9d
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0750.md
@@ -0,0 +1,4 @@
+Negative impls cannot be default impls. A default impl supplies
+default values for the items within to be used by other impls, whereas
+a negative impl declares that there are no other impls. These don't
+make sense to combine.
diff --git a/src/librustc_error_codes/error_codes/E0751.md b/src/librustc_error_codes/error_codes/E0751.md
new file mode 100644
index 0000000000000..a440f82e4b6b3
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0751.md
@@ -0,0 +1,12 @@
+There are both a positive and negative trait implementation for the same type.
+
+Erroneous code example:
+
+```compile_fail,E0748
+trait MyTrait {}
+impl MyTrait for i32 { }
+impl !MyTrait for i32 { }
+```
+
+Negative implementations are a promise that the trait will never be
+implemented for the given types.
diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index bed26c3736b83..7cd4ce4c17623 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -802,13 +802,13 @@ impl HandlerInner {
                     ));
                     self.failure(&format!(
                         "For more information about an error, try \
-                                           `rustc --explain {}`.",
+                         `rustc --explain {}`.",
                         &error_codes[0]
                     ));
                 } else {
                     self.failure(&format!(
                         "For more information about this error, try \
-                                           `rustc --explain {}`.",
+                         `rustc --explain {}`.",
                         &error_codes[0]
                     ));
                 }
diff --git a/src/librustc_feature/active.rs b/src/librustc_feature/active.rs
index 9f1fee8fc09d8..4dfa309540cad 100644
--- a/src/librustc_feature/active.rs
+++ b/src/librustc_feature/active.rs
@@ -554,6 +554,9 @@ declare_features! (
     // Allows limiting the evaluation steps of const expressions
     (active, const_eval_limit, "1.43.0", Some(67217), None),
 
+    /// Allow negative trait implementations.
+    (active, negative_impls, "1.44.0", Some(68318), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
diff --git a/src/librustc_incremental/assert_module_sources.rs b/src/librustc_incremental/assert_module_sources.rs
index 54d7e0ece5031..c5446116f4c50 100644
--- a/src/librustc_incremental/assert_module_sources.rs
+++ b/src/librustc_incremental/assert_module_sources.rs
@@ -175,6 +175,6 @@ impl AssertModuleSource<'tcx> {
             return true;
         }
         debug!("check_config: no match found");
-        return false;
+        false
     }
 }
diff --git a/src/librustc_incremental/persist/save.rs b/src/librustc_incremental/persist/save.rs
index b465a11c99c06..ba586d0cfba04 100644
--- a/src/librustc_incremental/persist/save.rs
+++ b/src/librustc_incremental/persist/save.rs
@@ -132,7 +132,6 @@ where
         }
         Err(err) => {
             sess.err(&format!("failed to write dep-graph to `{}`: {}", path_buf.display(), err));
-            return;
         }
     }
 }
diff --git a/src/librustc_infer/infer/equate.rs b/src/librustc_infer/infer/equate.rs
index bb0c124a1892d..8f8fc4f137b73 100644
--- a/src/librustc_infer/infer/equate.rs
+++ b/src/librustc_infer/infer/equate.rs
@@ -136,7 +136,7 @@ impl TypeRelation<'tcx> for Equate<'combine, 'infcx, 'tcx> {
         } else {
             // Fast path for the common case.
             self.relate(a.skip_binder(), b.skip_binder())?;
-            return Ok(a.clone());
+            Ok(a.clone())
         }
     }
 }
diff --git a/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs b/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs
index 50b324c72278e..689323ce48346 100644
--- a/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs
+++ b/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs
@@ -142,6 +142,6 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             .span_label(span_2, String::new())
             .span_label(span, span_label)
             .emit();
-        return Some(ErrorReported);
+        Some(ErrorReported)
     }
 }
diff --git a/src/librustc_infer/infer/higher_ranked/mod.rs b/src/librustc_infer/infer/higher_ranked/mod.rs
index 105b987f85e96..6a5a1c46d4caf 100644
--- a/src/librustc_infer/infer/higher_ranked/mod.rs
+++ b/src/librustc_infer/infer/higher_ranked/mod.rs
@@ -30,7 +30,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
 
         let span = self.trace.cause.span;
 
-        return self.infcx.commit_if_ok(|snapshot| {
+        self.infcx.commit_if_ok(|snapshot| {
             // First, we instantiate each bound region in the supertype with a
             // fresh placeholder region.
             let (b_prime, placeholder_map) = self.infcx.replace_bound_vars_with_placeholders(b);
@@ -53,7 +53,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
             debug!("higher_ranked_sub: OK result={:?}", result);
 
             Ok(ty::Binder::bind(result))
-        });
+        })
     }
 }
 
diff --git a/src/librustc_infer/infer/lexical_region_resolve/mod.rs b/src/librustc_infer/infer/lexical_region_resolve/mod.rs
index 3af10e850d534..821b9f72c0b20 100644
--- a/src/librustc_infer/infer/lexical_region_resolve/mod.rs
+++ b/src/librustc_infer/infer/lexical_region_resolve/mod.rs
@@ -452,12 +452,10 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                 debug!("Expanding value of {:?} from {:?} to {:?}", b_vid, cur_region, lub);
 
                 *b_data = VarValue::Value(lub);
-                return true;
+                true
             }
 
-            VarValue::ErrorValue => {
-                return false;
-            }
+            VarValue::ErrorValue => false,
         }
     }
 
@@ -804,7 +802,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
             }
         }
 
-        return graph;
+        graph
     }
 
     fn collect_error_for_expanding_node(
diff --git a/src/librustc_infer/infer/nll_relate/mod.rs b/src/librustc_infer/infer/nll_relate/mod.rs
index 50bea300c5064..c194e968013eb 100644
--- a/src/librustc_infer/infer/nll_relate/mod.rs
+++ b/src/librustc_infer/infer/nll_relate/mod.rs
@@ -877,7 +877,7 @@ where
                     // If sub-roots are equal, then `for_vid` and
                     // `vid` are related via subtyping.
                     debug!("TypeGeneralizer::tys: occurs check failed");
-                    return Err(TypeError::Mismatch);
+                    Err(TypeError::Mismatch)
                 } else {
                     match variables.probe(vid) {
                         TypeVariableValue::Known { value: u } => {
@@ -898,7 +898,7 @@ where
 
                             let u = self.tcx().mk_ty_var(new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
-                            return Ok(u);
+                            Ok(u)
                         }
                     }
                 }
diff --git a/src/librustc_infer/infer/region_constraints/mod.rs b/src/librustc_infer/infer/region_constraints/mod.rs
index 868b95043796b..38475b02e5db8 100644
--- a/src/librustc_infer/infer/region_constraints/mod.rs
+++ b/src/librustc_infer/infer/region_constraints/mod.rs
@@ -505,7 +505,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
             self.undo_log.push(AddVar(vid));
         }
         debug!("created new region variable {:?} in {:?} with origin {:?}", vid, universe, origin);
-        return vid;
+        vid
     }
 
     /// Returns the universe for the given variable.
diff --git a/src/librustc_lexer/src/lib.rs b/src/librustc_lexer/src/lib.rs
index 25334461a113b..d3ac58a49c8d5 100644
--- a/src/librustc_lexer/src/lib.rs
+++ b/src/librustc_lexer/src/lib.rs
@@ -527,10 +527,10 @@ impl Cursor<'_> {
         if self.first() == '\'' {
             self.bump();
             let kind = Char { terminated: true };
-            return Literal { kind, suffix_start: self.len_consumed() };
+            Literal { kind, suffix_start: self.len_consumed() }
+        } else {
+            Lifetime { starts_with_number }
         }
-
-        return Lifetime { starts_with_number };
     }
 
     fn single_quoted_string(&mut self) -> bool {
diff --git a/src/librustc_lint/builtin.rs b/src/librustc_lint/builtin.rs
index 408031028b102..88f2284cd6154 100644
--- a/src/librustc_lint/builtin.rs
+++ b/src/librustc_lint/builtin.rs
@@ -269,7 +269,7 @@ impl EarlyLintPass for UnsafeCode {
                 })
             }
 
-            _ => return,
+            _ => {}
         }
     }
 
diff --git a/src/librustc_lint/types.rs b/src/librustc_lint/types.rs
index 4949c93d45eed..fcd50001cb3a9 100644
--- a/src/librustc_lint/types.rs
+++ b/src/librustc_lint/types.rs
@@ -273,7 +273,6 @@ fn lint_int_literal<'a, 'tcx>(
                     cx.sess()
                         .source_map()
                         .span_to_snippet(lit.span)
-                        .ok()
                         .expect("must get snippet from literal"),
                     t.name_str(),
                     min,
@@ -338,7 +337,6 @@ fn lint_uint_literal<'a, 'tcx>(
                     cx.sess()
                         .source_map()
                         .span_to_snippet(lit.span)
-                        .ok()
                         .expect("must get snippet from literal"),
                     t.name_str(),
                     min,
diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index 229740615f707..b5826d6a5efa6 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -543,7 +543,7 @@ impl EarlyLintPass for UnusedParens {
             // Do not lint on `(..)` as that will result in the other arms being useless.
             Paren(_)
             // The other cases do not contain sub-patterns.
-            | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => return,
+            | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
             // These are list-like patterns; parens can always be removed.
             TupleStruct(_, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
                 self.check_unused_parens_pat(cx, p, false, false);
diff --git a/src/librustc_metadata/creader.rs b/src/librustc_metadata/creader.rs
index f20cdfcba15ca..9b6e427abc1fd 100644
--- a/src/librustc_metadata/creader.rs
+++ b/src/librustc_metadata/creader.rs
@@ -264,7 +264,7 @@ impl<'a> CrateLoader<'a> {
                 ret = Some(cnum);
             }
         });
-        return ret;
+        ret
     }
 
     fn verify_no_symbol_conflicts(&self, span: Span, root: &CrateRoot<'_>) {
diff --git a/src/librustc_metadata/foreign_modules.rs b/src/librustc_metadata/foreign_modules.rs
index fc988ec15cee9..60b8239a82155 100644
--- a/src/librustc_metadata/foreign_modules.rs
+++ b/src/librustc_metadata/foreign_modules.rs
@@ -6,7 +6,7 @@ use rustc_hir::itemlikevisit::ItemLikeVisitor;
 crate fn collect(tcx: TyCtxt<'_>) -> Vec<ForeignModule> {
     let mut collector = Collector { tcx, modules: Vec::new() };
     tcx.hir().krate().visit_all_item_likes(&mut collector);
-    return collector.modules;
+    collector.modules
 }
 
 struct Collector<'tcx> {
diff --git a/src/librustc_metadata/link_args.rs b/src/librustc_metadata/link_args.rs
index 13668b2423fdd..56b26efe5bf1e 100644
--- a/src/librustc_metadata/link_args.rs
+++ b/src/librustc_metadata/link_args.rs
@@ -16,7 +16,7 @@ crate fn collect(tcx: TyCtxt<'_>) -> Vec<String> {
         }
     }
 
-    return collector.args;
+    collector.args
 }
 
 struct Collector {
diff --git a/src/librustc_metadata/locator.rs b/src/librustc_metadata/locator.rs
index 1ede629e7ef7d..2f9be599ba94b 100644
--- a/src/librustc_metadata/locator.rs
+++ b/src/librustc_metadata/locator.rs
@@ -949,7 +949,7 @@ fn get_metadata_section(
     let start = Instant::now();
     let ret = get_metadata_section_imp(target, flavor, filename, loader);
     info!("reading {:?} => {:?}", filename.file_name().unwrap(), start.elapsed());
-    return ret;
+    ret
 }
 
 /// A trivial wrapper for `Mmap` that implements `StableDeref`.
diff --git a/src/librustc_metadata/native_libs.rs b/src/librustc_metadata/native_libs.rs
index 64bbf393ba0f1..19d2d620f58a7 100644
--- a/src/librustc_metadata/native_libs.rs
+++ b/src/librustc_metadata/native_libs.rs
@@ -15,7 +15,7 @@ crate fn collect(tcx: TyCtxt<'_>) -> Vec<NativeLibrary> {
     let mut collector = Collector { tcx, libs: Vec::new() };
     tcx.hir().krate().visit_all_item_likes(&mut collector);
     collector.process_command_line();
-    return collector.libs;
+    collector.libs
 }
 
 crate fn relevant_lib(sess: &Session, lib: &NativeLibrary) -> bool {
diff --git a/src/librustc_metadata/rmeta/decoder/cstore_impl.rs b/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
index cc2bd51f92f3e..ca75daf1aa9b2 100644
--- a/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
+++ b/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
@@ -170,7 +170,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
             .iter()
             .filter_map(|&(exported_symbol, export_level)| {
                 if let ExportedSymbol::NonGeneric(def_id) = exported_symbol {
-                    return Some((def_id, export_level))
+                    Some((def_id, export_level))
                 } else {
                     None
                 }
diff --git a/src/librustc_mir/borrow_check/borrow_set.rs b/src/librustc_mir/borrow_check/borrow_set.rs
index 9d5cf3ec4bec0..9f4f0ce5620b5 100644
--- a/src/librustc_mir/borrow_check/borrow_set.rs
+++ b/src/librustc_mir/borrow_check/borrow_set.rs
@@ -273,7 +273,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
             assert_eq!(borrow_data.borrowed_place, *place);
         }
 
-        return self.super_rvalue(rvalue, location);
+        self.super_rvalue(rvalue, location)
     }
 }
 
diff --git a/src/librustc_mir/borrow_check/diagnostics/region_name.rs b/src/librustc_mir/borrow_check/diagnostics/region_name.rs
index 7103fc596c922..d1d0ba215e08e 100644
--- a/src/librustc_mir/borrow_check/diagnostics/region_name.rs
+++ b/src/librustc_mir/borrow_check/diagnostics/region_name.rs
@@ -500,7 +500,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
             }
         }
 
-        return None;
+        None
     }
 
     /// We've found an enum/struct/union type with the substitutions
diff --git a/src/librustc_mir/const_eval/machine.rs b/src/librustc_mir/const_eval/machine.rs
index 28889486c383b..d81aae6523a45 100644
--- a/src/librustc_mir/const_eval/machine.rs
+++ b/src/librustc_mir/const_eval/machine.rs
@@ -56,7 +56,7 @@ impl<'mir, 'tcx> InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>> {
 
         self.return_to_block(ret.map(|r| r.1))?;
         self.dump_place(*dest);
-        return Ok(true);
+        Ok(true)
     }
 
     /// "Intercept" a function call to a panic-related function
diff --git a/src/librustc_mir/dataflow/mod.rs b/src/librustc_mir/dataflow/mod.rs
index dd0f9ff75b9fe..c98a5e84729ab 100644
--- a/src/librustc_mir/dataflow/mod.rs
+++ b/src/librustc_mir/dataflow/mod.rs
@@ -122,7 +122,7 @@ pub(crate) fn has_rustc_mir_with(attrs: &[ast::Attribute], name: Symbol) -> Opti
             }
         }
     }
-    return None;
+    None
 }
 
 pub struct MoveDataParamEnv<'tcx> {
@@ -171,7 +171,7 @@ where
                     return None;
                 }
             }
-            return None;
+            None
         };
 
         let print_preflow_to = name_found(tcx.sess, attributes, sym::borrowck_graphviz_preflow);
diff --git a/src/librustc_mir/interpret/operator.rs b/src/librustc_mir/interpret/operator.rs
index f2ee5e047a88e..76a5aecb9db62 100644
--- a/src/librustc_mir/interpret/operator.rs
+++ b/src/librustc_mir/interpret/operator.rs
@@ -64,7 +64,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Ge => l >= r,
             _ => bug!("Invalid operation on char: {:?}", bin_op),
         };
-        return (Scalar::from_bool(res), false, self.tcx.types.bool);
+        (Scalar::from_bool(res), false, self.tcx.types.bool)
     }
 
     fn binary_bool_op(
@@ -87,7 +87,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             BitXor => l ^ r,
             _ => bug!("Invalid operation on bool: {:?}", bin_op),
         };
-        return (Scalar::from_bool(res), false, self.tcx.types.bool);
+        (Scalar::from_bool(res), false, self.tcx.types.bool)
     }
 
     fn binary_float_op<F: Float + Into<Scalar<M::PointerTag>>>(
@@ -113,7 +113,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Rem => ((l % r).value.into(), ty),
             _ => bug!("invalid float op: `{:?}`", bin_op),
         };
-        return (val, false, ty);
+        (val, false, ty)
     }
 
     fn binary_int_op(
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 5313446c253c8..933e74ee9ed7e 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -212,9 +212,7 @@ impl<'tcx, Tag> MPlaceTy<'tcx, Tag> {
         if self.layout.is_unsized() {
             // We need to consult `meta` metadata
             match self.layout.ty.kind {
-                ty::Slice(..) | ty::Str => {
-                    return self.mplace.meta.unwrap_meta().to_machine_usize(cx);
-                }
+                ty::Slice(..) | ty::Str => self.mplace.meta.unwrap_meta().to_machine_usize(cx),
                 _ => bug!("len not supported on unsized type {:?}", self.layout.ty),
             }
         } else {
diff --git a/src/librustc_mir/interpret/terminator.rs b/src/librustc_mir/interpret/terminator.rs
index 22a081a9c8e0b..a9e45a032a6be 100644
--- a/src/librustc_mir/interpret/terminator.rs
+++ b/src/librustc_mir/interpret/terminator.rs
@@ -240,7 +240,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match instance.def {
             ty::InstanceDef::Intrinsic(..) => {
                 assert!(caller_abi == Abi::RustIntrinsic || caller_abi == Abi::PlatformIntrinsic);
-                return M::call_intrinsic(self, span, instance, args, ret, unwind);
+                M::call_intrinsic(self, span, instance, args, ret, unwind)
             }
             ty::InstanceDef::VtableShim(..)
             | ty::InstanceDef::ReifyShim(..)
diff --git a/src/librustc_mir/monomorphize/collector.rs b/src/librustc_mir/monomorphize/collector.rs
index 4dd037d93ce9b..a592e8d9c05fe 100644
--- a/src/librustc_mir/monomorphize/collector.rs
+++ b/src/librustc_mir/monomorphize/collector.rs
@@ -751,7 +751,7 @@ fn should_monomorphize_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: &Instance<'tcx
         bug!("cannot create local mono-item for {:?}", def_id)
     }
 
-    return true;
+    true
 }
 
 /// For a given pair of source and target type that occur in an unsizing coercion,
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index ca23c44f64668..43876380c840e 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -483,7 +483,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             err.span_label(source_info.span, format!("{:?}", panic));
             err.emit()
         });
-        return None;
+        None
     }
 
     fn check_unary_op(
diff --git a/src/librustc_mir_build/build/mod.rs b/src/librustc_mir_build/build/mod.rs
index 821c4d68c7e8a..f35b50d484bff 100644
--- a/src/librustc_mir_build/build/mod.rs
+++ b/src/librustc_mir_build/build/mod.rs
@@ -43,8 +43,7 @@ fn mir_build(tcx: TyCtxt<'_>, def_id: DefId) -> BodyAndCache<'_> {
             ..
         })
         | Node::TraitItem(hir::TraitItem {
-            kind:
-                hir::TraitItemKind::Fn(hir::FnSig { decl, .. }, hir::TraitFn::Provided(body_id)),
+            kind: hir::TraitItemKind::Fn(hir::FnSig { decl, .. }, hir::TraitFn::Provided(body_id)),
             ..
         }) => (*body_id, decl.output.span()),
         Node::Item(hir::Item { kind: hir::ItemKind::Static(ty, _, body_id), .. })
@@ -368,7 +367,7 @@ impl BlockContext {
             }
         }
 
-        return None;
+        None
     }
 
     /// Looks at the topmost frame on the BlockContext and reports
diff --git a/src/librustc_mir_build/hair/cx/block.rs b/src/librustc_mir_build/hair/cx/block.rs
index 8d7225c8c7b51..07a9d91cd746d 100644
--- a/src/librustc_mir_build/hair/cx/block.rs
+++ b/src/librustc_mir_build/hair/cx/block.rs
@@ -98,7 +98,7 @@ fn mirror_stmts<'a, 'tcx>(
             }
         }
     }
-    return result;
+    result
 }
 
 crate fn to_expr_ref<'a, 'tcx>(
diff --git a/src/librustc_parse/lib.rs b/src/librustc_parse/lib.rs
index c31cc1b4c9f00..58db7d286e7e6 100644
--- a/src/librustc_parse/lib.rs
+++ b/src/librustc_parse/lib.rs
@@ -320,7 +320,7 @@ pub fn nt_to_tokenstream(nt: &Nonterminal, sess: &ParseSess, span: Span) -> Toke
                 going with stringified version"
         );
     }
-    return tokens_for_real;
+    tokens_for_real
 }
 
 fn prepend_attrs(
diff --git a/src/librustc_parse/parser/expr.rs b/src/librustc_parse/parser/expr.rs
index c65e99842c5dd..5dac461441d64 100644
--- a/src/librustc_parse/parser/expr.rs
+++ b/src/librustc_parse/parser/expr.rs
@@ -996,7 +996,7 @@ impl<'a> Parser<'a> {
                 let expr = self.mk_expr(lo.to(self.prev_token.span), ExprKind::Lit(literal), attrs);
                 self.maybe_recover_from_bad_qpath(expr, true)
             }
-            None => return Err(self.expected_expression_found()),
+            None => Err(self.expected_expression_found()),
         }
     }
 
@@ -1713,7 +1713,7 @@ impl<'a> Parser<'a> {
         }
         let hi = self.token.span;
         self.bump();
-        return Ok(self.mk_expr(lo.to(hi), ExprKind::Match(scrutinee, arms), attrs));
+        Ok(self.mk_expr(lo.to(hi), ExprKind::Match(scrutinee, arms), attrs))
     }
 
     pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
diff --git a/src/librustc_parse/parser/item.rs b/src/librustc_parse/parser/item.rs
index 9d70f606f3ef4..2b1799f5c4885 100644
--- a/src/librustc_parse/parser/item.rs
+++ b/src/librustc_parse/parser/item.rs
@@ -314,7 +314,7 @@ impl<'a> Parser<'a> {
                 " struct ".into(),
                 Applicability::MaybeIncorrect, // speculative
             );
-            return Err(err);
+            Err(err)
         } else if self.look_ahead(1, |t| *t == token::OpenDelim(token::Paren)) {
             let ident = self.parse_ident().unwrap();
             self.bump(); // `(`
@@ -362,7 +362,7 @@ impl<'a> Parser<'a> {
                     );
                 }
             }
-            return Err(err);
+            Err(err)
         } else if self.look_ahead(1, |t| *t == token::Lt) {
             let ident = self.parse_ident().unwrap();
             self.eat_to_tokens(&[&token::Gt]);
@@ -384,7 +384,7 @@ impl<'a> Parser<'a> {
                     Applicability::MachineApplicable,
                 );
             }
-            return Err(err);
+            Err(err)
         } else {
             Ok(())
         }
@@ -910,7 +910,7 @@ impl<'a> Parser<'a> {
         let span = self.sess.source_map().def_span(span);
         let msg = format!("{} is not supported in {}", kind.descr(), ctx);
         self.struct_span_err(span, &msg).emit();
-        return None;
+        None
     }
 
     fn error_on_foreign_const(&self, span: Span, ident: Ident) {
diff --git a/src/librustc_parse/parser/pat.rs b/src/librustc_parse/parser/pat.rs
index 4585941943b74..6b987ff1cce68 100644
--- a/src/librustc_parse/parser/pat.rs
+++ b/src/librustc_parse/parser/pat.rs
@@ -918,7 +918,7 @@ impl<'a> Parser<'a> {
             }
             err.emit();
         }
-        return Ok((fields, etc));
+        Ok((fields, etc))
     }
 
     /// Recover on `...` as if it were `..` to avoid further errors.
diff --git a/src/librustc_parse/parser/stmt.rs b/src/librustc_parse/parser/stmt.rs
index d40597d8fcb0c..d43f5d67113a1 100644
--- a/src/librustc_parse/parser/stmt.rs
+++ b/src/librustc_parse/parser/stmt.rs
@@ -278,7 +278,7 @@ impl<'a> Parser<'a> {
             _ => {}
         }
         e.span_label(sp, "expected `{`");
-        return Err(e);
+        Err(e)
     }
 
     /// Parses a block. Inner attributes are allowed.
diff --git a/src/librustc_passes/layout_test.rs b/src/librustc_passes/layout_test.rs
index 9f6598a0ec1fe..9d8b1422bdf24 100644
--- a/src/librustc_passes/layout_test.rs
+++ b/src/librustc_passes/layout_test.rs
@@ -17,24 +17,30 @@ use rustc_span::symbol::sym;
 pub fn test_layout(tcx: TyCtxt<'_>) {
     if tcx.features().rustc_attrs {
         // if the `rustc_attrs` feature is not enabled, don't bother testing layout
-        tcx.hir().krate().visit_all_item_likes(&mut VarianceTest { tcx });
+        tcx.hir().krate().visit_all_item_likes(&mut LayoutTest { tcx });
     }
 }
 
-struct VarianceTest<'tcx> {
+struct LayoutTest<'tcx> {
     tcx: TyCtxt<'tcx>,
 }
 
-impl ItemLikeVisitor<'tcx> for VarianceTest<'tcx> {
+impl ItemLikeVisitor<'tcx> for LayoutTest<'tcx> {
     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
         let item_def_id = self.tcx.hir().local_def_id(item.hir_id);
 
-        if let ItemKind::TyAlias(..) = item.kind {
-            for attr in self.tcx.get_attrs(item_def_id).iter() {
-                if attr.check_name(sym::rustc_layout) {
-                    self.dump_layout_of(item_def_id, item, attr);
+        match item.kind {
+            ItemKind::TyAlias(..)
+            | ItemKind::Enum(..)
+            | ItemKind::Struct(..)
+            | ItemKind::Union(..) => {
+                for attr in self.tcx.get_attrs(item_def_id).iter() {
+                    if attr.check_name(sym::rustc_layout) {
+                        self.dump_layout_of(item_def_id, item, attr);
+                    }
                 }
             }
+            _ => {}
         }
     }
 
@@ -42,7 +48,7 @@ impl ItemLikeVisitor<'tcx> for VarianceTest<'tcx> {
     fn visit_impl_item(&mut self, _: &'tcx hir::ImplItem<'tcx>) {}
 }
 
-impl VarianceTest<'tcx> {
+impl LayoutTest<'tcx> {
     fn dump_layout_of(&self, item_def_id: DefId, item: &hir::Item<'tcx>, attr: &Attribute) {
         let tcx = self.tcx;
         let param_env = self.tcx.param_env(item_def_id);
@@ -81,6 +87,13 @@ impl VarianceTest<'tcx> {
                             );
                         }
 
+                        sym::debug => {
+                            self.tcx.sess.span_err(
+                                item.span,
+                                &format!("layout debugging: {:#?}", *ty_layout),
+                            );
+                        }
+
                         name => {
                             self.tcx.sess.span_err(
                                 meta_item.span(),
diff --git a/src/librustc_passes/liveness.rs b/src/librustc_passes/liveness.rs
index 70b106f5d2332..bf577d26b0fed 100644
--- a/src/librustc_passes/liveness.rs
+++ b/src/librustc_passes/liveness.rs
@@ -864,7 +864,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             first_merge,
             any_changed
         );
-        return any_changed;
+        any_changed
     }
 
     // Indicates that a local variable was *defined*; we know that no
diff --git a/src/librustc_passes/loops.rs b/src/librustc_passes/loops.rs
index 1daef45a1f591..fa2afae469c04 100644
--- a/src/librustc_passes/loops.rs
+++ b/src/librustc_passes/loops.rs
@@ -222,7 +222,7 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
                 return true;
             }
         }
-        return false;
+        false
     }
     fn emit_unlabled_cf_in_while_condition(&mut self, span: Span, cf_type: &str) {
         struct_span_err!(
diff --git a/src/librustc_passes/reachable.rs b/src/librustc_passes/reachable.rs
index 835e7cfb62816..1e9c6c91d385f 100644
--- a/src/librustc_passes/reachable.rs
+++ b/src/librustc_passes/reachable.rs
@@ -30,9 +30,7 @@ fn item_might_be_inlined(tcx: TyCtxt<'tcx>, item: &hir::Item<'_>, attrs: Codegen
     }
 
     match item.kind {
-        hir::ItemKind::Fn(ref sig, ..) if sig.header.is_const() => {
-            return true;
-        }
+        hir::ItemKind::Fn(ref sig, ..) if sig.header.is_const() => true,
         hir::ItemKind::Impl { .. } | hir::ItemKind::Fn(..) => {
             let generics = tcx.generics_of(tcx.hir().local_def_id(item.hir_id));
             generics.requires_monomorphization(tcx)
diff --git a/src/librustc_passes/stability.rs b/src/librustc_passes/stability.rs
index 11311a3e8aa68..8fa5a4fbc61f4 100644
--- a/src/librustc_passes/stability.rs
+++ b/src/librustc_passes/stability.rs
@@ -465,7 +465,7 @@ fn new_index(tcx: TyCtxt<'tcx>) -> Index<'tcx> {
             |v| intravisit::walk_crate(v, krate),
         );
     }
-    return index;
+    index
 }
 
 /// Cross-references the feature names of unstable APIs with enabled
diff --git a/src/librustc_privacy/lib.rs b/src/librustc_privacy/lib.rs
index c8c8c2299305b..a3510737b7edc 100644
--- a/src/librustc_privacy/lib.rs
+++ b/src/librustc_privacy/lib.rs
@@ -1423,7 +1423,7 @@ impl<'a, 'tcx> ObsoleteVisiblePrivateTypesVisitor<'a, 'tcx> {
                 Some(_) | None => false,
             }
         } else {
-            return false;
+            false
         }
     }
 
@@ -1837,7 +1837,7 @@ impl SearchInterfaceForPrivateItemsVisitor<'tcx> {
             && self.tcx.is_private_dep(item_id.krate);
 
         log::debug!("leaks_private_dep(item_id={:?})={}", item_id, ret);
-        return ret;
+        ret
     }
 }
 
diff --git a/src/librustc_resolve/def_collector.rs b/src/librustc_resolve/def_collector.rs
index 505cd331a2509..0dee997f2ed96 100644
--- a/src/librustc_resolve/def_collector.rs
+++ b/src/librustc_resolve/def_collector.rs
@@ -200,7 +200,7 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
 
     fn visit_pat(&mut self, pat: &'a Pat) {
         match pat.kind {
-            PatKind::MacCall(..) => return self.visit_macro_invoc(pat.id),
+            PatKind::MacCall(..) => self.visit_macro_invoc(pat.id),
             _ => visit::walk_pat(self, pat),
         }
     }
diff --git a/src/librustc_resolve/imports.rs b/src/librustc_resolve/imports.rs
index 663e61ad2add4..95597e8ebf1e9 100644
--- a/src/librustc_resolve/imports.rs
+++ b/src/librustc_resolve/imports.rs
@@ -1108,7 +1108,7 @@ impl<'a, 'b> ImportResolver<'a, 'b> {
                                     match binding.kind {
                                         // Never suggest the name that has binding error
                                         // i.e., the name that cannot be previously resolved
-                                        NameBindingKind::Res(Res::Err, _) => return None,
+                                        NameBindingKind::Res(Res::Err, _) => None,
                                         _ => Some(&i.name),
                                     }
                                 }
diff --git a/src/librustc_resolve/late/diagnostics.rs b/src/librustc_resolve/late/diagnostics.rs
index 41380b2a4b78a..e1256551e24da 100644
--- a/src/librustc_resolve/late/diagnostics.rs
+++ b/src/librustc_resolve/late/diagnostics.rs
@@ -380,7 +380,7 @@ impl<'a> LateResolutionVisitor<'a, '_, '_> {
                 _ => (),
             }
         };
-        return has_self_arg;
+        has_self_arg
     }
 
     fn followed_by_brace(&self, span: Span) -> (bool, Option<(Span, String)>) {
@@ -430,7 +430,7 @@ impl<'a> LateResolutionVisitor<'a, '_, '_> {
                 break;
             }
         }
-        return (followed_by_brace, closing_brace);
+        (followed_by_brace, closing_brace)
     }
 
     /// Provides context-dependent help for errors reported by the `smart_resolve_path_fragment`
diff --git a/src/librustc_resolve/late/lifetimes.rs b/src/librustc_resolve/late/lifetimes.rs
index 51bf1f4843972..bc843fccc4c71 100644
--- a/src/librustc_resolve/late/lifetimes.rs
+++ b/src/librustc_resolve/late/lifetimes.rs
@@ -133,7 +133,7 @@ impl RegionExt for Region {
 /// that it corresponds to.
 ///
 /// FIXME. This struct gets converted to a `ResolveLifetimes` for
-/// actual use. It has the same data, but indexed by `DefIndex`.  This
+/// actual use. It has the same data, but indexed by `LocalDefId`.  This
 /// is silly.
 #[derive(Default)]
 struct NamedRegionMap {
diff --git a/src/librustc_resolve/macros.rs b/src/librustc_resolve/macros.rs
index 6f2e0bce3acaf..166fc48b44c4c 100644
--- a/src/librustc_resolve/macros.rs
+++ b/src/librustc_resolve/macros.rs
@@ -83,7 +83,7 @@ fn sub_namespace_match(candidate: Option<MacroKind>, requirement: Option<MacroKi
 // line-breaks and is slow.
 fn fast_print_path(path: &ast::Path) -> Symbol {
     if path.segments.len() == 1 {
-        return path.segments[0].ident.name;
+        path.segments[0].ident.name
     } else {
         let mut path_str = String::with_capacity(64);
         for (i, segment) in path.segments.iter().enumerate() {
diff --git a/src/librustc_save_analysis/lib.rs b/src/librustc_save_analysis/lib.rs
index 98d81c6252242..59084f1904545 100644
--- a/src/librustc_save_analysis/lib.rs
+++ b/src/librustc_save_analysis/lib.rs
@@ -534,7 +534,7 @@ impl<'l, 'tcx> SaveContext<'l, 'tcx> {
                         let variant = &def.non_enum_variant();
                         filter!(self.span_utils, ident.span);
                         let span = self.span_from_span(ident.span);
-                        return Some(Data::RefData(Ref {
+                        Some(Data::RefData(Ref {
                             kind: RefKind::Variable,
                             span,
                             ref_id: self
@@ -542,7 +542,7 @@ impl<'l, 'tcx> SaveContext<'l, 'tcx> {
                                 .find_field_index(ident, variant)
                                 .map(|index| id_from_def_id(variant.fields[index].did))
                                 .unwrap_or_else(|| null_id()),
-                        }));
+                        }))
                     }
                     ty::Tuple(..) => None,
                     _ => {
diff --git a/src/librustc_session/config.rs b/src/librustc_session/config.rs
index c273e7fdbf916..f16e4ca93d80a 100644
--- a/src/librustc_session/config.rs
+++ b/src/librustc_session/config.rs
@@ -1140,7 +1140,7 @@ pub fn parse_error_format(
         _ => {}
     }
 
-    return error_format;
+    error_format
 }
 
 fn parse_crate_edition(matches: &getopts::Matches) -> Edition {
diff --git a/src/librustc_span/caching_source_map_view.rs b/src/librustc_span/caching_source_map_view.rs
index d6725160a5d02..68b0bd1a57418 100644
--- a/src/librustc_span/caching_source_map_view.rs
+++ b/src/librustc_span/caching_source_map_view.rs
@@ -99,10 +99,6 @@ impl<'sm> CachingSourceMapView<'sm> {
         cache_entry.line_end = line_bounds.1;
         cache_entry.time_stamp = self.time_stamp;
 
-        return Some((
-            cache_entry.file.clone(),
-            cache_entry.line_number,
-            pos - cache_entry.line_start,
-        ));
+        Some((cache_entry.file.clone(), cache_entry.line_number, pos - cache_entry.line_start))
     }
 }
diff --git a/src/librustc_span/lib.rs b/src/librustc_span/lib.rs
index dbc180114f1c1..627a894d80209 100644
--- a/src/librustc_span/lib.rs
+++ b/src/librustc_span/lib.rs
@@ -9,6 +9,7 @@
 #![feature(const_if_match)]
 #![feature(const_fn)]
 #![feature(const_panic)]
+#![cfg_attr(not(bootstrap), feature(negative_impls))]
 #![feature(nll)]
 #![feature(optin_builtin_traits)]
 #![feature(specialization)]
diff --git a/src/librustc_span/source_map.rs b/src/librustc_span/source_map.rs
index 7dd9e2f6316b4..39eb318a785e7 100644
--- a/src/librustc_span/source_map.rs
+++ b/src/librustc_span/source_map.rs
@@ -368,7 +368,7 @@ impl SourceMap {
 
     // If there is a doctest offset, applies it to the line.
     pub fn doctest_offset_line(&self, file: &FileName, orig: usize) -> usize {
-        return match file {
+        match file {
             FileName::DocTest(_, offset) => {
                 return if *offset >= 0 {
                     orig + *offset as usize
@@ -377,7 +377,7 @@ impl SourceMap {
                 };
             }
             _ => orig,
-        };
+        }
     }
 
     /// Looks up source information about a `BytePos`.
@@ -569,10 +569,10 @@ impl SourceMap {
         let local_end = self.lookup_byte_offset(sp.hi());
 
         if local_begin.sf.start_pos != local_end.sf.start_pos {
-            return Err(SpanSnippetError::DistinctSources(DistinctSources {
+            Err(SpanSnippetError::DistinctSources(DistinctSources {
                 begin: (local_begin.sf.name.clone(), local_begin.sf.start_pos),
                 end: (local_end.sf.name.clone(), local_end.sf.start_pos),
-            }));
+            }))
         } else {
             self.ensure_source_file_source_present(local_begin.sf.clone());
 
@@ -590,13 +590,11 @@ impl SourceMap {
             }
 
             if let Some(ref src) = local_begin.sf.src {
-                return extract_source(src, start_index, end_index);
+                extract_source(src, start_index, end_index)
             } else if let Some(src) = local_begin.sf.external_src.borrow().get_source() {
-                return extract_source(src, start_index, end_index);
+                extract_source(src, start_index, end_index)
             } else {
-                return Err(SpanSnippetError::SourceNotAvailable {
-                    filename: local_begin.sf.name.clone(),
-                });
+                Err(SpanSnippetError::SourceNotAvailable { filename: local_begin.sf.name.clone() })
             }
         }
     }
diff --git a/src/librustc_span/symbol.rs b/src/librustc_span/symbol.rs
index 5685505f6948d..7cc41c66c31f1 100644
--- a/src/librustc_span/symbol.rs
+++ b/src/librustc_span/symbol.rs
@@ -253,6 +253,7 @@ symbols! {
         debug_trait,
         declare_lint_pass,
         decl_macro,
+        debug,
         Debug,
         Decodable,
         Default,
@@ -472,6 +473,7 @@ symbols! {
         needs_drop,
         needs_panic_runtime,
         negate_unsigned,
+        negative_impls,
         never,
         never_type,
         never_type_fallback,
diff --git a/src/librustc_trait_selection/traits/auto_trait.rs b/src/librustc_trait_selection/traits/auto_trait.rs
index d221d6886e9fb..bd980e6eb8b8a 100644
--- a/src/librustc_trait_selection/traits/auto_trait.rs
+++ b/src/librustc_trait_selection/traits/auto_trait.rs
@@ -113,7 +113,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             return AutoTraitResult::ExplicitImpl;
         }
 
-        return tcx.infer_ctxt().enter(|infcx| {
+        tcx.infer_ctxt().enter(|infcx| {
             let mut fresh_preds = FxHashSet::default();
 
             // Due to the way projections are handled by SelectionContext, we need to run
@@ -219,8 +219,8 @@ impl<'tcx> AutoTraitFinder<'tcx> {
 
             let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
 
-            return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info));
-        });
+            AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info))
+        })
     }
 }
 
@@ -384,7 +384,7 @@ impl AutoTraitFinder<'tcx> {
             ty, trait_did, new_env, final_user_env
         );
 
-        return Some((new_env, final_user_env));
+        Some((new_env, final_user_env))
     }
 
     /// This method is designed to work around the following issue:
@@ -492,7 +492,7 @@ impl AutoTraitFinder<'tcx> {
                 }
                 _ => {}
             }
-            return true;
+            true
         });
 
         if should_add_new {
@@ -591,15 +591,15 @@ impl AutoTraitFinder<'tcx> {
     }
 
     fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
-        return self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types());
+        self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
     }
 
     pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
-        return match ty.kind {
+        match ty.kind {
             ty::Param(_) => true,
             ty::Projection(p) => self.is_of_param(p.self_ty()),
             _ => false,
-        };
+        }
     }
 
     fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
@@ -804,7 +804,7 @@ impl AutoTraitFinder<'tcx> {
                 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
             };
         }
-        return true;
+        true
     }
 
     pub fn clean_pred(
diff --git a/src/librustc_trait_selection/traits/coherence.rs b/src/librustc_trait_selection/traits/coherence.rs
index 5f542e7e13be5..dc13af99fec6c 100644
--- a/src/librustc_trait_selection/traits/coherence.rs
+++ b/src/librustc_trait_selection/traits/coherence.rs
@@ -221,10 +221,10 @@ pub fn trait_ref_is_knowable<'tcx>(
     // we are an owner.
     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
         debug!("trait_ref_is_knowable: orphan check passed");
-        return None;
+        None
     } else {
         debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
-        return Some(Conflict::Upstream);
+        Some(Conflict::Upstream)
     }
 }
 
diff --git a/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs b/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs
index 3d0dd73f03c18..18b2ca8983720 100644
--- a/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs
@@ -111,11 +111,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             }),
             hir::Node::Expr(hir::Expr { .. }) => {
                 let parent_hid = hir.get_parent_node(hir_id);
-                if parent_hid != hir_id {
-                    return self.describe_enclosure(parent_hid);
-                } else {
-                    None
-                }
+                if parent_hid != hir_id { self.describe_enclosure(parent_hid) } else { None }
             }
             _ => None,
         }
diff --git a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
index 0523a2019861c..522a8084cdc0d 100644
--- a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
@@ -351,7 +351,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             // Different to previous arm because one is `&hir::Local` and the other
             // is `P<hir::Local>`.
             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
-            _ => return None,
+            _ => None,
         }
     }
 
diff --git a/src/librustc_trait_selection/traits/query/normalize.rs b/src/librustc_trait_selection/traits/query/normalize.rs
index adec2ddb25322..99412fafcfa8d 100644
--- a/src/librustc_trait_selection/traits/query/normalize.rs
+++ b/src/librustc_trait_selection/traits/query/normalize.rs
@@ -169,12 +169,12 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                                 debug!("QueryNormalizer: result = {:#?}", result);
                                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
                                 self.obligations.extend(obligations);
-                                return result.normalized_ty;
+                                result.normalized_ty
                             }
 
                             Err(_) => {
                                 self.error = true;
-                                return ty;
+                                ty
                             }
                         }
                     }
diff --git a/src/librustc_trait_selection/traits/specialize/mod.rs b/src/librustc_trait_selection/traits/specialize/mod.rs
index b763851b86ef6..09b0a156f0b2e 100644
--- a/src/librustc_trait_selection/traits/specialize/mod.rs
+++ b/src/librustc_trait_selection/traits/specialize/mod.rs
@@ -324,86 +324,7 @@ pub(super) fn specialization_graph_provider(
             };
 
             if let Some(overlap) = overlap {
-                let impl_span =
-                    tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
-
-                // Work to be done after we've built the DiagnosticBuilder. We have to define it
-                // now because the struct_lint methods don't return back the DiagnosticBuilder
-                // that's passed in.
-                let decorate = |err: LintDiagnosticBuilder<'_>| {
-                    let msg = format!(
-                        "conflicting implementations of trait `{}`{}:{}",
-                        overlap.trait_desc,
-                        overlap
-                            .self_desc
-                            .clone()
-                            .map_or(String::new(), |ty| { format!(" for type `{}`", ty) }),
-                        match used_to_be_allowed {
-                            Some(FutureCompatOverlapErrorKind::Issue33140) => " (E0119)",
-                            _ => "",
-                        }
-                    );
-                    let mut err = err.build(&msg);
-                    match tcx.span_of_impl(overlap.with_impl) {
-                        Ok(span) => {
-                            err.span_label(
-                                tcx.sess.source_map().def_span(span),
-                                "first implementation here".to_string(),
-                            );
-
-                            err.span_label(
-                                impl_span,
-                                format!(
-                                    "conflicting implementation{}",
-                                    overlap
-                                        .self_desc
-                                        .map_or(String::new(), |ty| format!(" for `{}`", ty))
-                                ),
-                            );
-                        }
-                        Err(cname) => {
-                            let msg = match to_pretty_impl_header(tcx, overlap.with_impl) {
-                                Some(s) => format!(
-                                    "conflicting implementation in crate `{}`:\n- {}",
-                                    cname, s
-                                ),
-                                None => format!("conflicting implementation in crate `{}`", cname),
-                            };
-                            err.note(&msg);
-                        }
-                    }
-
-                    for cause in &overlap.intercrate_ambiguity_causes {
-                        cause.add_intercrate_ambiguity_hint(&mut err);
-                    }
-
-                    if overlap.involves_placeholder {
-                        coherence::add_placeholder_note(&mut err);
-                    }
-                    err.emit()
-                };
-
-                match used_to_be_allowed {
-                    None => {
-                        sg.has_errored = true;
-                        let err = struct_span_err!(tcx.sess, impl_span, E0119, "");
-                        decorate(LintDiagnosticBuilder::new(err));
-                    }
-                    Some(kind) => {
-                        let lint = match kind {
-                            FutureCompatOverlapErrorKind::Issue33140 => {
-                                ORDER_DEPENDENT_TRAIT_OBJECTS
-                            }
-                            FutureCompatOverlapErrorKind::LeakCheck => COHERENCE_LEAK_CHECK,
-                        };
-                        tcx.struct_span_lint_hir(
-                            lint,
-                            tcx.hir().as_local_hir_id(impl_def_id).unwrap(),
-                            impl_span,
-                            decorate,
-                        )
-                    }
-                };
+                report_overlap_conflict(tcx, overlap, impl_def_id, used_to_be_allowed, &mut sg);
             }
         } else {
             let parent = tcx.impl_parent(impl_def_id).unwrap_or(trait_id);
@@ -414,6 +335,168 @@ pub(super) fn specialization_graph_provider(
     tcx.arena.alloc(sg)
 }
 
+fn report_overlap_conflict(
+    tcx: TyCtxt<'_>,
+    overlap: OverlapError,
+    impl_def_id: DefId,
+    used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
+    sg: &mut specialization_graph::Graph,
+) {
+    let impl_polarity = tcx.impl_polarity(impl_def_id);
+    let other_polarity = tcx.impl_polarity(overlap.with_impl);
+    match (impl_polarity, other_polarity) {
+        (ty::ImplPolarity::Negative, ty::ImplPolarity::Positive) => {
+            report_negative_positive_conflict(
+                tcx,
+                &overlap,
+                impl_def_id,
+                impl_def_id,
+                overlap.with_impl,
+                sg,
+            );
+        }
+
+        (ty::ImplPolarity::Positive, ty::ImplPolarity::Negative) => {
+            report_negative_positive_conflict(
+                tcx,
+                &overlap,
+                impl_def_id,
+                overlap.with_impl,
+                impl_def_id,
+                sg,
+            );
+        }
+
+        _ => {
+            report_conflicting_impls(tcx, overlap, impl_def_id, used_to_be_allowed, sg);
+        }
+    }
+}
+
+fn report_negative_positive_conflict(
+    tcx: TyCtxt<'_>,
+    overlap: &OverlapError,
+    local_impl_def_id: DefId,
+    negative_impl_def_id: DefId,
+    positive_impl_def_id: DefId,
+    sg: &mut specialization_graph::Graph,
+) {
+    let impl_span = tcx.sess.source_map().def_span(tcx.span_of_impl(local_impl_def_id).unwrap());
+    let mut err = struct_span_err!(
+        tcx.sess,
+        impl_span,
+        E0748,
+        "found both positive and negative implementation of trait `{}`{}:",
+        overlap.trait_desc,
+        overlap.self_desc.clone().map_or(String::new(), |ty| format!(" for type `{}`", ty))
+    );
+
+    match tcx.span_of_impl(negative_impl_def_id) {
+        Ok(span) => {
+            err.span_label(
+                tcx.sess.source_map().def_span(span),
+                "negative implementation here".to_string(),
+            );
+        }
+        Err(cname) => {
+            err.note(&format!("negative implementation in crate `{}`", cname));
+        }
+    }
+
+    match tcx.span_of_impl(positive_impl_def_id) {
+        Ok(span) => {
+            err.span_label(
+                tcx.sess.source_map().def_span(span),
+                "positive implementation here".to_string(),
+            );
+        }
+        Err(cname) => {
+            err.note(&format!("positive implementation in crate `{}`", cname));
+        }
+    }
+
+    sg.has_errored = true;
+    err.emit();
+}
+
+fn report_conflicting_impls(
+    tcx: TyCtxt<'_>,
+    overlap: OverlapError,
+    impl_def_id: DefId,
+    used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
+    sg: &mut specialization_graph::Graph,
+) {
+    let impl_span = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
+
+    // Work to be done after we've built the DiagnosticBuilder. We have to define it
+    // now because the struct_lint methods don't return back the DiagnosticBuilder
+    // that's passed in.
+    let decorate = |err: LintDiagnosticBuilder<'_>| {
+        let msg = format!(
+            "conflicting implementations of trait `{}`{}:{}",
+            overlap.trait_desc,
+            overlap.self_desc.clone().map_or(String::new(), |ty| { format!(" for type `{}`", ty) }),
+            match used_to_be_allowed {
+                Some(FutureCompatOverlapErrorKind::Issue33140) => " (E0119)",
+                _ => "",
+            }
+        );
+        let mut err = err.build(&msg);
+        match tcx.span_of_impl(overlap.with_impl) {
+            Ok(span) => {
+                err.span_label(
+                    tcx.sess.source_map().def_span(span),
+                    "first implementation here".to_string(),
+                );
+
+                err.span_label(
+                    impl_span,
+                    format!(
+                        "conflicting implementation{}",
+                        overlap.self_desc.map_or(String::new(), |ty| format!(" for `{}`", ty))
+                    ),
+                );
+            }
+            Err(cname) => {
+                let msg = match to_pretty_impl_header(tcx, overlap.with_impl) {
+                    Some(s) => format!("conflicting implementation in crate `{}`:\n- {}", cname, s),
+                    None => format!("conflicting implementation in crate `{}`", cname),
+                };
+                err.note(&msg);
+            }
+        }
+
+        for cause in &overlap.intercrate_ambiguity_causes {
+            cause.add_intercrate_ambiguity_hint(&mut err);
+        }
+
+        if overlap.involves_placeholder {
+            coherence::add_placeholder_note(&mut err);
+        }
+        err.emit()
+    };
+
+    match used_to_be_allowed {
+        None => {
+            sg.has_errored = true;
+            let err = struct_span_err!(tcx.sess, impl_span, E0119, "");
+            decorate(LintDiagnosticBuilder::new(err));
+        }
+        Some(kind) => {
+            let lint = match kind {
+                FutureCompatOverlapErrorKind::Issue33140 => ORDER_DEPENDENT_TRAIT_OBJECTS,
+                FutureCompatOverlapErrorKind::LeakCheck => COHERENCE_LEAK_CHECK,
+            };
+            tcx.struct_span_lint_hir(
+                lint,
+                tcx.hir().as_local_hir_id(impl_def_id).unwrap(),
+                impl_span,
+                decorate,
+            )
+        }
+    };
+}
+
 /// Recovers the "impl X for Y" signature from `impl_def_id` and returns it as a
 /// string.
 fn to_pretty_impl_header(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Option<String> {
diff --git a/src/librustc_trait_selection/traits/wf.rs b/src/librustc_trait_selection/traits/wf.rs
index b69c5bdce2abc..5f40c1cefca45 100644
--- a/src/librustc_trait_selection/traits/wf.rs
+++ b/src/librustc_trait_selection/traits/wf.rs
@@ -595,7 +595,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         }
 
         // if we made it through that loop above, we made progress!
-        return true;
+        true
     }
 
     fn nominal_obligations(
diff --git a/src/librustc_traits/lowering/environment.rs b/src/librustc_traits/lowering/environment.rs
index 69d0bd0929687..ed6259d457361 100644
--- a/src/librustc_traits/lowering/environment.rs
+++ b/src/librustc_traits/lowering/environment.rs
@@ -146,7 +146,7 @@ crate fn program_clauses_for_env<'tcx>(
 
     debug!("program_clauses_for_env: closure = {:#?}", closure);
 
-    return tcx.mk_clauses(closure.into_iter());
+    tcx.mk_clauses(closure.into_iter())
 }
 
 crate fn environment(tcx: TyCtxt<'_>, def_id: DefId) -> Environment<'_> {
diff --git a/src/librustc_ty/needs_drop.rs b/src/librustc_ty/needs_drop.rs
index 0f71246c73759..3b72da23bafae 100644
--- a/src/librustc_ty/needs_drop.rs
+++ b/src/librustc_ty/needs_drop.rs
@@ -132,7 +132,7 @@ where
             }
         }
 
-        return None;
+        None
     }
 }
 
diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index 9a8d161572bcf..3ee6d5df7356b 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -2128,7 +2128,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 return Err(ErrorReported);
             }
         }
-        return Ok(bound);
+        Ok(bound)
     }
 
     fn complain_about_assoc_type_not_found<I>(
@@ -2709,7 +2709,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
             Res::Err => {
                 self.set_tainted_by_errors();
-                return self.tcx().types.err;
+                self.tcx().types.err
             }
             _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
         }
@@ -3047,7 +3047,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             )
             .emit();
         }
-        return Some(r);
+        Some(r)
     }
 }
 
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 368f64e4d41aa..fa21f31c3069a 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -1580,7 +1580,7 @@ fn check_union_fields(tcx: TyCtxt<'_>, span: Span, item_def_id: DefId) -> bool {
     } else {
         span_bug!(span, "unions must be ty::Adt, but got {:?}", item_type.kind);
     }
-    return true;
+    true
 }
 
 /// Checks that an opaque type does not contain cycles and does not use `Self` or `T::Foo`
@@ -1656,11 +1656,14 @@ fn check_opaque_for_inheriting_lifetimes(tcx: TyCtxt<'tcx>, def_id: DefId, span:
             _ => unreachable!(),
         };
 
-        tcx.sess.span_err(span, &format!(
+        tcx.sess.span_err(
+            span,
+            &format!(
             "`{}` return type cannot contain a projection or `Self` that references lifetimes from \
              a parent scope",
             if is_async { "async fn" } else { "impl Trait" },
-        ));
+        ),
+        );
     }
 }
 
@@ -1838,8 +1841,8 @@ fn maybe_check_static_with_link_section(tcx: TyCtxt<'_>, id: DefId, span: Span)
         Ok(ConstValue::ByRef { alloc, .. }) => {
             if alloc.relocations().len() != 0 {
                 let msg = "statics with a custom `#[link_section]` must be a \
-                       simple list of bytes on the wasm target with no \
-                       extra levels of indirection such as references";
+                           simple list of bytes on the wasm target with no \
+                           extra levels of indirection such as references";
                 tcx.sess.span_err(span, msg);
             }
         }
@@ -1968,6 +1971,24 @@ fn check_impl_items_against_trait<'tcx>(
         return;
     }
 
+    // Negative impls are not expected to have any items
+    match tcx.impl_polarity(impl_id) {
+        ty::ImplPolarity::Reservation | ty::ImplPolarity::Positive => {}
+        ty::ImplPolarity::Negative => {
+            if let [first_item_ref, ..] = impl_item_refs {
+                let first_item_span = tcx.hir().impl_item(first_item_ref.id).span;
+                struct_span_err!(
+                    tcx.sess,
+                    first_item_span,
+                    E0749,
+                    "negative impls cannot have any items"
+                )
+                .emit();
+            }
+            return;
+        }
+    }
+
     // Locate trait definition and items
     let trait_def = tcx.trait_def(impl_trait_ref.def_id);
 
@@ -2007,7 +2028,7 @@ fn check_impl_items_against_trait<'tcx>(
                             impl_item.span,
                             E0323,
                             "item `{}` is an associated const, \
-                              which doesn't match its trait `{}`",
+                             which doesn't match its trait `{}`",
                             ty_impl_item.ident,
                             impl_trait_ref.print_only_trait_path()
                         );
@@ -2313,7 +2334,7 @@ fn check_representable(tcx: TyCtxt<'_>, sp: Span, item_def_id: DefId) -> bool {
         }
         Representability::Representable | Representability::ContainsRecursive => (),
     }
-    return true;
+    true
 }
 
 pub fn check_simd(tcx: TyCtxt<'_>, sp: Span, def_id: DefId) {
@@ -3555,7 +3576,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let adjusted_ty = autoderef.unambiguous_final_ty(self);
         debug!(
             "try_index_step(expr={:?}, base_expr={:?}, adjusted_ty={:?}, \
-                               index_ty={:?})",
+             index_ty={:?})",
             expr, base_expr, adjusted_ty, index_ty
         );
 
@@ -4708,7 +4729,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 err.span_label(
                                     fn_span,
                                     "implicitly returns `()` as its body has no tail or `return` \
-                                 expression",
+                                     expression",
                                 );
                             }
                         },
@@ -5593,11 +5614,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             match self.at(&self.misc(span), self.param_env).sup(impl_ty, self_ty) {
                 Ok(ok) => self.register_infer_ok_obligations(ok),
                 Err(_) => {
-                    self.tcx.sess.delay_span_bug(span, &format!(
+                    self.tcx.sess.delay_span_bug(
+                        span,
+                        &format!(
                         "instantiate_value_path: (UFCS) {:?} was a subtype of {:?} but now is not?",
                         self_ty,
                         impl_ty,
-                    ));
+                    ),
+                    );
                 }
             }
         }
@@ -5783,7 +5807,7 @@ fn fatally_break_rust(sess: &Session) {
     handler.note_without_error("the compiler expectedly panicked. this is a feature.");
     handler.note_without_error(
         "we would appreciate a joke overview: \
-        https://github.com/rust-lang/rust/issues/43162#issuecomment-320764675",
+         https://github.com/rust-lang/rust/issues/43162#issuecomment-320764675",
     );
     handler.note_without_error(&format!(
         "rustc {} running on {}",
diff --git a/src/librustc_typeck/check/wfcheck.rs b/src/librustc_typeck/check/wfcheck.rs
index 3255d7b435c8f..45d0eea8628c4 100644
--- a/src/librustc_typeck/check/wfcheck.rs
+++ b/src/librustc_typeck/check/wfcheck.rs
@@ -112,15 +112,14 @@ pub fn check_item_well_formed(tcx: TyCtxt<'_>, def_id: DefId) {
                 }
                 ty::ImplPolarity::Negative => {
                     // FIXME(#27579): what amount of WF checking do we need for neg impls?
-                    if of_trait.is_some() && !is_auto {
+                    if let hir::Defaultness::Default { .. } = defaultness {
                         struct_span_err!(
                             tcx.sess,
                             item.span,
-                            E0192,
-                            "negative impls are only allowed for \
-                                   auto traits (e.g., `Send` and `Sync`)"
+                            E0750,
+                            "negative impls cannot be default impls"
                         )
-                        .emit()
+                        .emit();
                     }
                 }
                 ty::ImplPolarity::Reservation => {
@@ -886,13 +885,13 @@ fn check_opaque_types<'fcx, 'tcx>(
                                             .struct_span_err(
                                                 span,
                                                 "non-defining opaque type use \
-                                                    in defining scope",
+                                                 in defining scope",
                                             )
                                             .span_label(
                                                 param_span,
                                                 "cannot use static lifetime; use a bound lifetime \
-                                                instead or remove the lifetime parameter from the \
-                                                opaque type",
+                                                 instead or remove the lifetime parameter from the \
+                                                 opaque type",
                                             )
                                             .emit();
                                     } else {
@@ -907,13 +906,13 @@ fn check_opaque_types<'fcx, 'tcx>(
                                             .struct_span_err(
                                                 span,
                                                 "non-defining opaque type use \
-                                                in defining scope",
+                                                 in defining scope",
                                             )
                                             .span_note(
                                                 tcx.def_span(param.def_id),
                                                 &format!(
                                                     "used non-generic const {} for \
-                                                    generic parameter",
+                                                     generic parameter",
                                                     ty,
                                                 ),
                                             )
@@ -928,7 +927,7 @@ fn check_opaque_types<'fcx, 'tcx>(
                                     .struct_span_err(
                                         span,
                                         "non-defining opaque type use \
-                                            in defining scope",
+                                         in defining scope",
                                     )
                                     .span_note(spans, "lifetime used multiple times")
                                     .emit();
@@ -1014,7 +1013,7 @@ fn check_method_receiver<'fcx, 'tcx>(
                     span,
                     &format!(
                         "`{}` cannot be used as the type of `self` without \
-                            the `arbitrary_self_types` feature",
+                         the `arbitrary_self_types` feature",
                         receiver_ty,
                     ),
                 )
diff --git a/src/librustc_typeck/coherence/inherent_impls.rs b/src/librustc_typeck/coherence/inherent_impls.rs
index 60e5df68b5842..c6ee9ab60abf3 100644
--- a/src/librustc_typeck/coherence/inherent_impls.rs
+++ b/src/librustc_typeck/coherence/inherent_impls.rs
@@ -272,9 +272,7 @@ impl ItemLikeVisitor<'v> for InherentCollect<'tcx> {
                     item.span,
                 );
             }
-            ty::Error => {
-                return;
-            }
+            ty::Error => {}
             _ => {
                 struct_span_err!(
                     self.tcx.sess,
@@ -288,7 +286,6 @@ impl ItemLikeVisitor<'v> for InherentCollect<'tcx> {
                        to wrap it instead",
                 )
                 .emit();
-                return;
             }
         }
     }
diff --git a/src/librustc_typeck/collect/type_of.rs b/src/librustc_typeck/collect/type_of.rs
index 41c205bc11b35..44ef4ebd463d7 100644
--- a/src/librustc_typeck/collect/type_of.rs
+++ b/src/librustc_typeck/collect/type_of.rs
@@ -290,7 +290,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                             DUMMY_SP,
                             &format!("unexpected const parent path {:?}", parent_node,),
                         );
-                        return tcx.types.err;
+                        tcx.types.err
                     }
                 }
 
diff --git a/src/librustc_typeck/impl_wf_check/min_specialization.rs b/src/librustc_typeck/impl_wf_check/min_specialization.rs
index cae8837611846..ef94500f5c444 100644
--- a/src/librustc_typeck/impl_wf_check/min_specialization.rs
+++ b/src/librustc_typeck/impl_wf_check/min_specialization.rs
@@ -270,7 +270,7 @@ fn check_static_lifetimes<'tcx>(
     span: Span,
 ) {
     if tcx.any_free_region_meets(parent_substs, |r| *r == ty::ReStatic) {
-        tcx.sess.struct_span_err(span, &format!("cannot specialize on `'static` lifetime")).emit();
+        tcx.sess.struct_span_err(span, "cannot specialize on `'static` lifetime").emit();
     }
 }
 
diff --git a/src/librustdoc/passes/calculate_doc_coverage.rs b/src/librustdoc/passes/calculate_doc_coverage.rs
index f48224512ba4f..98300385c8fb8 100644
--- a/src/librustdoc/passes/calculate_doc_coverage.rs
+++ b/src/librustdoc/passes/calculate_doc_coverage.rs
@@ -8,7 +8,6 @@ use rustc_ast::attr;
 use rustc_span::symbol::sym;
 use rustc_span::FileName;
 use serde::Serialize;
-use serde_json;
 
 use std::collections::BTreeMap;
 use std::ops;
diff --git a/src/libstd/backtrace.rs b/src/libstd/backtrace.rs
index 34317c7a2ee3d..e10d466030f0b 100644
--- a/src/libstd/backtrace.rs
+++ b/src/libstd/backtrace.rs
@@ -250,7 +250,7 @@ impl Backtrace {
             },
         };
         ENABLED.store(enabled as usize + 1, SeqCst);
-        return enabled;
+        enabled
     }
 
     /// Capture a stack backtrace of the current thread.
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 1f122b02b6a29..1db5926908cb0 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -278,6 +278,7 @@
 #![feature(maybe_uninit_ref)]
 #![feature(maybe_uninit_slice)]
 #![feature(needs_panic_runtime)]
+#![cfg_attr(not(bootstrap), feature(negative_impls))]
 #![feature(never_type)]
 #![feature(nll)]
 #![feature(optin_builtin_traits)]
diff --git a/src/libstd/sys_common/backtrace.rs b/src/libstd/sys_common/backtrace.rs
index 2c7ba8f8ea1fd..e9b1e86d7ae49 100644
--- a/src/libstd/sys_common/backtrace.rs
+++ b/src/libstd/sys_common/backtrace.rs
@@ -28,7 +28,7 @@ pub fn lock() -> impl Drop {
 
     unsafe {
         LOCK.lock();
-        return Guard;
+        Guard
     }
 }
 
diff --git a/src/test/debuginfo/pretty-std-collections.rs b/src/test/debuginfo/pretty-std-collections.rs
index f8997fad9a53f..3d2d88a676d0d 100644
--- a/src/test/debuginfo/pretty-std-collections.rs
+++ b/src/test/debuginfo/pretty-std-collections.rs
@@ -17,35 +17,43 @@
 // gdb-command: print btree_set
 // gdb-check:$1 = BTreeSet<i32>(len: 15) = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}
 
+// gdb-command: print empty_btree_set
+// gdb-check:$2 = BTreeSet<i32>(len: 0)
+
 // gdb-command: print btree_map
-// gdb-check:$2 = BTreeMap<i32, i32>(len: 15) = {[0] = 0, [1] = 1, [2] = 2, [3] = 3, [4] = 4, [5] = 5, [6] = 6, [7] = 7, [8] = 8, [9] = 9, [10] = 10, [11] = 11, [12] = 12, [13] = 13, [14] = 14}
+// gdb-check:$3 = BTreeMap<i32, i32>(len: 15) = {[0] = 0, [1] = 1, [2] = 2, [3] = 3, [4] = 4, [5] = 5, [6] = 6, [7] = 7, [8] = 8, [9] = 9, [10] = 10, [11] = 11, [12] = 12, [13] = 13, [14] = 14}
+
+// gdb-command: print empty_btree_map
+// gdb-check:$4 = BTreeMap<i32, u32>(len: 0)
 
 // gdb-command: print vec_deque
-// gdb-check:$3 = VecDeque<i32>(len: 3, cap: 8) = {5, 3, 7}
+// gdb-check:$5 = VecDeque<i32>(len: 3, cap: 8) = {5, 3, 7}
 
 // gdb-command: print vec_deque2
-// gdb-check:$4 = VecDeque<i32>(len: 7, cap: 8) = {2, 3, 4, 5, 6, 7, 8}
+// gdb-check:$6 = VecDeque<i32>(len: 7, cap: 8) = {2, 3, 4, 5, 6, 7, 8}
 
 #![allow(unused_variables)]
-use std::collections::BTreeSet;
 use std::collections::BTreeMap;
+use std::collections::BTreeSet;
 use std::collections::VecDeque;
 
-
 fn main() {
-
     // BTreeSet
     let mut btree_set = BTreeSet::new();
     for i in 0..15 {
         btree_set.insert(i);
     }
 
+    let mut empty_btree_set: BTreeSet<i32> = BTreeSet::new();
+
     // BTreeMap
     let mut btree_map = BTreeMap::new();
     for i in 0..15 {
         btree_map.insert(i, i);
     }
 
+    let mut empty_btree_map: BTreeMap<i32, u32> = BTreeMap::new();
+
     // VecDeque
     let mut vec_deque = VecDeque::new();
     vec_deque.push_back(5);
@@ -63,4 +71,6 @@ fn main() {
     zzz(); // #break
 }
 
-fn zzz() { () }
+fn zzz() {
+    ()
+}
diff --git a/src/test/pretty/trait-polarity.rs b/src/test/pretty/trait-polarity.rs
index 3aab99bf6a0ae..df1a7946afb11 100644
--- a/src/test/pretty/trait-polarity.rs
+++ b/src/test/pretty/trait-polarity.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 // pp-exact
 
diff --git a/src/test/rustdoc/empty-section.rs b/src/test/rustdoc/empty-section.rs
index d95f3a80365cd..665aa38b11eba 100644
--- a/src/test/rustdoc/empty-section.rs
+++ b/src/test/rustdoc/empty-section.rs
@@ -1,6 +1,6 @@
 #![crate_name = "foo"]
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 pub struct Foo;
 
diff --git a/src/test/rustdoc/impl-parts-crosscrate.rs b/src/test/rustdoc/impl-parts-crosscrate.rs
index f9583d1a72296..a68db9c70ad2d 100644
--- a/src/test/rustdoc/impl-parts-crosscrate.rs
+++ b/src/test/rustdoc/impl-parts-crosscrate.rs
@@ -1,7 +1,7 @@
 // aux-build:rustdoc-impl-parts-crosscrate.rs
 // ignore-cross-compile
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 extern crate rustdoc_impl_parts_crosscrate;
 
diff --git a/src/test/rustdoc/impl-parts.rs b/src/test/rustdoc/impl-parts.rs
index fbb4e725481c0..68baca9a04e9a 100644
--- a/src/test/rustdoc/impl-parts.rs
+++ b/src/test/rustdoc/impl-parts.rs
@@ -1,3 +1,4 @@
+#![feature(negative_impls)]
 #![feature(optin_builtin_traits)]
 
 pub auto trait AnOibit {}
diff --git a/src/test/rustdoc/issue-55321.rs b/src/test/rustdoc/issue-55321.rs
index 257cb32c65c25..8c001db06c5bf 100644
--- a/src/test/rustdoc/issue-55321.rs
+++ b/src/test/rustdoc/issue-55321.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 // @has issue_55321/struct.A.html
 // @has - '//*[@id="implementations-list"]/*[@class="impl"]//code' "impl !Send for A"
diff --git a/src/test/rustdoc/negative-impl-sidebar.rs b/src/test/rustdoc/negative-impl-sidebar.rs
index 838ca0402e48a..cb46d1778d924 100644
--- a/src/test/rustdoc/negative-impl-sidebar.rs
+++ b/src/test/rustdoc/negative-impl-sidebar.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![crate_name = "foo"]
 
 pub struct Foo;
diff --git a/src/test/rustdoc/negative-impl.rs b/src/test/rustdoc/negative-impl.rs
index 8ac87f4f0cb97..d76aac6906c46 100644
--- a/src/test/rustdoc/negative-impl.rs
+++ b/src/test/rustdoc/negative-impl.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 // @matches negative_impl/struct.Alpha.html '//pre' "pub struct Alpha"
 pub struct Alpha;
diff --git a/src/test/ui/async-await/issue-64130-1-sync.rs b/src/test/ui/async-await/issue-64130-1-sync.rs
index cc5ca89f03af0..af83f14bbda5d 100644
--- a/src/test/ui/async-await/issue-64130-1-sync.rs
+++ b/src/test/ui/async-await/issue-64130-1-sync.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 // edition:2018
 
 // This tests the the specialized async-await-specific error when futures don't implement an
diff --git a/src/test/ui/async-await/issue-64130-2-send.rs b/src/test/ui/async-await/issue-64130-2-send.rs
index 1efe2ab3f85e2..2362831d8b8f6 100644
--- a/src/test/ui/async-await/issue-64130-2-send.rs
+++ b/src/test/ui/async-await/issue-64130-2-send.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 // edition:2018
 
 // This tests the the specialized async-await-specific error when futures don't implement an
diff --git a/src/test/ui/async-await/issue-64130-3-other.rs b/src/test/ui/async-await/issue-64130-3-other.rs
index 901544edba18a..b819970d59d50 100644
--- a/src/test/ui/async-await/issue-64130-3-other.rs
+++ b/src/test/ui/async-await/issue-64130-3-other.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 // edition:2018
 
 // This tests the the unspecialized async-await-specific error when futures don't implement an
diff --git a/src/test/ui/async-await/issue-64130-3-other.stderr b/src/test/ui/async-await/issue-64130-3-other.stderr
index d6828172928dd..6456e7abd745d 100644
--- a/src/test/ui/async-await/issue-64130-3-other.stderr
+++ b/src/test/ui/async-await/issue-64130-3-other.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `Foo: Qux` is not satisfied in `impl std::future::Future`
-  --> $DIR/issue-64130-3-other.rs:23:5
+  --> $DIR/issue-64130-3-other.rs:24:5
    |
 LL | fn is_qux<T: Qux>(t: T) { }
    |    ------    --- required by this bound in `is_qux`
@@ -13,7 +13,7 @@ LL |     is_qux(bar());
    = help: the following implementations were found:
              <Foo as Qux>
 note: future does not implement `Qux` as this value is used across an await
-  --> $DIR/issue-64130-3-other.rs:17:5
+  --> $DIR/issue-64130-3-other.rs:18:5
    |
 LL |     let x = Foo;
    |         - has type `Foo`
diff --git a/src/test/ui/codemap_tests/empty_span.rs b/src/test/ui/codemap_tests/empty_span.rs
index 4d52b391280a7..7753e2eceb543 100644
--- a/src/test/ui/codemap_tests/empty_span.rs
+++ b/src/test/ui/codemap_tests/empty_span.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 fn main() {
     struct Foo;
 
diff --git a/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.rs b/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.rs
index b4f5f9ef56bb1..24b878927530c 100644
--- a/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.rs
+++ b/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![feature(marker_trait_attr)]
 
 #[marker]
@@ -6,13 +6,11 @@ trait MyTrait {}
 
 struct TestType<T>(::std::marker::PhantomData<T>);
 
-unsafe impl<T: MyTrait+'static> Send for TestType<T> {}
+unsafe impl<T: MyTrait + 'static> Send for TestType<T> {}
 
-impl<T: MyTrait> !Send for TestType<T> {}
-//~^ ERROR conflicting implementations
+impl<T: MyTrait> !Send for TestType<T> {} //~ ERROR found both positive and negative implementation
 
-unsafe impl<T:'static> Send for TestType<T> {}
-//~^ ERROR conflicting implementations
+unsafe impl<T: 'static> Send for TestType<T> {} //~ ERROR conflicting implementations
 
 impl !Send for TestType<i32> {}
 
diff --git a/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.stderr b/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.stderr
index 25d3d3ee997a5..5081536b70245 100644
--- a/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.stderr
+++ b/src/test/ui/coherence/coherence-conflicting-negative-trait-impl.stderr
@@ -1,21 +1,22 @@
-error[E0119]: conflicting implementations of trait `std::marker::Send` for type `TestType<_>`:
+error[E0748]: found both positive and negative implementation of trait `std::marker::Send` for type `TestType<_>`:
   --> $DIR/coherence-conflicting-negative-trait-impl.rs:11:1
    |
-LL | unsafe impl<T: MyTrait+'static> Send for TestType<T> {}
-   | ---------------------------------------------------- first implementation here
+LL | unsafe impl<T: MyTrait + 'static> Send for TestType<T> {}
+   | ------------------------------------------------------ positive implementation here
 LL | 
 LL | impl<T: MyTrait> !Send for TestType<T> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<_>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ negative implementation here
 
 error[E0119]: conflicting implementations of trait `std::marker::Send` for type `TestType<_>`:
-  --> $DIR/coherence-conflicting-negative-trait-impl.rs:14:1
+  --> $DIR/coherence-conflicting-negative-trait-impl.rs:13:1
    |
-LL | unsafe impl<T: MyTrait+'static> Send for TestType<T> {}
-   | ---------------------------------------------------- first implementation here
+LL | unsafe impl<T: MyTrait + 'static> Send for TestType<T> {}
+   | ------------------------------------------------------ first implementation here
 ...
-LL | unsafe impl<T:'static> Send for TestType<T> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<_>`
+LL | unsafe impl<T: 'static> Send for TestType<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<_>`
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0119`.
+Some errors have detailed explanations: E0119, E0748.
+For more information about an error, try `rustc --explain E0119`.
diff --git a/src/test/ui/coherence/coherence-default-trait-impl.rs b/src/test/ui/coherence/coherence-default-trait-impl.rs
index db24662e2d5ad..4115ba34e17b2 100644
--- a/src/test/ui/coherence/coherence-default-trait-impl.rs
+++ b/src/test/ui/coherence/coherence-default-trait-impl.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait MySafeTrait {}
 
diff --git a/src/test/ui/coherence/coherence-default-trait-impl.stderr b/src/test/ui/coherence/coherence-default-trait-impl.stderr
index f6a163268a14a..b08ccb087d91c 100644
--- a/src/test/ui/coherence/coherence-default-trait-impl.stderr
+++ b/src/test/ui/coherence/coherence-default-trait-impl.stderr
@@ -1,11 +1,11 @@
 error[E0199]: implementing the trait `MySafeTrait` is not unsafe
-  --> $DIR/coherence-default-trait-impl.rs:7:1
+  --> $DIR/coherence-default-trait-impl.rs:8:1
    |
 LL | unsafe impl MySafeTrait for Foo {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0200]: the trait `MyUnsafeTrait` requires an `unsafe impl` declaration
-  --> $DIR/coherence-default-trait-impl.rs:12:1
+  --> $DIR/coherence-default-trait-impl.rs:13:1
    |
 LL | impl MyUnsafeTrait for Foo {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs
index 5ea69190951e1..a9c8d20a79d75 100644
--- a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 // Test for issue #56934 - that it is impossible to redundantly
 // implement an auto-trait for a trait object type that contains it.
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr
index b8137b36948cd..23db5328a728b 100644
--- a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr
@@ -1,17 +1,17 @@
 error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:14:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:15:1
    |
 LL | impl !Marker1 for dyn Object + Marker2 { }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
 
 error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:16:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:17:1
    |
 LL | impl !Marker2 for dyn Object + Marker2 { }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
 
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:22:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:23:1
    |
 LL | impl !Send for dyn Marker2 {}
    | ^^^^^^^^^^^^^^^-----------
@@ -22,13 +22,13 @@ LL | impl !Send for dyn Marker2 {}
    = note: define and implement a trait or new type instead
 
 error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:26:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:27:1
    |
 LL | impl !Send for dyn Object {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
 
 error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:27:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:28:1
    |
 LL | impl !Send for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs
index 6b5689e8260f0..c565f9c83e89e 100644
--- a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 // Test for issue #56934 - that it is impossible to redundantly
 // implement an auto-trait for a trait object type that contains it.
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr
index d68337bed0066..141ab7771f325 100644
--- a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr
@@ -1,17 +1,17 @@
 error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:14:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:15:1
    |
 LL | impl Marker1 for dyn Object + Marker2 { }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
 
 error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:16:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:17:1
    |
 LL | impl Marker2 for dyn Object + Marker2 { }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
 
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:22:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:23:1
    |
 LL | unsafe impl Send for dyn Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^-----------
@@ -22,13 +22,13 @@ LL | unsafe impl Send for dyn Marker2 {}
    = note: define and implement a trait or new type instead
 
 error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:26:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:27:1
    |
 LL | unsafe impl Send for dyn Object {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
 
 error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:27:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:28:1
    |
 LL | unsafe impl Send for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
diff --git a/src/test/ui/coherence/coherence-impls-copy.rs b/src/test/ui/coherence/coherence-impls-copy.rs
index dec40f9dd40aa..a86ca0e5eacdc 100644
--- a/src/test/ui/coherence/coherence-impls-copy.rs
+++ b/src/test/ui/coherence/coherence-impls-copy.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Copy;
 
diff --git a/src/test/ui/coherence/coherence-impls-send.rs b/src/test/ui/coherence/coherence-impls-send.rs
index 7898dc9831da2..e00cb9a7c5b51 100644
--- a/src/test/ui/coherence/coherence-impls-send.rs
+++ b/src/test/ui/coherence/coherence-impls-send.rs
@@ -1,9 +1,9 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Copy;
 
 enum TestE {
-  A
+    A,
 }
 
 struct MyType;
@@ -26,5 +26,4 @@ unsafe impl Send for &'static [NotSync] {}
 //~^ ERROR conflicting implementations of trait
 //~| ERROR only traits defined in the current crate
 
-fn main() {
-}
+fn main() {}
diff --git a/src/test/ui/coherence/coherence-impls-sized.rs b/src/test/ui/coherence/coherence-impls-sized.rs
index 19e7349c507ef..231b96ad42efb 100644
--- a/src/test/ui/coherence/coherence-impls-sized.rs
+++ b/src/test/ui/coherence/coherence-impls-sized.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Copy;
 
diff --git a/src/test/ui/coherence/coherence-negative-impls-safe-rpass.rs b/src/test/ui/coherence/coherence-negative-impls-safe-rpass.rs
index 695a71cbd2d7c..b87e162aca096 100644
--- a/src/test/ui/coherence/coherence-negative-impls-safe-rpass.rs
+++ b/src/test/ui/coherence/coherence-negative-impls-safe-rpass.rs
@@ -2,7 +2,7 @@
 #![allow(dead_code)]
 // pretty-expanded FIXME #23616
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Send;
 
diff --git a/src/test/ui/coherence/coherence-negative-impls-safe.rs b/src/test/ui/coherence/coherence-negative-impls-safe.rs
index 45c478ecc0362..4821aa6b5ad3d 100644
--- a/src/test/ui/coherence/coherence-negative-impls-safe.rs
+++ b/src/test/ui/coherence/coherence-negative-impls-safe.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Send;
 
diff --git a/src/test/ui/coherence/coherence-orphan.rs b/src/test/ui/coherence/coherence-orphan.rs
index a7b48825d7c02..3beac04c7e829 100644
--- a/src/test/ui/coherence/coherence-orphan.rs
+++ b/src/test/ui/coherence/coherence-orphan.rs
@@ -1,5 +1,5 @@
 // aux-build:coherence_orphan_lib.rs
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 extern crate coherence_orphan_lib as lib;
 
diff --git a/src/test/ui/consts/const_forget.rs b/src/test/ui/consts/const_forget.rs
new file mode 100644
index 0000000000000..2dcb72a5a09cb
--- /dev/null
+++ b/src/test/ui/consts/const_forget.rs
@@ -0,0 +1,22 @@
+// check-pass
+
+#![feature(const_forget)]
+
+use std::mem::forget;
+
+const _: () = forget(0i32);
+const _: () = forget(Vec::<Vec<Box<i32>>>::new());
+
+// Writing this function signature without const-forget
+// triggers compiler errors:
+// 1) That we use a non-const fn inside a const fn
+// 2) without the forget, it complains about the destructor of Box
+//
+// FIXME: this method cannot be called in const-eval yet, as Box isn't
+// const constructable
+#[allow(unused)]
+const fn const_forget_box<T: ?Sized>(b: Box<T>) {
+    forget(b);
+}
+
+fn main() {}
diff --git a/src/test/ui/error-codes/E0192.rs b/src/test/ui/error-codes/E0192.rs
deleted file mode 100644
index c52977e49b457..0000000000000
--- a/src/test/ui/error-codes/E0192.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-#![feature(optin_builtin_traits)]
-
-trait Trait {
-    type Bar;
-}
-
-struct Foo;
-
-impl !Trait for Foo { } //~ ERROR E0192
-
-fn main() {
-}
diff --git a/src/test/ui/error-codes/E0192.stderr b/src/test/ui/error-codes/E0192.stderr
deleted file mode 100644
index 8faa550a50935..0000000000000
--- a/src/test/ui/error-codes/E0192.stderr
+++ /dev/null
@@ -1,9 +0,0 @@
-error[E0192]: negative impls are only allowed for auto traits (e.g., `Send` and `Sync`)
-  --> $DIR/E0192.rs:9:1
-   |
-LL | impl !Trait for Foo { }
-   | ^^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0192`.
diff --git a/src/test/ui/error-codes/E0198.rs b/src/test/ui/error-codes/E0198.rs
index 00ab0c3562378..041bbe8fdcf69 100644
--- a/src/test/ui/error-codes/E0198.rs
+++ b/src/test/ui/error-codes/E0198.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 struct Foo;
 
diff --git a/src/test/ui/error-codes/E0199.rs b/src/test/ui/error-codes/E0199.rs
index c95afa3f97d4a..2421bf0a55f65 100644
--- a/src/test/ui/error-codes/E0199.rs
+++ b/src/test/ui/error-codes/E0199.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 struct Foo;
 
diff --git a/src/test/ui/feature-gates/feature-gate-optin-builtin-traits.stderr b/src/test/ui/feature-gates/feature-gate-optin-builtin-traits.stderr
index 490d29ad8a35f..1553d0531dc09 100644
--- a/src/test/ui/feature-gates/feature-gate-optin-builtin-traits.stderr
+++ b/src/test/ui/feature-gates/feature-gate-optin-builtin-traits.stderr
@@ -13,8 +13,8 @@ error[E0658]: negative trait bounds are not yet fully implemented; use marker ty
 LL | impl !AutoDummyTrait for DummyStruct {}
    |      ^^^^^^^^^^^^^^^
    |
-   = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
-   = help: add `#![feature(optin_builtin_traits)]` to the crate attributes to enable
+   = note: see issue #68318 <https://github.com/rust-lang/rust/issues/68318> for more information
+   = help: add `#![feature(negative_impls)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/generator/auto-trait-regions.rs b/src/test/ui/generator/auto-trait-regions.rs
index dbd8965dcf0d0..1e77d8058a72b 100644
--- a/src/test/ui/generator/auto-trait-regions.rs
+++ b/src/test/ui/generator/auto-trait-regions.rs
@@ -1,5 +1,6 @@
 #![feature(generators)]
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait Foo {}
 
diff --git a/src/test/ui/generator/auto-trait-regions.stderr b/src/test/ui/generator/auto-trait-regions.stderr
index 29a3907d93c84..5ec462e10465f 100644
--- a/src/test/ui/generator/auto-trait-regions.stderr
+++ b/src/test/ui/generator/auto-trait-regions.stderr
@@ -1,5 +1,5 @@
 error: implementation of `Foo` is not general enough
-  --> $DIR/auto-trait-regions.rs:30:5
+  --> $DIR/auto-trait-regions.rs:31:5
    |
 LL | auto trait Foo {}
    | ----------------- trait `Foo` defined here
@@ -11,7 +11,7 @@ LL |     assert_foo(gen);
    = note: ...but `Foo` is actually implemented for the type `&'1 OnlyFooIfStaticRef`, for some specific lifetime `'1`
 
 error: implementation of `Foo` is not general enough
-  --> $DIR/auto-trait-regions.rs:30:5
+  --> $DIR/auto-trait-regions.rs:31:5
    |
 LL | auto trait Foo {}
    | ----------------- trait `Foo` defined here
@@ -23,7 +23,7 @@ LL |     assert_foo(gen);
    = note: ...but `Foo` is actually implemented for the type `&'1 OnlyFooIfStaticRef`, for some specific lifetime `'1`
 
 error: implementation of `Foo` is not general enough
-  --> $DIR/auto-trait-regions.rs:49:5
+  --> $DIR/auto-trait-regions.rs:50:5
    |
 LL | auto trait Foo {}
    | ----------------- trait `Foo` defined here
@@ -35,7 +35,7 @@ LL |     assert_foo(gen);
    = note: ...but `Foo` is actually implemented for the type `A<'_, '2>`, for some specific lifetime `'2`
 
 error: implementation of `Foo` is not general enough
-  --> $DIR/auto-trait-regions.rs:49:5
+  --> $DIR/auto-trait-regions.rs:50:5
    |
 LL | auto trait Foo {}
    | ----------------- trait `Foo` defined here
diff --git a/src/test/ui/issues/issue-17718-static-sync.rs b/src/test/ui/issues/issue-17718-static-sync.rs
index dccbde6a3c532..6f278d76bb18b 100644
--- a/src/test/ui/issues/issue-17718-static-sync.rs
+++ b/src/test/ui/issues/issue-17718-static-sync.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Sync;
 
diff --git a/src/test/ui/issues/issue-23080-2.rs b/src/test/ui/issues/issue-23080-2.rs
index d20bb4bd90726..7f6b9e3fba79f 100644
--- a/src/test/ui/issues/issue-23080-2.rs
+++ b/src/test/ui/issues/issue-23080-2.rs
@@ -1,6 +1,7 @@
 //~ ERROR
 
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 unsafe auto trait Trait {
     type Output; //~ ERROR E0380
diff --git a/src/test/ui/issues/issue-23080-2.stderr b/src/test/ui/issues/issue-23080-2.stderr
index fcd1ecfa98288..48ce09aaa34da 100644
--- a/src/test/ui/issues/issue-23080-2.stderr
+++ b/src/test/ui/issues/issue-23080-2.stderr
@@ -1,5 +1,5 @@
 error[E0380]: auto traits cannot have methods or associated items
-  --> $DIR/issue-23080-2.rs:6:10
+  --> $DIR/issue-23080-2.rs:7:10
    |
 LL | unsafe auto trait Trait {
    |                   ----- auto trait cannot have items
diff --git a/src/test/ui/issues/issue-23080.rs b/src/test/ui/issues/issue-23080.rs
index fa5c35316bc28..035db82ba5de0 100644
--- a/src/test/ui/issues/issue-23080.rs
+++ b/src/test/ui/issues/issue-23080.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 unsafe auto trait Trait {
     fn method(&self) { //~ ERROR E0380
diff --git a/src/test/ui/issues/issue-23080.stderr b/src/test/ui/issues/issue-23080.stderr
index dbb9861b5784a..73ecb1c362e17 100644
--- a/src/test/ui/issues/issue-23080.stderr
+++ b/src/test/ui/issues/issue-23080.stderr
@@ -1,5 +1,5 @@
 error[E0380]: auto traits cannot have methods or associated items
-  --> $DIR/issue-23080.rs:4:8
+  --> $DIR/issue-23080.rs:5:8
    |
 LL | unsafe auto trait Trait {
    |                   ----- auto trait cannot have items
diff --git a/src/test/ui/issues/issue-29516.rs b/src/test/ui/issues/issue-29516.rs
index d43367e34529b..035f904b15bb3 100644
--- a/src/test/ui/issues/issue-29516.rs
+++ b/src/test/ui/issues/issue-29516.rs
@@ -1,5 +1,6 @@
 // check-pass
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait NotSame {}
 
diff --git a/src/test/ui/issues/issue-33140-hack-boundaries.rs b/src/test/ui/issues/issue-33140-hack-boundaries.rs
index fbdef51c13255..5984a256a3086 100644
--- a/src/test/ui/issues/issue-33140-hack-boundaries.rs
+++ b/src/test/ui/issues/issue-33140-hack-boundaries.rs
@@ -1,11 +1,10 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![allow(order_dependent_trait_objects)]
 
 // Check that the issue #33140 hack does not allow unintended things.
 
 // OK
-trait Trait0 {
-}
+trait Trait0 {}
 
 impl Trait0 for dyn Send {}
 impl Trait0 for dyn Send {}
@@ -20,58 +19,49 @@ impl Trait1 for dyn Send {}
 //~^ ERROR E0119
 
 // Problem 2: negative impl
-trait Trait2 {
-}
+trait Trait2 {}
 
 impl Trait2 for dyn Send {}
 impl !Trait2 for dyn Send {}
-//~^ ERROR E0119
-
+//~^ ERROR E0748
 
 // Problem 3: type parameter
-trait Trait3<T: ?Sized> {
-}
+trait Trait3<T: ?Sized> {}
 
 impl Trait3<dyn Sync> for dyn Send {}
 impl Trait3<dyn Sync> for dyn Send {}
 //~^ ERROR E0119
 
 // Problem 4a: not a trait object - generic
-trait Trait4a {
-}
+trait Trait4a {}
 
 impl<T: ?Sized> Trait4a for T {}
 impl Trait4a for dyn Send {}
 //~^ ERROR E0119
 
 // Problem 4b: not a trait object - misc
-trait Trait4b {
-}
+trait Trait4b {}
 
 impl Trait4b for () {}
 impl Trait4b for () {}
 //~^ ERROR E0119
 
 // Problem 4c: not a principal-less trait object
-trait Trait4c {
-}
+trait Trait4c {}
 
 impl Trait4c for dyn Trait1 + Send {}
 impl Trait4c for dyn Trait1 + Send {}
 //~^ ERROR E0119
 
 // Problem 4d: lifetimes
-trait Trait4d {
-}
+trait Trait4d {}
 
 impl<'a> Trait4d for dyn Send + 'a {}
 impl<'a> Trait4d for dyn Send + 'a {}
 //~^ ERROR E0119
 
-
 // Problem 5: where-clauses
-trait Trait5 {
-}
+trait Trait5 {}
 
 impl Trait5 for dyn Send {}
 impl Trait5 for dyn Send where u32: Copy {}
diff --git a/src/test/ui/issues/issue-33140-hack-boundaries.stderr b/src/test/ui/issues/issue-33140-hack-boundaries.stderr
index 95aaa55ba7c67..b3bf34a2a9031 100644
--- a/src/test/ui/issues/issue-33140-hack-boundaries.stderr
+++ b/src/test/ui/issues/issue-33140-hack-boundaries.stderr
@@ -1,21 +1,21 @@
 error[E0119]: conflicting implementations of trait `Trait1` for type `(dyn std::marker::Send + 'static)`:
-  --> $DIR/issue-33140-hack-boundaries.rs:19:1
+  --> $DIR/issue-33140-hack-boundaries.rs:18:1
    |
 LL | impl Trait1 for dyn Send {}
    | ------------------------ first implementation here
 LL | impl Trait1 for dyn Send {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + 'static)`
 
-error[E0119]: conflicting implementations of trait `Trait2` for type `(dyn std::marker::Send + 'static)`:
-  --> $DIR/issue-33140-hack-boundaries.rs:27:1
+error[E0748]: found both positive and negative implementation of trait `Trait2` for type `(dyn std::marker::Send + 'static)`:
+  --> $DIR/issue-33140-hack-boundaries.rs:25:1
    |
 LL | impl Trait2 for dyn Send {}
-   | ------------------------ first implementation here
+   | ------------------------ positive implementation here
 LL | impl !Trait2 for dyn Send {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + 'static)`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ negative implementation here
 
 error[E0119]: conflicting implementations of trait `Trait3<(dyn std::marker::Sync + 'static)>` for type `(dyn std::marker::Send + 'static)`:
-  --> $DIR/issue-33140-hack-boundaries.rs:36:1
+  --> $DIR/issue-33140-hack-boundaries.rs:32:1
    |
 LL | impl Trait3<dyn Sync> for dyn Send {}
    | ---------------------------------- first implementation here
@@ -23,7 +23,7 @@ LL | impl Trait3<dyn Sync> for dyn Send {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + 'static)`
 
 error[E0119]: conflicting implementations of trait `Trait4a` for type `(dyn std::marker::Send + 'static)`:
-  --> $DIR/issue-33140-hack-boundaries.rs:44:1
+  --> $DIR/issue-33140-hack-boundaries.rs:39:1
    |
 LL | impl<T: ?Sized> Trait4a for T {}
    | ----------------------------- first implementation here
@@ -31,7 +31,7 @@ LL | impl Trait4a for dyn Send {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + 'static)`
 
 error[E0119]: conflicting implementations of trait `Trait4b` for type `()`:
-  --> $DIR/issue-33140-hack-boundaries.rs:52:1
+  --> $DIR/issue-33140-hack-boundaries.rs:46:1
    |
 LL | impl Trait4b for () {}
    | ------------------- first implementation here
@@ -39,7 +39,7 @@ LL | impl Trait4b for () {}
    | ^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
 
 error[E0119]: conflicting implementations of trait `Trait4c` for type `(dyn Trait1 + std::marker::Send + 'static)`:
-  --> $DIR/issue-33140-hack-boundaries.rs:60:1
+  --> $DIR/issue-33140-hack-boundaries.rs:53:1
    |
 LL | impl Trait4c for dyn Trait1 + Send {}
    | ---------------------------------- first implementation here
@@ -47,7 +47,7 @@ LL | impl Trait4c for dyn Trait1 + Send {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn Trait1 + std::marker::Send + 'static)`
 
 error[E0119]: conflicting implementations of trait `Trait4d` for type `dyn std::marker::Send`:
-  --> $DIR/issue-33140-hack-boundaries.rs:68:1
+  --> $DIR/issue-33140-hack-boundaries.rs:60:1
    |
 LL | impl<'a> Trait4d for dyn Send + 'a {}
    | ---------------------------------- first implementation here
@@ -55,7 +55,7 @@ LL | impl<'a> Trait4d for dyn Send + 'a {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `dyn std::marker::Send`
 
 error[E0119]: conflicting implementations of trait `Trait5` for type `(dyn std::marker::Send + 'static)`:
-  --> $DIR/issue-33140-hack-boundaries.rs:77:1
+  --> $DIR/issue-33140-hack-boundaries.rs:67:1
    |
 LL | impl Trait5 for dyn Send {}
    | ------------------------ first implementation here
@@ -64,4 +64,5 @@ LL | impl Trait5 for dyn Send where u32: Copy {}
 
 error: aborting due to 8 previous errors
 
-For more information about this error, try `rustc --explain E0119`.
+Some errors have detailed explanations: E0119, E0748.
+For more information about an error, try `rustc --explain E0119`.
diff --git a/src/test/ui/layout/debug.rs b/src/test/ui/layout/debug.rs
new file mode 100644
index 0000000000000..047002c9c99e2
--- /dev/null
+++ b/src/test/ui/layout/debug.rs
@@ -0,0 +1,14 @@
+#![feature(never_type, rustc_attrs)]
+#![crate_type = "lib"]
+
+#[rustc_layout(debug)]
+enum E { Foo, Bar(!, i32, i32) } //~ ERROR: layout debugging
+
+#[rustc_layout(debug)]
+struct S { f1: i32, f2: (), f3: i32 } //~ ERROR: layout debugging
+
+#[rustc_layout(debug)]
+union U { f1: (i32, i32), f3: i32 } //~ ERROR: layout debugging
+
+#[rustc_layout(debug)]
+type Test = Result<i32, i32>; //~ ERROR: layout debugging
diff --git a/src/test/ui/layout/debug.stderr b/src/test/ui/layout/debug.stderr
new file mode 100644
index 0000000000000..6e704528c4130
--- /dev/null
+++ b/src/test/ui/layout/debug.stderr
@@ -0,0 +1,335 @@
+error: layout debugging: LayoutDetails {
+    fields: Arbitrary {
+        offsets: [
+            Size {
+                raw: 0,
+            },
+        ],
+        memory_index: [
+            0,
+        ],
+    },
+    variants: Multiple {
+        discr: Scalar {
+            value: Int(
+                I32,
+                false,
+            ),
+            valid_range: 0..=0,
+        },
+        discr_kind: Tag,
+        discr_index: 0,
+        variants: [
+            LayoutDetails {
+                fields: Arbitrary {
+                    offsets: [],
+                    memory_index: [],
+                },
+                variants: Single {
+                    index: 0,
+                },
+                abi: Aggregate {
+                    sized: true,
+                },
+                largest_niche: None,
+                align: AbiAndPrefAlign {
+                    abi: Align {
+                        pow2: 0,
+                    },
+                    pref: Align {
+                        pow2: 3,
+                    },
+                },
+                size: Size {
+                    raw: 4,
+                },
+            },
+            LayoutDetails {
+                fields: Arbitrary {
+                    offsets: [
+                        Size {
+                            raw: 4,
+                        },
+                        Size {
+                            raw: 4,
+                        },
+                        Size {
+                            raw: 8,
+                        },
+                    ],
+                    memory_index: [
+                        0,
+                        1,
+                        2,
+                    ],
+                },
+                variants: Single {
+                    index: 1,
+                },
+                abi: Uninhabited,
+                largest_niche: None,
+                align: AbiAndPrefAlign {
+                    abi: Align {
+                        pow2: 2,
+                    },
+                    pref: Align {
+                        pow2: 3,
+                    },
+                },
+                size: Size {
+                    raw: 12,
+                },
+            },
+        ],
+    },
+    abi: Aggregate {
+        sized: true,
+    },
+    largest_niche: Some(
+        Niche {
+            offset: Size {
+                raw: 0,
+            },
+            scalar: Scalar {
+                value: Int(
+                    I32,
+                    false,
+                ),
+                valid_range: 0..=0,
+            },
+        },
+    ),
+    align: AbiAndPrefAlign {
+        abi: Align {
+            pow2: 2,
+        },
+        pref: Align {
+            pow2: 3,
+        },
+    },
+    size: Size {
+        raw: 12,
+    },
+}
+  --> $DIR/debug.rs:5:1
+   |
+LL | enum E { Foo, Bar(!, i32, i32) }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: layout debugging: LayoutDetails {
+    fields: Arbitrary {
+        offsets: [
+            Size {
+                raw: 0,
+            },
+            Size {
+                raw: 0,
+            },
+            Size {
+                raw: 4,
+            },
+        ],
+        memory_index: [
+            1,
+            0,
+            2,
+        ],
+    },
+    variants: Single {
+        index: 0,
+    },
+    abi: ScalarPair(
+        Scalar {
+            value: Int(
+                I32,
+                true,
+            ),
+            valid_range: 0..=4294967295,
+        },
+        Scalar {
+            value: Int(
+                I32,
+                true,
+            ),
+            valid_range: 0..=4294967295,
+        },
+    ),
+    largest_niche: None,
+    align: AbiAndPrefAlign {
+        abi: Align {
+            pow2: 2,
+        },
+        pref: Align {
+            pow2: 3,
+        },
+    },
+    size: Size {
+        raw: 8,
+    },
+}
+  --> $DIR/debug.rs:8:1
+   |
+LL | struct S { f1: i32, f2: (), f3: i32 }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: layout debugging: LayoutDetails {
+    fields: Union(
+        2,
+    ),
+    variants: Single {
+        index: 0,
+    },
+    abi: Aggregate {
+        sized: true,
+    },
+    largest_niche: None,
+    align: AbiAndPrefAlign {
+        abi: Align {
+            pow2: 2,
+        },
+        pref: Align {
+            pow2: 3,
+        },
+    },
+    size: Size {
+        raw: 8,
+    },
+}
+  --> $DIR/debug.rs:11:1
+   |
+LL | union U { f1: (i32, i32), f3: i32 }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: layout debugging: LayoutDetails {
+    fields: Arbitrary {
+        offsets: [
+            Size {
+                raw: 0,
+            },
+        ],
+        memory_index: [
+            0,
+        ],
+    },
+    variants: Multiple {
+        discr: Scalar {
+            value: Int(
+                I32,
+                false,
+            ),
+            valid_range: 0..=1,
+        },
+        discr_kind: Tag,
+        discr_index: 0,
+        variants: [
+            LayoutDetails {
+                fields: Arbitrary {
+                    offsets: [
+                        Size {
+                            raw: 4,
+                        },
+                    ],
+                    memory_index: [
+                        0,
+                    ],
+                },
+                variants: Single {
+                    index: 0,
+                },
+                abi: Aggregate {
+                    sized: true,
+                },
+                largest_niche: None,
+                align: AbiAndPrefAlign {
+                    abi: Align {
+                        pow2: 2,
+                    },
+                    pref: Align {
+                        pow2: 3,
+                    },
+                },
+                size: Size {
+                    raw: 8,
+                },
+            },
+            LayoutDetails {
+                fields: Arbitrary {
+                    offsets: [
+                        Size {
+                            raw: 4,
+                        },
+                    ],
+                    memory_index: [
+                        0,
+                    ],
+                },
+                variants: Single {
+                    index: 1,
+                },
+                abi: Aggregate {
+                    sized: true,
+                },
+                largest_niche: None,
+                align: AbiAndPrefAlign {
+                    abi: Align {
+                        pow2: 2,
+                    },
+                    pref: Align {
+                        pow2: 3,
+                    },
+                },
+                size: Size {
+                    raw: 8,
+                },
+            },
+        ],
+    },
+    abi: ScalarPair(
+        Scalar {
+            value: Int(
+                I32,
+                false,
+            ),
+            valid_range: 0..=1,
+        },
+        Scalar {
+            value: Int(
+                I32,
+                true,
+            ),
+            valid_range: 0..=4294967295,
+        },
+    ),
+    largest_niche: Some(
+        Niche {
+            offset: Size {
+                raw: 0,
+            },
+            scalar: Scalar {
+                value: Int(
+                    I32,
+                    false,
+                ),
+                valid_range: 0..=1,
+            },
+        },
+    ),
+    align: AbiAndPrefAlign {
+        abi: Align {
+            pow2: 2,
+        },
+        pref: Align {
+            pow2: 3,
+        },
+    },
+    size: Size {
+        raw: 8,
+    },
+}
+  --> $DIR/debug.rs:14:1
+   |
+LL | type Test = Result<i32, i32>;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+
diff --git a/src/test/ui/macros/issue-69838-dir/bar.rs b/src/test/ui/macros/issue-69838-dir/bar.rs
new file mode 100644
index 0000000000000..ec12f8c5cb442
--- /dev/null
+++ b/src/test/ui/macros/issue-69838-dir/bar.rs
@@ -0,0 +1,3 @@
+// ignore-test -- this is an auxiliary file as part of another test.
+
+pub fn i_am_in_bar() {}
diff --git a/src/test/ui/macros/issue-69838-dir/included.rs b/src/test/ui/macros/issue-69838-dir/included.rs
new file mode 100644
index 0000000000000..9900b8fd5092c
--- /dev/null
+++ b/src/test/ui/macros/issue-69838-dir/included.rs
@@ -0,0 +1,3 @@
+// ignore-test -- this is an auxiliary file as part of another test.
+
+pub mod bar;
diff --git a/src/test/ui/macros/issue-69838-mods-relative-to-included-path.rs b/src/test/ui/macros/issue-69838-mods-relative-to-included-path.rs
new file mode 100644
index 0000000000000..2a4e97f0ef5f1
--- /dev/null
+++ b/src/test/ui/macros/issue-69838-mods-relative-to-included-path.rs
@@ -0,0 +1,7 @@
+// check-pass
+
+include!("issue-69838-dir/included.rs");
+
+fn main() {
+    bar::i_am_in_bar();
+}
diff --git a/src/test/ui/mut/mutable-enum-indirect.rs b/src/test/ui/mut/mutable-enum-indirect.rs
index 611ff0d66a24e..502859c041353 100644
--- a/src/test/ui/mut/mutable-enum-indirect.rs
+++ b/src/test/ui/mut/mutable-enum-indirect.rs
@@ -1,7 +1,7 @@
 // Tests that an `&` pointer to something inherently mutable is itself
 // to be considered mutable.
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Sync;
 
diff --git a/src/test/ui/never_type/auto-traits.rs b/src/test/ui/never_type/auto-traits.rs
index 2d9689888cb30..84c8db4053e4f 100644
--- a/src/test/ui/never_type/auto-traits.rs
+++ b/src/test/ui/never_type/auto-traits.rs
@@ -1,6 +1,7 @@
 // check-pass
 
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![feature(never_type)]
 
 fn main() {
diff --git a/src/test/ui/no_send-enum.rs b/src/test/ui/no_send-enum.rs
index 4b4d06f1e32cd..bd560649b990e 100644
--- a/src/test/ui/no_send-enum.rs
+++ b/src/test/ui/no_send-enum.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Send;
 
diff --git a/src/test/ui/no_send-struct.rs b/src/test/ui/no_send-struct.rs
index 67816bfee5d02..75a363f9f7639 100644
--- a/src/test/ui/no_send-struct.rs
+++ b/src/test/ui/no_send-struct.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Send;
 
diff --git a/src/test/ui/no_share-enum.rs b/src/test/ui/no_share-enum.rs
index f5edb63cf86e9..44bf1913e7aac 100644
--- a/src/test/ui/no_share-enum.rs
+++ b/src/test/ui/no_share-enum.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Sync;
 
diff --git a/src/test/ui/no_share-struct.rs b/src/test/ui/no_share-struct.rs
index 35867d0f2166d..7d8a36a76f274 100644
--- a/src/test/ui/no_share-struct.rs
+++ b/src/test/ui/no_share-struct.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Sync;
 
diff --git a/src/test/ui/parser/trait-object-bad-parens.rs b/src/test/ui/parser/trait-object-bad-parens.rs
index 048e028be1ca1..0a2836d691f5f 100644
--- a/src/test/ui/parser/trait-object-bad-parens.rs
+++ b/src/test/ui/parser/trait-object-bad-parens.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![allow(bare_trait_objects)]
 
 auto trait Auto {}
diff --git a/src/test/ui/parser/trait-object-bad-parens.stderr b/src/test/ui/parser/trait-object-bad-parens.stderr
index f53afdff5e7c2..74e484eebee1f 100644
--- a/src/test/ui/parser/trait-object-bad-parens.stderr
+++ b/src/test/ui/parser/trait-object-bad-parens.stderr
@@ -1,23 +1,23 @@
 error[E0178]: expected a path on the left-hand side of `+`, not `((Auto))`
-  --> $DIR/trait-object-bad-parens.rs:7:16
+  --> $DIR/trait-object-bad-parens.rs:8:16
    |
 LL |     let _: Box<((Auto)) + Auto>;
    |                ^^^^^^^^^^^^^^^ expected a path
 
 error[E0178]: expected a path on the left-hand side of `+`, not `(Auto + Auto)`
-  --> $DIR/trait-object-bad-parens.rs:9:16
+  --> $DIR/trait-object-bad-parens.rs:10:16
    |
 LL |     let _: Box<(Auto + Auto) + Auto>;
    |                ^^^^^^^^^^^^^^^^^^^^ expected a path
 
 error[E0178]: expected a path on the left-hand side of `+`, not `(Auto)`
-  --> $DIR/trait-object-bad-parens.rs:11:16
+  --> $DIR/trait-object-bad-parens.rs:12:16
    |
 LL |     let _: Box<(Auto +) + Auto>;
    |                ^^^^^^^^^^^^^^^ expected a path
 
 error[E0178]: expected a path on the left-hand side of `+`, not `(dyn Auto)`
-  --> $DIR/trait-object-bad-parens.rs:13:16
+  --> $DIR/trait-object-bad-parens.rs:14:16
    |
 LL |     let _: Box<(dyn Auto) + Auto>;
    |                ^^^^^^^^^^^^^^^^^ expected a path
diff --git a/src/test/ui/privacy/privacy-sanity.rs b/src/test/ui/privacy/privacy-sanity.rs
index f83b24c49a10a..8bbf1ab5d1f30 100644
--- a/src/test/ui/privacy/privacy-sanity.rs
+++ b/src/test/ui/privacy/privacy-sanity.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 pub trait Tr {
     fn f();
diff --git a/src/test/ui/privacy/private-in-public-non-principal-2.rs b/src/test/ui/privacy/private-in-public-non-principal-2.rs
index 8a59073fa6c7e..cd3d609ca3ea6 100644
--- a/src/test/ui/privacy/private-in-public-non-principal-2.rs
+++ b/src/test/ui/privacy/private-in-public-non-principal-2.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 #[allow(private_in_public)]
 mod m {
diff --git a/src/test/ui/privacy/private-in-public-non-principal-2.stderr b/src/test/ui/privacy/private-in-public-non-principal-2.stderr
index 2db4925722642..b7513cc729418 100644
--- a/src/test/ui/privacy/private-in-public-non-principal-2.stderr
+++ b/src/test/ui/privacy/private-in-public-non-principal-2.stderr
@@ -1,5 +1,5 @@
 error: trait `m::PrivNonPrincipal` is private
-  --> $DIR/private-in-public-non-principal-2.rs:11:5
+  --> $DIR/private-in-public-non-principal-2.rs:12:5
    |
 LL |     m::leak_dyn_nonprincipal();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/src/test/ui/privacy/private-in-public-non-principal.rs b/src/test/ui/privacy/private-in-public-non-principal.rs
index 5d89d8105b119..f902552b7de25 100644
--- a/src/test/ui/privacy/private-in-public-non-principal.rs
+++ b/src/test/ui/privacy/private-in-public-non-principal.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 pub trait PubPrincipal {}
 auto trait PrivNonPrincipal {}
diff --git a/src/test/ui/privacy/private-in-public-non-principal.stderr b/src/test/ui/privacy/private-in-public-non-principal.stderr
index 2a41fae43c629..05233a0ad61f9 100644
--- a/src/test/ui/privacy/private-in-public-non-principal.stderr
+++ b/src/test/ui/privacy/private-in-public-non-principal.stderr
@@ -1,5 +1,5 @@
 warning: private trait `PrivNonPrincipal` in public interface (error E0445)
-  --> $DIR/private-in-public-non-principal.rs:6:1
+  --> $DIR/private-in-public-non-principal.rs:7:1
    |
 LL | pub fn leak_dyn_nonprincipal() -> Box<dyn PubPrincipal + PrivNonPrincipal> { loop {} }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -9,13 +9,13 @@ LL | pub fn leak_dyn_nonprincipal() -> Box<dyn PubPrincipal + PrivNonPrincipal>
    = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
 
 error: missing documentation for a method
-  --> $DIR/private-in-public-non-principal.rs:13:9
+  --> $DIR/private-in-public-non-principal.rs:14:9
    |
 LL |         pub fn check_doc_lint() {}
    |         ^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: the lint level is defined here
-  --> $DIR/private-in-public-non-principal.rs:10:8
+  --> $DIR/private-in-public-non-principal.rs:11:8
    |
 LL | #[deny(missing_docs)]
    |        ^^^^^^^^^^^^
diff --git a/src/test/ui/specialization/defaultimpl/validation.rs b/src/test/ui/specialization/defaultimpl/validation.rs
index 26b3f1ec41491..bcc464d9a5ddc 100644
--- a/src/test/ui/specialization/defaultimpl/validation.rs
+++ b/src/test/ui/specialization/defaultimpl/validation.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![feature(specialization)]
 
 struct S;
@@ -8,8 +8,9 @@ default impl S {} //~ ERROR inherent impls cannot be `default`
 
 default unsafe impl Send for S {} //~ ERROR impls of auto traits cannot be default
 default impl !Send for Z {} //~ ERROR impls of auto traits cannot be default
+//~^ ERROR negative impls cannot be default impls
 
 trait Tr {}
-default impl !Tr for S {} //~ ERROR negative impls are only allowed for auto traits
+default impl !Tr for S {} //~ ERROR negative impls cannot be default impls
 
 fn main() {}
diff --git a/src/test/ui/specialization/defaultimpl/validation.stderr b/src/test/ui/specialization/defaultimpl/validation.stderr
index 6e19d79e48f6b..448df5c3783be 100644
--- a/src/test/ui/specialization/defaultimpl/validation.stderr
+++ b/src/test/ui/specialization/defaultimpl/validation.stderr
@@ -20,12 +20,18 @@ error: impls of auto traits cannot be default
 LL | default impl !Send for Z {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0192]: negative impls are only allowed for auto traits (e.g., `Send` and `Sync`)
-  --> $DIR/validation.rs:13:1
+error[E0750]: negative impls cannot be default impls
+  --> $DIR/validation.rs:10:1
+   |
+LL | default impl !Send for Z {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0750]: negative impls cannot be default impls
+  --> $DIR/validation.rs:14:1
    |
 LL | default impl !Tr for S {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0192`.
+For more information about this error, try `rustc --explain E0750`.
diff --git a/src/test/ui/specialization/specialization-overlap-negative.rs b/src/test/ui/specialization/specialization-overlap-negative.rs
index af80d6686e3a1..dff2b28bf003c 100644
--- a/src/test/ui/specialization/specialization-overlap-negative.rs
+++ b/src/test/ui/specialization/specialization-overlap-negative.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![feature(specialization)]
 
 trait MyTrait {}
@@ -6,6 +6,6 @@ trait MyTrait {}
 struct TestType<T>(::std::marker::PhantomData<T>);
 
 unsafe impl<T: Clone> Send for TestType<T> {}
-impl<T: MyTrait> !Send for TestType<T> {} //~ ERROR E0119
+impl<T: MyTrait> !Send for TestType<T> {} //~ ERROR E0748
 
 fn main() {}
diff --git a/src/test/ui/specialization/specialization-overlap-negative.stderr b/src/test/ui/specialization/specialization-overlap-negative.stderr
index 947aad824ea88..cbc7271b199a6 100644
--- a/src/test/ui/specialization/specialization-overlap-negative.stderr
+++ b/src/test/ui/specialization/specialization-overlap-negative.stderr
@@ -1,11 +1,11 @@
-error[E0119]: conflicting implementations of trait `std::marker::Send` for type `TestType<_>`:
+error[E0748]: found both positive and negative implementation of trait `std::marker::Send` for type `TestType<_>`:
   --> $DIR/specialization-overlap-negative.rs:9:1
    |
 LL | unsafe impl<T: Clone> Send for TestType<T> {}
-   | ------------------------------------------ first implementation here
+   | ------------------------------------------ positive implementation here
 LL | impl<T: MyTrait> !Send for TestType<T> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<_>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ negative implementation here
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0119`.
+For more information about this error, try `rustc --explain E0748`.
diff --git a/src/test/ui/specialization/specialization-polarity.rs b/src/test/ui/specialization/specialization-polarity.rs
index d574480674571..4a733fb3c497e 100644
--- a/src/test/ui/specialization/specialization-polarity.rs
+++ b/src/test/ui/specialization/specialization-polarity.rs
@@ -1,16 +1,17 @@
 // Make sure specialization cannot change impl polarity
 
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![feature(specialization)]
 
 auto trait Foo {}
 
 impl<T> Foo for T {}
-impl !Foo for u8 {} //~ ERROR E0119
+impl !Foo for u8 {} //~ ERROR E0748
 
 auto trait Bar {}
 
 impl<T> !Bar for T {}
-impl Bar for u8 {} //~ ERROR E0119
+impl Bar for u8 {} //~ ERROR E0748
 
 fn main() {}
diff --git a/src/test/ui/specialization/specialization-polarity.stderr b/src/test/ui/specialization/specialization-polarity.stderr
index bc1b2aeb70fc7..30655d87d5500 100644
--- a/src/test/ui/specialization/specialization-polarity.stderr
+++ b/src/test/ui/specialization/specialization-polarity.stderr
@@ -1,19 +1,19 @@
-error[E0119]: conflicting implementations of trait `Foo` for type `u8`:
-  --> $DIR/specialization-polarity.rs:9:1
+error[E0748]: found both positive and negative implementation of trait `Foo` for type `u8`:
+  --> $DIR/specialization-polarity.rs:10:1
    |
 LL | impl<T> Foo for T {}
-   | ----------------- first implementation here
+   | ----------------- positive implementation here
 LL | impl !Foo for u8 {}
-   | ^^^^^^^^^^^^^^^^ conflicting implementation for `u8`
+   | ^^^^^^^^^^^^^^^^ negative implementation here
 
-error[E0119]: conflicting implementations of trait `Bar` for type `u8`:
-  --> $DIR/specialization-polarity.rs:14:1
+error[E0748]: found both positive and negative implementation of trait `Bar` for type `u8`:
+  --> $DIR/specialization-polarity.rs:15:1
    |
 LL | impl<T> !Bar for T {}
-   | ------------------ first implementation here
+   | ------------------ negative implementation here
 LL | impl Bar for u8 {}
-   | ^^^^^^^^^^^^^^^ conflicting implementation for `u8`
+   | ^^^^^^^^^^^^^^^ positive implementation here
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0119`.
+For more information about this error, try `rustc --explain E0748`.
diff --git a/src/test/ui/syntax-trait-polarity-feature-gate.stderr b/src/test/ui/syntax-trait-polarity-feature-gate.stderr
index 5d4c1b354f700..3562deecbd522 100644
--- a/src/test/ui/syntax-trait-polarity-feature-gate.stderr
+++ b/src/test/ui/syntax-trait-polarity-feature-gate.stderr
@@ -4,8 +4,8 @@ error[E0658]: negative trait bounds are not yet fully implemented; use marker ty
 LL | impl !Send for TestType {}
    |      ^^^^^
    |
-   = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
-   = help: add `#![feature(optin_builtin_traits)]` to the crate attributes to enable
+   = note: see issue #68318 <https://github.com/rust-lang/rust/issues/68318> for more information
+   = help: add `#![feature(negative_impls)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/syntax-trait-polarity.rs b/src/test/ui/syntax-trait-polarity.rs
index 1b7fc1587e6d2..ed2947493b02c 100644
--- a/src/test/ui/syntax-trait-polarity.rs
+++ b/src/test/ui/syntax-trait-polarity.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Send;
 
@@ -12,7 +12,6 @@ trait TestTrait {}
 unsafe impl !Send for TestType {}
 //~^ ERROR negative impls cannot be unsafe
 impl !TestTrait for TestType {}
-//~^ ERROR negative impls are only allowed for auto traits
 
 struct TestType2<T>(T);
 
@@ -22,6 +21,5 @@ impl<T> !TestType2<T> {}
 unsafe impl<T> !Send for TestType2<T> {}
 //~^ ERROR negative impls cannot be unsafe
 impl<T> !TestTrait for TestType2<T> {}
-//~^ ERROR negative impls are only allowed for auto traits
 
 fn main() {}
diff --git a/src/test/ui/syntax-trait-polarity.stderr b/src/test/ui/syntax-trait-polarity.stderr
index 5777e0ade908e..1fd40fb66570d 100644
--- a/src/test/ui/syntax-trait-polarity.stderr
+++ b/src/test/ui/syntax-trait-polarity.stderr
@@ -16,7 +16,7 @@ LL | unsafe impl !Send for TestType {}
    | unsafe because of this
 
 error: inherent impls cannot be negative
-  --> $DIR/syntax-trait-polarity.rs:19:10
+  --> $DIR/syntax-trait-polarity.rs:18:10
    |
 LL | impl<T> !TestType2<T> {}
    |         -^^^^^^^^^^^^ inherent impl for this type
@@ -24,7 +24,7 @@ LL | impl<T> !TestType2<T> {}
    |         negative because of this
 
 error[E0198]: negative impls cannot be unsafe
-  --> $DIR/syntax-trait-polarity.rs:22:16
+  --> $DIR/syntax-trait-polarity.rs:21:16
    |
 LL | unsafe impl<T> !Send for TestType2<T> {}
    | ------         -^^^^
@@ -32,19 +32,6 @@ LL | unsafe impl<T> !Send for TestType2<T> {}
    | |              negative because of this
    | unsafe because of this
 
-error[E0192]: negative impls are only allowed for auto traits (e.g., `Send` and `Sync`)
-  --> $DIR/syntax-trait-polarity.rs:14:1
-   |
-LL | impl !TestTrait for TestType {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error[E0192]: negative impls are only allowed for auto traits (e.g., `Send` and `Sync`)
-  --> $DIR/syntax-trait-polarity.rs:24:1
-   |
-LL | impl<T> !TestTrait for TestType2<T> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to 6 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0192, E0198.
-For more information about an error, try `rustc --explain E0192`.
+For more information about this error, try `rustc --explain E0198`.
diff --git a/src/test/ui/traits/auto-traits.rs b/src/test/ui/traits/auto-traits.rs
index c495b97b25bfe..15fdddc5f3f5c 100644
--- a/src/test/ui/traits/auto-traits.rs
+++ b/src/test/ui/traits/auto-traits.rs
@@ -1,6 +1,7 @@
 // run-pass
 #![allow(unused_doc_comments)]
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait Auto {}
 unsafe auto trait AutoUnsafe {}
diff --git a/src/test/ui/traits/negative-impls/auxiliary/foreign_trait.rs b/src/test/ui/traits/negative-impls/auxiliary/foreign_trait.rs
new file mode 100644
index 0000000000000..681f26438e695
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/auxiliary/foreign_trait.rs
@@ -0,0 +1,6 @@
+#![feature(negative_impls)]
+
+pub trait ForeignTrait {}
+
+impl ForeignTrait for u32 {}
+impl !ForeignTrait for String {}
diff --git a/src/test/ui/traits/negative-impls/feature-gate-negative_impls.rs b/src/test/ui/traits/negative-impls/feature-gate-negative_impls.rs
new file mode 100644
index 0000000000000..683fd6db6f29d
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/feature-gate-negative_impls.rs
@@ -0,0 +1,3 @@
+trait MyTrait {}
+impl !MyTrait for u32 {} //~ ERROR negative trait bounds are not yet fully implemented
+fn main() {}
diff --git a/src/test/ui/traits/negative-impls/feature-gate-negative_impls.stderr b/src/test/ui/traits/negative-impls/feature-gate-negative_impls.stderr
new file mode 100644
index 0000000000000..b253fbd0da7f9
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/feature-gate-negative_impls.stderr
@@ -0,0 +1,12 @@
+error[E0658]: negative trait bounds are not yet fully implemented; use marker types for now
+  --> $DIR/feature-gate-negative_impls.rs:2:6
+   |
+LL | impl !MyTrait for u32 {}
+   |      ^^^^^^^^
+   |
+   = note: see issue #68318 <https://github.com/rust-lang/rust/issues/68318> for more information
+   = help: add `#![feature(negative_impls)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/traits/traits-negative-impls.rs b/src/test/ui/traits/negative-impls/negated-auto-traits-error.rs
similarity index 98%
rename from src/test/ui/traits/traits-negative-impls.rs
rename to src/test/ui/traits/negative-impls/negated-auto-traits-error.rs
index fb9a3a99748d0..4bdad5dc591c5 100644
--- a/src/test/ui/traits/traits-negative-impls.rs
+++ b/src/test/ui/traits/negative-impls/negated-auto-traits-error.rs
@@ -3,7 +3,7 @@
 // errors are not reported. This way, we make sure that, for each function, different
 // typeck phases are involved and all errors are reported.
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Send;
 
diff --git a/src/test/ui/traits/traits-negative-impls.stderr b/src/test/ui/traits/negative-impls/negated-auto-traits-error.stderr
similarity index 92%
rename from src/test/ui/traits/traits-negative-impls.stderr
rename to src/test/ui/traits/negative-impls/negated-auto-traits-error.stderr
index 599bbfe222546..69a91b09e3ee0 100644
--- a/src/test/ui/traits/traits-negative-impls.stderr
+++ b/src/test/ui/traits/negative-impls/negated-auto-traits-error.stderr
@@ -1,5 +1,5 @@
 error[E0277]: `dummy::TestType` cannot be sent between threads safely
-  --> $DIR/traits-negative-impls.rs:23:11
+  --> $DIR/negated-auto-traits-error.rs:23:11
    |
 LL | struct Outer<T: Send>(T);
    | ------------------------- required by `Outer`
@@ -10,7 +10,7 @@ LL |     Outer(TestType);
    = help: the trait `std::marker::Send` is not implemented for `dummy::TestType`
 
 error[E0277]: `dummy::TestType` cannot be sent between threads safely
-  --> $DIR/traits-negative-impls.rs:23:5
+  --> $DIR/negated-auto-traits-error.rs:23:5
    |
 LL | struct Outer<T: Send>(T);
    | ------------------------- required by `Outer`
@@ -21,7 +21,7 @@ LL |     Outer(TestType);
    = help: the trait `std::marker::Send` is not implemented for `dummy::TestType`
 
 error[E0277]: `dummy1b::TestType` cannot be sent between threads safely
-  --> $DIR/traits-negative-impls.rs:32:13
+  --> $DIR/negated-auto-traits-error.rs:32:13
    |
 LL | fn is_send<T: Send>(_: T) {}
    |    -------    ---- required by this bound in `is_send`
@@ -32,7 +32,7 @@ LL |     is_send(TestType);
    = help: the trait `std::marker::Send` is not implemented for `dummy1b::TestType`
 
 error[E0277]: `dummy1c::TestType` cannot be sent between threads safely
-  --> $DIR/traits-negative-impls.rs:40:13
+  --> $DIR/negated-auto-traits-error.rs:40:13
    |
 LL | fn is_send<T: Send>(_: T) {}
    |    -------    ---- required by this bound in `is_send`
@@ -44,7 +44,7 @@ LL |     is_send((8, TestType));
    = note: required because it appears within the type `({integer}, dummy1c::TestType)`
 
 error[E0277]: `dummy2::TestType` cannot be sent between threads safely
-  --> $DIR/traits-negative-impls.rs:48:13
+  --> $DIR/negated-auto-traits-error.rs:48:13
    |
 LL | fn is_send<T: Send>(_: T) {}
    |    -------    ---- required by this bound in `is_send`
@@ -60,7 +60,7 @@ LL |     is_send(Box::new(TestType));
    = note: required because it appears within the type `std::boxed::Box<dummy2::TestType>`
 
 error[E0277]: `dummy3::TestType` cannot be sent between threads safely
-  --> $DIR/traits-negative-impls.rs:56:13
+  --> $DIR/negated-auto-traits-error.rs:56:13
    |
 LL | fn is_send<T: Send>(_: T) {}
    |    -------    ---- required by this bound in `is_send`
@@ -74,7 +74,7 @@ LL |     is_send(Box::new(Outer2(TestType)));
    = note: required because it appears within the type `std::boxed::Box<Outer2<dummy3::TestType>>`
 
 error[E0277]: `main::TestType` cannot be sent between threads safely
-  --> $DIR/traits-negative-impls.rs:66:13
+  --> $DIR/negated-auto-traits-error.rs:66:13
    |
 LL | fn is_sync<T: Sync>(_: T) {}
    |    -------    ---- required by this bound in `is_sync`
diff --git a/src/test/ui/traits/traits-negative-impls-rpass.rs b/src/test/ui/traits/negative-impls/negated-auto-traits-rpass.rs
similarity index 91%
rename from src/test/ui/traits/traits-negative-impls-rpass.rs
rename to src/test/ui/traits/negative-impls/negated-auto-traits-rpass.rs
index 8664b6a6a6e36..010dbf2466403 100644
--- a/src/test/ui/traits/traits-negative-impls-rpass.rs
+++ b/src/test/ui/traits/negative-impls/negated-auto-traits-rpass.rs
@@ -1,6 +1,6 @@
 // run-pass
 #![allow(unused_variables)]
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Send;
 
diff --git a/src/test/ui/traits/negative-impls/negative-impls-basic.rs b/src/test/ui/traits/negative-impls/negative-impls-basic.rs
new file mode 100644
index 0000000000000..474e0381799bd
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/negative-impls-basic.rs
@@ -0,0 +1,17 @@
+// A simple test that we are able to create negative impls, when the
+// feature gate is given.
+//
+// run-pass
+
+#![feature(negative_impls)]
+#![allow(dead_code)]
+
+struct TestType;
+
+trait TestTrait {
+    fn dummy(&self) {}
+}
+
+impl !TestTrait for TestType {}
+
+fn main() {}
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-negative.rs b/src/test/ui/traits/negative-impls/negative-specializes-negative.rs
new file mode 100644
index 0000000000000..877c3e8af4f1a
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/negative-specializes-negative.rs
@@ -0,0 +1,13 @@
+#![feature(specialization)]
+#![feature(negative_impls)]
+
+// Test a negative impl that "specializes" another negative impl.
+//
+// run-pass
+
+trait MyTrait {}
+
+impl<T> !MyTrait for T {}
+impl !MyTrait for u32 {}
+
+fn main() {}
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive-item.rs b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.rs
new file mode 100644
index 0000000000000..043273e624189
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.rs
@@ -0,0 +1,13 @@
+#![feature(specialization)]
+#![feature(negative_impls)]
+
+// Negative impl for u32 cannot "specialize" the base impl.
+trait MyTrait {
+    fn foo();
+}
+impl<T> MyTrait for T {
+    default fn foo() {}
+}
+impl !MyTrait for u32 {} //~ ERROR E0748
+
+fn main() {}
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive-item.stderr b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.stderr
new file mode 100644
index 0000000000000..21fd08251d474
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive-item.stderr
@@ -0,0 +1,12 @@
+error[E0748]: found both positive and negative implementation of trait `MyTrait` for type `u32`:
+  --> $DIR/negative-specializes-positive-item.rs:11:1
+   |
+LL | impl<T> MyTrait for T {
+   | --------------------- positive implementation here
+...
+LL | impl !MyTrait for u32 {}
+   | ^^^^^^^^^^^^^^^^^^^^^ negative implementation here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0748`.
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive.rs b/src/test/ui/traits/negative-impls/negative-specializes-positive.rs
new file mode 100644
index 0000000000000..ac0fac10eef50
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive.rs
@@ -0,0 +1,14 @@
+#![feature(specialization)]
+#![feature(negative_impls)]
+
+// Negative impl for u32 cannot "specialize" the base impl.
+trait MyTrait {}
+impl<T> MyTrait for T {}
+impl !MyTrait for u32 {} //~ ERROR E0748
+
+// The second impl specializes the first, no error.
+trait MyTrait2 {}
+impl<T> MyTrait2 for T {}
+impl MyTrait2 for u32 {}
+
+fn main() {}
diff --git a/src/test/ui/traits/negative-impls/negative-specializes-positive.stderr b/src/test/ui/traits/negative-impls/negative-specializes-positive.stderr
new file mode 100644
index 0000000000000..0e7426c826028
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/negative-specializes-positive.stderr
@@ -0,0 +1,11 @@
+error[E0748]: found both positive and negative implementation of trait `MyTrait` for type `u32`:
+  --> $DIR/negative-specializes-positive.rs:7:1
+   |
+LL | impl<T> MyTrait for T {}
+   | --------------------- positive implementation here
+LL | impl !MyTrait for u32 {}
+   | ^^^^^^^^^^^^^^^^^^^^^ negative implementation here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0748`.
diff --git a/src/test/ui/traits/negative-impls/no-items.rs b/src/test/ui/traits/negative-impls/no-items.rs
new file mode 100644
index 0000000000000..5fc6be9b30007
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/no-items.rs
@@ -0,0 +1,11 @@
+#![feature(negative_impls)]
+
+trait MyTrait {
+    type Foo;
+}
+
+impl !MyTrait for u32 {
+    type Foo = i32; //~ ERROR negative impls cannot have any items
+}
+
+fn main() {}
diff --git a/src/test/ui/traits/negative-impls/no-items.stderr b/src/test/ui/traits/negative-impls/no-items.stderr
new file mode 100644
index 0000000000000..67b94bba12143
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/no-items.stderr
@@ -0,0 +1,9 @@
+error[E0749]: negative impls cannot have any items
+  --> $DIR/no-items.rs:8:5
+   |
+LL |     type Foo = i32;
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0749`.
diff --git a/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.rs b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.rs
new file mode 100644
index 0000000000000..d9c498fca6645
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.rs
@@ -0,0 +1,26 @@
+use std::cell::Cell;
+use std::marker::PhantomPinned;
+use std::pin::Pin;
+
+struct MyType<'a>(Cell<Option<&'a mut MyType<'a>>>, PhantomPinned);
+
+impl<'a> Clone for &'a mut MyType<'a> {
+    //~^ ERROR E0748
+    fn clone(&self) -> &'a mut MyType<'a> {
+        self.0.replace(None).unwrap()
+    }
+}
+
+fn main() {
+    let mut unpinned = MyType(Cell::new(None), PhantomPinned);
+    let bad_addr = &unpinned as *const MyType<'_> as usize;
+    let mut p = Box::pin(MyType(Cell::new(Some(&mut unpinned)), PhantomPinned));
+
+    // p_mut1 is okay: it does not point to the bad_addr
+    let p_mut1: Pin<&mut MyType<'_>> = p.as_mut();
+    assert_ne!(bad_addr, &*p_mut1 as *const _ as usize);
+
+    // but p_mut2 does point to bad_addr! this is unsound
+    let p_mut2: Pin<&mut MyType<'_>> = p_mut1.clone();
+    assert_eq!(bad_addr, &*p_mut2 as *const _ as usize);
+}
diff --git a/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.stderr b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.stderr
new file mode 100644
index 0000000000000..f3305722cf2d3
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.stderr
@@ -0,0 +1,11 @@
+error[E0748]: found both positive and negative implementation of trait `std::clone::Clone` for type `&mut MyType<'_>`:
+  --> $DIR/pin-unsound-issue-66544-clone.rs:7:1
+   |
+LL | impl<'a> Clone for &'a mut MyType<'a> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ positive implementation here
+   |
+   = note: negative implementation in crate `core`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0748`.
diff --git a/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-derefmut.rs b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-derefmut.rs
new file mode 100644
index 0000000000000..cc27c301ffa9d
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-derefmut.rs
@@ -0,0 +1,33 @@
+// Demonstrate that "rogue" `DerefMut` impls for `&T` are not allowed.
+//
+// https://github.com/rust-lang/rust/issues/66544
+
+use std::cell::Cell;
+use std::marker::PhantomPinned;
+use std::ops::DerefMut;
+use std::pin::Pin;
+
+struct MyType<'a>(Cell<Option<&'a mut MyType<'a>>>, PhantomPinned);
+
+impl<'a> DerefMut for &'a MyType<'a> {
+    //~^ ERROR E0748
+    fn deref_mut(&mut self) -> &mut MyType<'a> {
+        self.0.replace(None).unwrap()
+    }
+}
+
+fn main() {
+    let mut unpinned = MyType(Cell::new(None), PhantomPinned);
+    let bad_addr = &unpinned as *const MyType<'_> as usize;
+    let p = Box::pin(MyType(Cell::new(Some(&mut unpinned)), PhantomPinned));
+
+    // p_ref is okay: it does not point to the bad_addr
+    let mut p_ref: Pin<&MyType<'_>> = p.as_ref();
+    assert_ne!(bad_addr, &*p_ref as *const _ as usize);
+
+    // but p_mut does point to bad_addr! this is unsound
+    let p_mut: Pin<&mut MyType<'_>> = p_ref.as_mut();
+    assert_eq!(bad_addr, &*p_mut as *const _ as usize);
+
+    println!("oh no!");
+}
diff --git a/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-derefmut.stderr b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-derefmut.stderr
new file mode 100644
index 0000000000000..4e71c2468246b
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/pin-unsound-issue-66544-derefmut.stderr
@@ -0,0 +1,11 @@
+error[E0748]: found both positive and negative implementation of trait `std::ops::DerefMut` for type `&MyType<'_>`:
+  --> $DIR/pin-unsound-issue-66544-derefmut.rs:12:1
+   |
+LL | impl<'a> DerefMut for &'a MyType<'a> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ positive implementation here
+   |
+   = note: negative implementation in crate `core`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0748`.
diff --git a/src/test/ui/traits/negative-impls/positive-specializes-negative.rs b/src/test/ui/traits/negative-impls/positive-specializes-negative.rs
new file mode 100644
index 0000000000000..eac4c24d5d3d0
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/positive-specializes-negative.rs
@@ -0,0 +1,9 @@
+#![feature(specialization)]
+#![feature(negative_impls)]
+
+trait MyTrait {}
+
+impl<T> !MyTrait for T {}
+impl MyTrait for u32 {} //~ ERROR E0748
+
+fn main() {}
diff --git a/src/test/ui/traits/negative-impls/positive-specializes-negative.stderr b/src/test/ui/traits/negative-impls/positive-specializes-negative.stderr
new file mode 100644
index 0000000000000..ab371945a7eab
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/positive-specializes-negative.stderr
@@ -0,0 +1,11 @@
+error[E0748]: found both positive and negative implementation of trait `MyTrait` for type `u32`:
+  --> $DIR/positive-specializes-negative.rs:7:1
+   |
+LL | impl<T> !MyTrait for T {}
+   | ---------------------- negative implementation here
+LL | impl MyTrait for u32 {}
+   | ^^^^^^^^^^^^^^^^^^^^ positive implementation here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0748`.
diff --git a/src/test/ui/traits/negative-impls/rely-on-negative-impl-in-coherence.rs b/src/test/ui/traits/negative-impls/rely-on-negative-impl-in-coherence.rs
new file mode 100644
index 0000000000000..db72aaf18034f
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/rely-on-negative-impl-in-coherence.rs
@@ -0,0 +1,21 @@
+#![feature(negative_impls)]
+
+// aux-build: foreign_trait.rs
+
+// Test that we cannot implement `LocalTrait` for `String`,
+// even though there is a `String: !ForeignTrait` impl.
+//
+// This may not be the behavior we want long term, but it's the
+// current semantics that we implemented so as to land `!Foo` impls
+// quickly. See internals thread:
+//
+// https://internals.rust-lang.org/t/foo/11587/
+
+extern crate foreign_trait;
+use foreign_trait::ForeignTrait;
+
+trait LocalTrait { }
+impl<T: ForeignTrait> LocalTrait for T { }
+impl LocalTrait for String { } //~ ERROR conflicting implementations
+
+fn main() { }
diff --git a/src/test/ui/traits/negative-impls/rely-on-negative-impl-in-coherence.stderr b/src/test/ui/traits/negative-impls/rely-on-negative-impl-in-coherence.stderr
new file mode 100644
index 0000000000000..7cce45d2c8f8f
--- /dev/null
+++ b/src/test/ui/traits/negative-impls/rely-on-negative-impl-in-coherence.stderr
@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `LocalTrait` for type `std::string::String`:
+  --> $DIR/rely-on-negative-impl-in-coherence.rs:19:1
+   |
+LL | impl<T: ForeignTrait> LocalTrait for T { }
+   | -------------------------------------- first implementation here
+LL | impl LocalTrait for String { }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `std::string::String`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/traits/overlap-not-permitted-for-builtin-trait.rs b/src/test/ui/traits/overlap-not-permitted-for-builtin-trait.rs
index 86029473b513b..4106f56d64ac6 100644
--- a/src/test/ui/traits/overlap-not-permitted-for-builtin-trait.rs
+++ b/src/test/ui/traits/overlap-not-permitted-for-builtin-trait.rs
@@ -1,5 +1,5 @@
 #![allow(dead_code)]
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 // Overlapping negative impls for `MyStruct` are not permitted:
 struct MyStruct;
@@ -7,5 +7,4 @@ impl !Send for MyStruct {}
 impl !Send for MyStruct {}
 //~^ ERROR conflicting implementations of trait
 
-fn main() {
-}
+fn main() {}
diff --git a/src/test/ui/traits/overlap-permitted-for-marker-traits.rs b/src/test/ui/traits/overlap-permitted-for-marker-traits.rs
new file mode 100644
index 0000000000000..00823d13b3696
--- /dev/null
+++ b/src/test/ui/traits/overlap-permitted-for-marker-traits.rs
@@ -0,0 +1,28 @@
+// run-pass
+// Tests for RFC 1268: we allow overlapping impls of marker traits,
+// that is, traits without items. In this case, a type `T` is
+// `MyMarker` if it is either `Debug` or `Display`.
+
+#![feature(marker_trait_attr)]
+#![feature(negative_impls)]
+
+use std::fmt::{Debug, Display};
+
+#[marker]
+trait MyMarker {}
+
+impl<T: Debug> MyMarker for T {}
+impl<T: Display> MyMarker for T {}
+
+fn foo<T: MyMarker>(t: T) -> T {
+    t
+}
+
+fn main() {
+    // Debug && Display:
+    assert_eq!(1, foo(1));
+    assert_eq!(2.0, foo(2.0));
+
+    // Debug && !Display:
+    assert_eq!(vec![1], foo(vec![1]));
+}
diff --git a/src/test/ui/traits/syntax-trait-polarity.rs b/src/test/ui/traits/syntax-trait-polarity.rs
index c6524f5c8e782..c809f9e89f934 100644
--- a/src/test/ui/traits/syntax-trait-polarity.rs
+++ b/src/test/ui/traits/syntax-trait-polarity.rs
@@ -2,7 +2,7 @@
 #![allow(dead_code)]
 // pretty-expanded FIXME #23616
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 struct TestType;
 
diff --git a/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.rs b/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.rs
index c65242b1bb1c3..571f934fc5bc7 100644
--- a/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.rs
+++ b/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.rs
@@ -3,6 +3,7 @@
 // to be synthesized.
 
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait Magic: Copy {} //~ ERROR E0568
 
diff --git a/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.stderr b/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.stderr
index a83ff3701511d..f44986da0e230 100644
--- a/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.stderr
+++ b/src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.stderr
@@ -1,5 +1,5 @@
 error[E0568]: auto traits cannot have super traits
-  --> $DIR/traits-inductive-overflow-supertrait-oibit.rs:7:19
+  --> $DIR/traits-inductive-overflow-supertrait-oibit.rs:8:19
    |
 LL | auto trait Magic: Copy {}
    |            -----  ^^^^ help: remove the super traits
@@ -7,7 +7,7 @@ LL | auto trait Magic: Copy {}
    |            auto trait cannot have super traits
 
 error[E0277]: the trait bound `NoClone: std::marker::Copy` is not satisfied
-  --> $DIR/traits-inductive-overflow-supertrait-oibit.rs:15:23
+  --> $DIR/traits-inductive-overflow-supertrait-oibit.rs:16:23
    |
 LL | fn copy<T: Magic>(x: T) -> (T, T) { (x, x) }
    |    ----    ----- required by this bound in `copy`
diff --git a/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.rs b/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.rs
index 92d8ba887270e..8824a6d2767f7 100644
--- a/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.rs
+++ b/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait Magic : Sized where Option<Self> : Magic {} //~ ERROR E0568
 impl<T:Magic> Magic for T {}
diff --git a/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.stderr b/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.stderr
index e397629327754..63b3300f6dbf4 100644
--- a/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.stderr
+++ b/src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.stderr
@@ -1,5 +1,5 @@
 error[E0568]: auto traits cannot have super traits
-  --> $DIR/typeck-auto-trait-no-supertraits-2.rs:3:20
+  --> $DIR/typeck-auto-trait-no-supertraits-2.rs:4:20
    |
 LL | auto trait Magic : Sized where Option<Self> : Magic {}
    |            -----   ^^^^^ help: remove the super traits
diff --git a/src/test/ui/typeck/typeck-auto-trait-no-supertraits.rs b/src/test/ui/typeck/typeck-auto-trait-no-supertraits.rs
index e48017acfc718..edbca91512486 100644
--- a/src/test/ui/typeck/typeck-auto-trait-no-supertraits.rs
+++ b/src/test/ui/typeck/typeck-auto-trait-no-supertraits.rs
@@ -23,6 +23,7 @@
 // }
 
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait Magic: Copy {} //~ ERROR E0568
 impl<T:Magic> Magic for T {}
diff --git a/src/test/ui/typeck/typeck-auto-trait-no-supertraits.stderr b/src/test/ui/typeck/typeck-auto-trait-no-supertraits.stderr
index b1602e3642ecb..796638fc54dce 100644
--- a/src/test/ui/typeck/typeck-auto-trait-no-supertraits.stderr
+++ b/src/test/ui/typeck/typeck-auto-trait-no-supertraits.stderr
@@ -1,5 +1,5 @@
 error[E0568]: auto traits cannot have super traits
-  --> $DIR/typeck-auto-trait-no-supertraits.rs:27:19
+  --> $DIR/typeck-auto-trait-no-supertraits.rs:28:19
    |
 LL | auto trait Magic: Copy {}
    |            -----  ^^^^ help: remove the super traits
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.rs b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.rs
index 6c170fb5bae7d..71ac2b466c10a 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.rs
+++ b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait MyTrait {}
 
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.stderr b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.stderr
index f060afea24e7f..23401ca308676 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.stderr
+++ b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types-2.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `MyS2: MyTrait` is not satisfied in `(MyS2, MyS)`
-  --> $DIR/typeck-default-trait-impl-constituent-types-2.rs:16:5
+  --> $DIR/typeck-default-trait-impl-constituent-types-2.rs:17:5
    |
 LL | fn is_mytrait<T: MyTrait>() {}
    |    ----------    ------- required by this bound in `is_mytrait`
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.rs b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.rs
index d72f676121875..6483b9213dc53 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.rs
+++ b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait MyTrait {}
 
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.stderr b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.stderr
index 22a2cb3e0ecb8..a30b29a3893cc 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.stderr
+++ b/src/test/ui/typeck/typeck-default-trait-impl-constituent-types.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `MyS2: MyTrait` is not satisfied
-  --> $DIR/typeck-default-trait-impl-constituent-types.rs:20:18
+  --> $DIR/typeck-default-trait-impl-constituent-types.rs:21:18
    |
 LL | fn is_mytrait<T: MyTrait>() {}
    |    ----------    ------- required by this bound in `is_mytrait`
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-cross-crate-coherence.rs b/src/test/ui/typeck/typeck-default-trait-impl-cross-crate-coherence.rs
index 212e165151d81..772ac322032ec 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-cross-crate-coherence.rs
+++ b/src/test/ui/typeck/typeck-default-trait-impl-cross-crate-coherence.rs
@@ -3,7 +3,7 @@
 // Test that we do not consider associated types to be sendable without
 // some applicable trait bound (and we don't ICE).
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 extern crate tdticc_coherence_lib as lib;
 
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-negation-send.rs b/src/test/ui/typeck/typeck-default-trait-impl-negation-send.rs
index 617d0f3b565a8..3a2fc39d409d9 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-negation-send.rs
+++ b/src/test/ui/typeck/typeck-default-trait-impl-negation-send.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 struct MySendable {
    t: *mut u8
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-negation-sync.rs b/src/test/ui/typeck/typeck-default-trait-impl-negation-sync.rs
index e4487fb110cf1..2734b761e61b7 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-negation-sync.rs
+++ b/src/test/ui/typeck/typeck-default-trait-impl-negation-sync.rs
@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 struct Managed;
 impl !Send for Managed {}
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-negation.rs b/src/test/ui/typeck/typeck-default-trait-impl-negation.rs
index 0b6e13be6e108..47cab60625dce 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-negation.rs
+++ b/src/test/ui/typeck/typeck-default-trait-impl-negation.rs
@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait MyTrait {}
 
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-negation.stderr b/src/test/ui/typeck/typeck-default-trait-impl-negation.stderr
index 4b13fcc885a0d..e31bb81074469 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-negation.stderr
+++ b/src/test/ui/typeck/typeck-default-trait-impl-negation.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `ThisImplsUnsafeTrait: MyTrait` is not satisfied
-  --> $DIR/typeck-default-trait-impl-negation.rs:21:19
+  --> $DIR/typeck-default-trait-impl-negation.rs:22:19
    |
 LL | fn is_my_trait<T: MyTrait>() {}
    |    -----------    ------- required by this bound in `is_my_trait`
@@ -11,7 +11,7 @@ LL |     is_my_trait::<ThisImplsUnsafeTrait>();
              <ThisImplsUnsafeTrait as MyTrait>
 
 error[E0277]: the trait bound `ThisImplsTrait: MyUnsafeTrait` is not satisfied
-  --> $DIR/typeck-default-trait-impl-negation.rs:24:26
+  --> $DIR/typeck-default-trait-impl-negation.rs:25:26
    |
 LL | fn is_my_unsafe_trait<T: MyUnsafeTrait>() {}
    |    ------------------    ------------- required by this bound in `is_my_unsafe_trait`
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-precedence.rs b/src/test/ui/typeck/typeck-default-trait-impl-precedence.rs
index 9b228f706465b..614a5ff55b1ed 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-precedence.rs
+++ b/src/test/ui/typeck/typeck-default-trait-impl-precedence.rs
@@ -4,6 +4,7 @@
 // impls whose types unify.
 
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait Defaulted { }
 impl<'a,T:Signed> Defaulted for &'a T { }
diff --git a/src/test/ui/typeck/typeck-default-trait-impl-precedence.stderr b/src/test/ui/typeck/typeck-default-trait-impl-precedence.stderr
index 1587730441820..0d256094c60e0 100644
--- a/src/test/ui/typeck/typeck-default-trait-impl-precedence.stderr
+++ b/src/test/ui/typeck/typeck-default-trait-impl-precedence.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `u32: Signed` is not satisfied
-  --> $DIR/typeck-default-trait-impl-precedence.rs:18:5
+  --> $DIR/typeck-default-trait-impl-precedence.rs:19:5
    |
 LL | fn is_defaulted<T:Defaulted>() { }
    |    ------------   --------- required by this bound in `is_defaulted`
diff --git a/src/test/ui/typeck/typeck-negative-impls-builtin.rs b/src/test/ui/typeck/typeck-negative-impls-builtin.rs
deleted file mode 100644
index 7bdd1035a1bf2..0000000000000
--- a/src/test/ui/typeck/typeck-negative-impls-builtin.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-#![feature(optin_builtin_traits)]
-
-struct TestType;
-
-trait TestTrait {
-    fn dummy(&self) { }
-}
-
-impl !TestTrait for TestType {}
-//~^ ERROR negative impls are only allowed for auto traits (e.g., `Send` and `Sync`)
-
-fn main() {}
diff --git a/src/test/ui/typeck/typeck-negative-impls-builtin.stderr b/src/test/ui/typeck/typeck-negative-impls-builtin.stderr
deleted file mode 100644
index 4e3d054ff6fad..0000000000000
--- a/src/test/ui/typeck/typeck-negative-impls-builtin.stderr
+++ /dev/null
@@ -1,9 +0,0 @@
-error[E0192]: negative impls are only allowed for auto traits (e.g., `Send` and `Sync`)
-  --> $DIR/typeck-negative-impls-builtin.rs:9:1
-   |
-LL | impl !TestTrait for TestType {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0192`.
diff --git a/src/test/ui/typeck/typeck-unsafe-always-share.rs b/src/test/ui/typeck/typeck-unsafe-always-share.rs
index 7d1ff732983fe..dc5ddf5156302 100644
--- a/src/test/ui/typeck/typeck-unsafe-always-share.rs
+++ b/src/test/ui/typeck/typeck-unsafe-always-share.rs
@@ -1,6 +1,6 @@
 // Verify that UnsafeCell is *always* !Sync regardless if `T` is sync.
 
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::cell::UnsafeCell;
 use std::marker::Sync;