Auto merge of #55101 - alexreg:trait-aliases, r=nikomatsakis

Implement trait aliases (RFC 1733)

Extends groundwork done in #45047, and fully implements rust-lang/rfcs#1733.

CC @durka @nikomatsakis

Author: bors

src/doc/unstable-book/src/language-features/trait-alias.md

@@ -0,0 +1,34 @@
+# `trait_alias`
+
+The tracking issue for this feature is: [#41517]
+
+[#41417]: https://github.com/rust-lang/rust/issues/41517
+
+------------------------
+
+The `trait_alias` feature adds support for trait aliases. These allow aliases
+to be created for one or more traits (currently just a single regular trait plus
+any number of auto-traits), and used wherever traits would normally be used as
+either bounds or trait objects.
+
+```rust
+#![feature(trait_alias)]
+
+trait Foo = std::fmt::Debug + Send;
+trait Bar = Foo + Sync;
+
+// Use trait alias as bound on type parameter.
+fn foo<T: Foo>(v: &T) {
+    println!("{:?}", v);
+}
+
+pub fn main() {
+    foo(&1);
+
+    // Use trait alias for trait objects.
+    let a: &Bar = &123;
+    println!("{:?}", a);
+    let b = Box::new(456) as Box<dyn Foo>;
+    println!("{:?}", b);
+}
+```

src/librustc/hir/lowering.rs

@@ -4875,23 +4875,24 @@ impl<'a> LoweringContext<'a> {
         let node = match qpath {
             hir::QPath::Resolved(None, path) => {
                 // Turn trait object paths into `TyKind::TraitObject` instead.
-                if let Def::Trait(_) = path.def {
-                    let principal = hir::PolyTraitRef {
-                        bound_generic_params: hir::HirVec::new(),
-                        trait_ref: hir::TraitRef {
-                            path: path.and_then(|path| path),
-                            ref_id: id.node_id,
-                            hir_ref_id: id.hir_id,
-                        },
-                        span,
-                    };
+                match path.def {
+                    Def::Trait(_) | Def::TraitAlias(_) => {
+                        let principal = hir::PolyTraitRef {
+                            bound_generic_params: hir::HirVec::new(),
+                            trait_ref: hir::TraitRef {
+                                path: path.and_then(|path| path),
+                                ref_id: id.node_id,
+                                hir_ref_id: id.hir_id,
+                            },
+                            span,
+                        };
 
-                    // The original ID is taken by the `PolyTraitRef`,
-                    // so the `Ty` itself needs a different one.
-                    id = self.next_id();
-                    hir::TyKind::TraitObject(hir_vec![principal], self.elided_dyn_bound(span))
-                } else {
-                    hir::TyKind::Path(hir::QPath::Resolved(None, path))
+                        // The original ID is taken by the `PolyTraitRef`,
+                        // so the `Ty` itself needs a different one.
+                        id = self.next_id();
+                        hir::TyKind::TraitObject(hir_vec![principal], self.elided_dyn_bound(span))
+                    }
+                    _ => hir::TyKind::Path(hir::QPath::Resolved(None, path)),
                 }
             }
             _ => hir::TyKind::Path(qpath),

src/librustc/hir/map/mod.rs

@@ -301,9 +301,7 @@ impl<'hir> Map<'hir> {
                     ItemKind::Struct(..) => Some(Def::Struct(def_id())),
                     ItemKind::Union(..) => Some(Def::Union(def_id())),
                     ItemKind::Trait(..) => Some(Def::Trait(def_id())),
-                    ItemKind::TraitAlias(..) => {
-                        bug!("trait aliases are not yet implemented (see issue #41517)")
-                    },
+                    ItemKind::TraitAlias(..) => Some(Def::TraitAlias(def_id())),
                     ItemKind::ExternCrate(_) |
                     ItemKind::Use(..) |
                     ItemKind::ForeignMod(..) |

src/librustc/ich/impls_ty.rs

@@ -1119,6 +1119,7 @@ for traits::Vtable<'gcx, N> where N: HashStable<StableHashingContext<'a>> {
             &VtableClosure(ref table_closure) => table_closure.hash_stable(hcx, hasher),
             &VtableFnPointer(ref table_fn_pointer) => table_fn_pointer.hash_stable(hcx, hasher),
             &VtableGenerator(ref table_generator) => table_generator.hash_stable(hcx, hasher),
+            &VtableTraitAlias(ref table_alias) => table_alias.hash_stable(hcx, hasher),
         }
     }
 }
@@ -1227,6 +1228,22 @@ for traits::VtableGeneratorData<'gcx, N> where N: HashStable<StableHashingContex
     }
 }
 
+impl<'a, 'gcx, N> HashStable<StableHashingContext<'a>>
+for traits::VtableTraitAliasData<'gcx, N> where N: HashStable<StableHashingContext<'a>> {
+    fn hash_stable<W: StableHasherResult>(&self,
+                                          hcx: &mut StableHashingContext<'a>,
+                                          hasher: &mut StableHasher<W>) {
+        let traits::VtableTraitAliasData {
+            alias_def_id,
+            substs,
+            ref nested,
+        } = *self;
+        alias_def_id.hash_stable(hcx, hasher);
+        substs.hash_stable(hcx, hasher);
+        nested.hash_stable(hcx, hasher);
+    }
+}
+
 impl_stable_hash_for!(
     impl<'tcx, V> for struct infer::canonical::Canonical<'tcx, V> {
         max_universe, variables, value

src/librustc/middle/resolve_lifetime.rs

@@ -447,6 +447,17 @@ fn krate<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) -> NamedRegionMap {
     map
 }
 
+/// In traits, there is an implicit `Self` type parameter which comes before the generics.
+/// We have to account for this when computing the index of the other generic parameters.
+/// This function returns whether there is such an implicit parameter defined on the given item.
+fn sub_items_have_self_param(node: &hir::ItemKind) -> bool {
+    match *node {
+        hir::ItemKind::Trait(..) |
+        hir::ItemKind::TraitAlias(..) => true,
+        _ => false,
+    }
+}
+
 impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
         NestedVisitorMap::All(&self.tcx.hir)
@@ -522,8 +533,8 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     hir::ItemKind::Impl(..) => true,
                     _ => false,
                 };
-                // These kinds of items have only early bound lifetime parameters.
-                let mut index = if let hir::ItemKind::Trait(..) = item.node {
+                // These kinds of items have only early-bound lifetime parameters.
+                let mut index = if sub_items_have_self_param(&item.node) {
                     1 // Self comes before lifetimes
                 } else {
                     0
@@ -1602,8 +1613,8 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         let mut index = 0;
         if let Some(parent_id) = parent_id {
             let parent = self.tcx.hir.expect_item(parent_id);
-            if let hir::ItemKind::Trait(..) = parent.node {
-                index += 1; // Self comes first.
+            if sub_items_have_self_param(&parent.node) {
+                index += 1; // Self comes before lifetimes
             }
             match parent.node {
                 hir::ItemKind::Trait(_, _, ref generics, ..)

src/librustc/traits/mod.rs

@@ -534,8 +534,11 @@ pub enum Vtable<'tcx, N> {
     /// Same as above, but for a fn pointer type with the given signature.
     VtableFnPointer(VtableFnPointerData<'tcx, N>),
 
-    /// Vtable automatically generated for a generator
+    /// Vtable automatically generated for a generator.
     VtableGenerator(VtableGeneratorData<'tcx, N>),
+
+    /// Vtable for a trait alias.
+    VtableTraitAlias(VtableTraitAliasData<'tcx, N>),
 }
 
 /// Identifies a particular impl in the source, along with a set of
@@ -605,6 +608,13 @@ pub struct VtableFnPointerData<'tcx, N> {
     pub nested: Vec<N>
 }
 
+#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable)]
+pub struct VtableTraitAliasData<'tcx, N> {
+    pub alias_def_id: DefId,
+    pub substs: &'tcx Substs<'tcx>,
+    pub nested: Vec<N>,
+}
+
 /// Creates predicate obligations from the generic bounds.
 pub fn predicates_for_generics<'tcx>(cause: ObligationCause<'tcx>,
                                      param_env: ty::ParamEnv<'tcx>,
@@ -1067,6 +1077,7 @@ impl<'tcx, N> Vtable<'tcx, N> {
             VtableGenerator(c) => c.nested,
             VtableObject(d) => d.nested,
             VtableFnPointer(d) => d.nested,
+            VtableTraitAlias(d) => d.nested,
         }
     }
 
@@ -1090,20 +1101,25 @@ impl<'tcx, N> Vtable<'tcx, N> {
                 trait_def_id: d.trait_def_id,
                 nested: d.nested.into_iter().map(f).collect(),
             }),
-            VtableFnPointer(p) => VtableFnPointer(VtableFnPointerData {
-                fn_ty: p.fn_ty,
-                nested: p.nested.into_iter().map(f).collect(),
+            VtableClosure(c) => VtableClosure(VtableClosureData {
+                closure_def_id: c.closure_def_id,
+                substs: c.substs,
+                nested: c.nested.into_iter().map(f).collect(),
             }),
             VtableGenerator(c) => VtableGenerator(VtableGeneratorData {
                 generator_def_id: c.generator_def_id,
                 substs: c.substs,
                 nested: c.nested.into_iter().map(f).collect(),
             }),
-            VtableClosure(c) => VtableClosure(VtableClosureData {
-                closure_def_id: c.closure_def_id,
-                substs: c.substs,
-                nested: c.nested.into_iter().map(f).collect(),
-            })
+            VtableFnPointer(p) => VtableFnPointer(VtableFnPointerData {
+                fn_ty: p.fn_ty,
+                nested: p.nested.into_iter().map(f).collect(),
+            }),
+            VtableTraitAlias(d) => VtableTraitAlias(VtableTraitAliasData {
+                alias_def_id: d.alias_def_id,
+                substs: d.substs,
+                nested: d.nested.into_iter().map(f).collect(),
+            }),
         }
     }
 }

src/librustc/traits/project.rs

@@ -19,10 +19,7 @@ use super::PredicateObligation;
 use super::Selection;
 use super::SelectionContext;
 use super::SelectionError;
-use super::VtableClosureData;
-use super::VtableGeneratorData;
-use super::VtableFnPointerData;
-use super::VtableImplData;
+use super::{VtableImplData, VtableClosureData, VtableGeneratorData, VtableFnPointerData};
 use super::util;
 
 use hir::def_id::DefId;
@@ -1073,7 +1070,8 @@ fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
             super::VtableClosure(_) |
             super::VtableGenerator(_) |
             super::VtableFnPointer(_) |
-            super::VtableObject(_) => {
+            super::VtableObject(_) |
+            super::VtableTraitAlias(_) => {
                 debug!("assemble_candidates_from_impls: vtable={:?}",
                        vtable);
                 true
@@ -1235,7 +1233,8 @@ fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
             confirm_object_candidate(selcx, obligation, obligation_trait_ref),
         super::VtableAutoImpl(..) |
         super::VtableParam(..) |
-        super::VtableBuiltin(..) =>
+        super::VtableBuiltin(..) |
+        super::VtableTraitAlias(..) =>
             // we don't create Select candidates with this kind of resolution
             span_bug!(
                 obligation.cause.span,
@@ -1486,7 +1485,7 @@ fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
     impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
     -> Progress<'tcx>
 {
-    let VtableImplData { substs, nested, impl_def_id } = impl_vtable;
+    let VtableImplData { impl_def_id, substs, nested } = impl_vtable;
 
     let tcx = selcx.tcx();
     let param_env = obligation.param_env;