Rollup merge of #73055 - lcnr:skol-no-more, r=matthewjasper

remove leftover mentions of `skol` and `int` from the compiler

This PR mostly changes `skol` -> `placeholder` and all cases where `int` is used as a type to `i32`.

Author: RalfJung

src/librustc_infer/infer/higher_ranked/mod.rs

@@ -63,14 +63,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// placeholder region. This is the first step of checking subtyping
     /// when higher-ranked things are involved.
     ///
-    /// **Important:** you must call this function from within a snapshot.
-    /// Moreover, before committing the snapshot, you must eventually call
-    /// either `plug_leaks` or `pop_placeholders` to remove the placeholder
-    /// regions. If you rollback the snapshot (or are using a probe), then
-    /// the pop occurs as part of the rollback, so an explicit call is not
-    /// needed (but is also permitted).
-    ///
-    /// For more information about how placeholders and HRTBs work, see
+    /// **Important:** You have to be careful to not leak these placeholders,
+    /// for more information about how placeholders and HRTBs work, see
     /// the [rustc dev guide].
     ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html

src/librustc_infer/infer/region_constraints/leak_check.rs

@@ -128,7 +128,7 @@ impl<'tcx> TaintSet<'tcx> {
                         verifys[i].origin.span(),
                         "we never add verifications while doing higher-ranked things",
                     ),
-                    &Purged | &AddCombination(..) | &AddVar(..) => {}
+                    &AddCombination(..) | &AddVar(..) => {}
                 }
             }
         }

src/librustc_infer/infer/region_constraints/mod.rs

@@ -289,14 +289,6 @@ pub(crate) enum UndoLog<'tcx> {
 
     /// We added a GLB/LUB "combination variable".
     AddCombination(CombineMapType, TwoRegions<'tcx>),
-
-    /// During skolemization, we sometimes purge entries from the undo
-    /// log in a kind of minisnapshot (unlike other snapshots, this
-    /// purging actually takes place *on success*). In that case, we
-    /// replace the corresponding entry with `Noop` so as to avoid the
-    /// need to do a bunch of swapping. (We can't use `swap_remove` as
-    /// the order of the vector is important.)
-    Purged,
 }
 
 #[derive(Copy, Clone, PartialEq)]
@@ -357,9 +349,6 @@ impl<'tcx> RegionConstraintStorage<'tcx> {
 
     fn rollback_undo_entry(&mut self, undo_entry: UndoLog<'tcx>) {
         match undo_entry {
-            Purged => {
-                // nothing to do here
-            }
             AddVar(vid) => {
                 self.var_infos.pop().unwrap();
                 assert_eq!(self.var_infos.len(), vid.index() as usize);
@@ -488,62 +477,6 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         self.var_infos[vid].origin
     }
 
-    /// Removes all the edges to/from the placeholder regions that are
-    /// in `skols`. This is used after a higher-ranked operation
-    /// completes to remove all trace of the placeholder regions
-    /// created in that time.
-    pub fn pop_placeholders(&mut self, placeholders: &FxHashSet<ty::Region<'tcx>>) {
-        debug!("pop_placeholders(placeholders={:?})", placeholders);
-
-        assert!(UndoLogs::<super::UndoLog<'_>>::in_snapshot(&self.undo_log));
-
-        let constraints_to_kill: Vec<usize> = self
-            .undo_log
-            .iter()
-            .enumerate()
-            .rev()
-            .filter(|&(_, undo_entry)| match undo_entry {
-                super::UndoLog::RegionConstraintCollector(undo_entry) => {
-                    kill_constraint(placeholders, undo_entry)
-                }
-                _ => false,
-            })
-            .map(|(index, _)| index)
-            .collect();
-
-        for index in constraints_to_kill {
-            let undo_entry = match &mut self.undo_log[index] {
-                super::UndoLog::RegionConstraintCollector(undo_entry) => {
-                    mem::replace(undo_entry, Purged)
-                }
-                _ => unreachable!(),
-            };
-            self.rollback_undo_entry(undo_entry);
-        }
-
-        return;
-
-        fn kill_constraint<'tcx>(
-            placeholders: &FxHashSet<ty::Region<'tcx>>,
-            undo_entry: &UndoLog<'tcx>,
-        ) -> bool {
-            match undo_entry {
-                &AddConstraint(Constraint::VarSubVar(..)) => false,
-                &AddConstraint(Constraint::RegSubVar(a, _)) => placeholders.contains(&a),
-                &AddConstraint(Constraint::VarSubReg(_, b)) => placeholders.contains(&b),
-                &AddConstraint(Constraint::RegSubReg(a, b)) => {
-                    placeholders.contains(&a) || placeholders.contains(&b)
-                }
-                &AddGiven(..) => false,
-                &AddVerify(_) => false,
-                &AddCombination(_, ref two_regions) => {
-                    placeholders.contains(&two_regions.a) || placeholders.contains(&two_regions.b)
-                }
-                &AddVar(..) | &Purged => false,
-            }
-        }
-    }
-
     fn add_constraint(&mut self, constraint: Constraint<'tcx>, origin: SubregionOrigin<'tcx>) {
         // cannot add constraints once regions are resolved
         debug!("RegionConstraintCollector: add_constraint({:?})", constraint);

src/librustc_infer/infer/undo_log.rs

@@ -198,10 +198,6 @@ impl<'tcx> InferCtxtUndoLogs<'tcx> {
         assert!(self.logs.len() >= snapshot.undo_len);
         assert!(self.num_open_snapshots > 0);
     }
-
-    pub(crate) fn iter(&self) -> std::slice::Iter<'_, UndoLog<'tcx>> {
-        self.logs.iter()
-    }
 }
 
 impl<'tcx> std::ops::Index<usize> for InferCtxtUndoLogs<'tcx> {

src/librustc_infer/traits/mod.rs

@@ -29,10 +29,10 @@ crate use self::util::elaborate_predicates;
 
 pub use rustc_middle::traits::*;
 
-/// An `Obligation` represents some trait reference (e.g., `int: Eq`) for
+/// An `Obligation` represents some trait reference (e.g., `i32: Eq`) for
 /// which the "impl_source" must be found. The process of finding a "impl_source" is
 /// called "resolving" the `Obligation`. This process consists of
-/// either identifying an `impl` (e.g., `impl Eq for int`) that
+/// either identifying an `impl` (e.g., `impl Eq for i32`) that
 /// satisfies the obligation, or else finding a bound that is in
 /// scope. The eventual result is usually a `Selection` (defined below).
 #[derive(Clone, PartialEq, Eq, Hash)]

src/librustc_infer/traits/util.rs

@@ -63,11 +63,11 @@ impl PredicateSet<'tcx> {
     fn insert(&mut self, pred: ty::Predicate<'tcx>) -> bool {
         // We have to be careful here because we want
         //
-        //    for<'a> Foo<&'a int>
+        //    for<'a> Foo<&'a i32>
         //
         // and
         //
-        //    for<'b> Foo<&'b int>
+        //    for<'b> Foo<&'b i32>
         //
         // to be considered equivalent. So normalize all late-bound
         // regions before we throw things into the underlying set.

src/librustc_middle/traits/mod.rs

@@ -393,23 +393,25 @@ pub type SelectionResult<'tcx, T> = Result<Option<T>, SelectionError<'tcx>>;
 /// ```
 /// impl<T:Clone> Clone<T> for Option<T> { ... } // Impl_1
 /// impl<T:Clone> Clone<T> for Box<T> { ... }    // Impl_2
-/// impl Clone for int { ... }             // Impl_3
+/// impl Clone for i32 { ... }                   // Impl_3
 ///
-/// fn foo<T:Clone>(concrete: Option<Box<int>>,
-///                 param: T,
-///                 mixed: Option<T>) {
+/// fn foo<T: Clone>(concrete: Option<Box<i32>>, param: T, mixed: Option<T>) {
+///     // Case A: Vtable points at a specific impl. Only possible when
+///     // type is concretely known. If the impl itself has bounded
+///     // type parameters, Vtable will carry resolutions for those as well:
+///     concrete.clone(); // Vtable(Impl_1, [Vtable(Impl_2, [Vtable(Impl_3)])])
 ///
-///    // Case A: ImplSource points at a specific impl. Only possible when
-///    // type is concretely known. If the impl itself has bounded
-///    // type parameters, ImplSource will carry resolutions for those as well:
-///    concrete.clone(); // ImplSource(Impl_1, [ImplSource(Impl_2, [ImplSource(Impl_3)])])
+///     // Case A: ImplSource points at a specific impl. Only possible when
+///     // type is concretely known. If the impl itself has bounded
+///     // type parameters, ImplSource will carry resolutions for those as well:
+///     concrete.clone(); // ImplSource(Impl_1, [ImplSource(Impl_2, [ImplSource(Impl_3)])])
 ///
-///    // Case B: ImplSource must be provided by caller. This applies when
-///    // type is a type parameter.
-///    param.clone();    // ImplSourceParam
+///     // Case B: ImplSource must be provided by caller. This applies when
+///     // type is a type parameter.
+///     param.clone();    // ImplSourceParam
 ///
-///    // Case C: A mix of cases A and B.
-///    mixed.clone();    // ImplSource(Impl_1, [ImplSourceParam])
+///     // Case C: A mix of cases A and B.
+///     mixed.clone();    // ImplSource(Impl_1, [ImplSourceParam])
 /// }
 /// ```
 ///

src/librustc_middle/ty/subst.rs

@@ -599,12 +599,12 @@ impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
     ///
     /// ```
     /// type Func<A> = fn(A);
-    /// type MetaFunc = for<'a> fn(Func<&'a int>)
+    /// type MetaFunc = for<'a> fn(Func<&'a i32>)
     /// ```
     ///
     /// The type `MetaFunc`, when fully expanded, will be
     ///
-    ///     for<'a> fn(fn(&'a int))
+    ///     for<'a> fn(fn(&'a i32))
     ///             ^~ ^~ ^~~
     ///             |  |  |
     ///             |  |  DebruijnIndex of 2
@@ -613,26 +613,26 @@ impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
     /// Here the `'a` lifetime is bound in the outer function, but appears as an argument of the
     /// inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
     /// over the inner binder (remember that we count De Bruijn indices from 1). However, in the
-    /// definition of `MetaFunc`, the binder is not visible, so the type `&'a int` will have a
+    /// definition of `MetaFunc`, the binder is not visible, so the type `&'a i32` will have a
     /// De Bruijn index of 1. It's only during the substitution that we can see we must increase the
     /// depth by 1 to account for the binder that we passed through.
     ///
     /// As a second example, consider this twist:
     ///
     /// ```
     /// type FuncTuple<A> = (A,fn(A));
-    /// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a int>)
+    /// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a i32>)
     /// ```
     ///
     /// Here the final type will be:
     ///
-    ///     for<'a> fn((&'a int, fn(&'a int)))
+    ///     for<'a> fn((&'a i32, fn(&'a i32)))
     ///                 ^~~         ^~~
     ///                 |           |
     ///          DebruijnIndex of 1 |
     ///                      DebruijnIndex of 2
     ///
-    /// As indicated in the diagram, here the same type `&'a int` is substituted once, but in the
+    /// As indicated in the diagram, here the same type `&'a i32` is substituted once, but in the
     /// first case we do not increase the De Bruijn index and in the second case we do. The reason
     /// is that only in the second case have we passed through a fn binder.
     fn shift_vars_through_binders<T: TypeFoldable<'tcx>>(&self, val: T) -> T {

src/librustc_middle/ty/walk.rs

@@ -22,13 +22,13 @@ impl<'tcx> TypeWalker<'tcx> {
     /// Skips the subtree corresponding to the last type
     /// returned by `next()`.
     ///
-    /// Example: Imagine you are walking `Foo<Bar<int>, usize>`.
+    /// Example: Imagine you are walking `Foo<Bar<i32>, usize>`.
     ///
     /// ```
     /// let mut iter: TypeWalker = ...;
     /// iter.next(); // yields Foo
-    /// iter.next(); // yields Bar<int>
-    /// iter.skip_current_subtree(); // skips int
+    /// iter.next(); // yields Bar<i32>
+    /// iter.skip_current_subtree(); // skips i32
     /// iter.next(); // yields usize
     /// ```
     pub fn skip_current_subtree(&mut self) {

src/librustc_trait_selection/traits/project.rs

@@ -361,7 +361,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                 // handle normalization within binders because
                 // otherwise we wind up a need to normalize when doing
                 // trait matching (since you can have a trait
-                // obligation like `for<'a> T::B : Fn(&'a int)`), but
+                // obligation like `for<'a> T::B: Fn(&'a i32)`), but
                 // we can't normalize with bound regions in scope. So
                 // far now we just ignore binders but only normalize
                 // if all bound regions are gone (and then we still

src/librustc_trait_selection/traits/query/normalize.rs

@@ -145,7 +145,7 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 // handle normalization within binders because
                 // otherwise we wind up a need to normalize when doing
                 // trait matching (since you can have a trait
-                // obligation like `for<'a> T::B : Fn(&'a int)`), but
+                // obligation like `for<'a> T::B: Fn(&'a i32)`), but
                 // we can't normalize with bound regions in scope. So
                 // far now we just ignore binders but only normalize
                 // if all bound regions are gone (and then we still

src/librustc_trait_selection/traits/select/confirmation.rs

@@ -553,14 +553,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ///
     /// Here is an example. Imagine we have a closure expression
     /// and we desugared it so that the type of the expression is
-    /// `Closure`, and `Closure` expects an int as argument. Then it
+    /// `Closure`, and `Closure` expects `i32` as argument. Then it
     /// is "as if" the compiler generated this impl:
     ///
-    ///     impl Fn(int) for Closure { ... }
+    ///     impl Fn(i32) for Closure { ... }
     ///
-    /// Now imagine our obligation is `Fn(usize) for Closure`. So far
+    /// Now imagine our obligation is `Closure: Fn(usize)`. So far
     /// we have matched the self type `Closure`. At this point we'll
-    /// compare the `int` to `usize` and generate an error.
+    /// compare the `i32` to `usize` and generate an error.
     ///
     /// Note that this checking occurs *after* the impl has selected,
     /// because these output type parameters should not affect the

src/librustc_trait_selection/traits/select/mod.rs

@@ -1754,38 +1754,37 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Vec<PredicateObligation<'tcx>> {
         // Because the types were potentially derived from
         // higher-ranked obligations they may reference late-bound
-        // regions. For example, `for<'a> Foo<&'a int> : Copy` would
-        // yield a type like `for<'a> &'a int`. In general, we
+        // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
+        // yield a type like `for<'a> &'a i32`. In general, we
         // maintain the invariant that we never manipulate bound
         // regions, so we have to process these bound regions somehow.
         //
         // The strategy is to:
         //
         // 1. Instantiate those regions to placeholder regions (e.g.,
-        //    `for<'a> &'a int` becomes `&0 int`.
-        // 2. Produce something like `&'0 int : Copy`
-        // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
+        //    `for<'a> &'a i32` becomes `&0 i32`.
+        // 2. Produce something like `&'0 i32 : Copy`
+        // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
 
         types
-            .skip_binder()
+            .skip_binder() // binder moved -\
             .iter()
             .flat_map(|ty| {
-                // binder moved -\
                 let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/
 
                 self.infcx.commit_unconditionally(|_| {
-                    let (skol_ty, _) = self.infcx.replace_bound_vars_with_placeholders(&ty);
+                    let (placeholder_ty, _) = self.infcx.replace_bound_vars_with_placeholders(&ty);
                     let Normalized { value: normalized_ty, mut obligations } =
                         ensure_sufficient_stack(|| {
                             project::normalize_with_depth(
                                 self,
                                 param_env,
                                 cause.clone(),
                                 recursion_depth,
-                                &skol_ty,
+                                &placeholder_ty,
                             )
                         });
-                    let skol_obligation = predicate_for_trait_def(
+                    let placeholder_obligation = predicate_for_trait_def(
                         self.tcx(),
                         param_env,
                         cause.clone(),
@@ -1794,7 +1793,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         normalized_ty,
                         &[],
                     );
-                    obligations.push(skol_obligation);
+                    obligations.push(placeholder_obligation);
                     obligations
                 })
             })
@@ -1844,9 +1843,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return Err(());
         }
 
-        let (skol_obligation, placeholder_map) =
+        let (placeholder_obligation, placeholder_map) =
             self.infcx().replace_bound_vars_with_placeholders(&obligation.predicate);
-        let skol_obligation_trait_ref = skol_obligation.trait_ref;
+        let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
 
         let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
 
@@ -1865,14 +1864,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(
             "match_impl(impl_def_id={:?}, obligation={:?}, \
-             impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
-            impl_def_id, obligation, impl_trait_ref, skol_obligation_trait_ref
+             impl_trait_ref={:?}, placeholder_obligation_trait_ref={:?})",
+            impl_def_id, obligation, impl_trait_ref, placeholder_obligation_trait_ref
         );
 
         let InferOk { obligations, .. } = self
             .infcx
             .at(&obligation.cause, obligation.param_env)
-            .eq(skol_obligation_trait_ref, impl_trait_ref)
+            .eq(placeholder_obligation_trait_ref, impl_trait_ref)
             .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
         nested_obligations.extend(obligations);
 

src/librustc_trait_selection/traits/specialize/mod.rs

@@ -130,7 +130,7 @@ pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId,
 
     // We determine whether there's a subset relationship by:
     //
-    // - skolemizing impl1,
+    // - replacing bound vars with placeholders in impl1,
     // - assuming the where clauses for impl1,
     // - instantiating impl2 with fresh inference variables,
     // - unifying,