reviews + rebase

compiler/rustc_infer/src/infer/mod.rs

@@ -1031,7 +1031,6 @@ impl<'tcx> InferCtxt<'tcx> {
             _ => {}
         }
 
-        // FIXME(tree_universes): leaking placeholders
         self.enter_forall(predicate, |ty::SubtypePredicate { a_is_expected, a, b }| {
             Ok(self.at(cause, param_env).sub_exp(DefineOpaqueTypes::No, a_is_expected, a, b))
         })
@@ -1042,7 +1041,6 @@ impl<'tcx> InferCtxt<'tcx> {
         cause: &traits::ObligationCause<'tcx>,
         predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
     ) {
-        // FIXME(tree_universes): leaking placeholders
         self.enter_forall(predicate, |ty::OutlivesPredicate(r_a, r_b)| {
             let origin = SubregionOrigin::from_obligation_cause(cause, || {
                 RelateRegionParamBound(cause.span)

compiler/rustc_infer/src/infer/relate/higher_ranked.rs

@@ -49,7 +49,6 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
             debug!("b_prime={:?}", sup_prime);
 
             // Compare types now that bound regions have been replaced.
-            // FIXME(tree_universes): leaked universes
             let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime);
             if result.is_ok() {
                 debug!("OK result={result:?}");
@@ -70,7 +69,7 @@ impl<'tcx> InferCtxt<'tcx> {
     /// This is the first step of checking subtyping when higher-ranked things are involved.
     /// For more details visit the relevant sections of the [rustc dev guide].
     ///
-    /// `enter_forall` should be preferred over this method.
+    /// fn `enter_forall` should be preferred over this method.
     ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
     #[instrument(level = "debug", skip(self), ret)]
@@ -111,21 +110,25 @@ impl<'tcx> InferCtxt<'tcx> {
     }
 
     /// Replaces all bound variables (lifetimes, types, and constants) bound by
-    /// `binder` with placeholder variables in a new universe. This means that the
-    /// new placeholders can only be named by inference variables created after
-    /// this method has been called.
+    /// `binder` with placeholder variables in a new universe and then calls the
+    /// closure `f` with the instantiated value. The new placeholders can only be
+    /// named by inference variables inside of the closure `f` or afterwards.
     ///
     /// This is the first step of checking subtyping when higher-ranked things are involved.
     /// For more details visit the relevant sections of the [rustc dev guide].
     ///
-    /// This method should be preferred over `enter_forall_and_leak_universe`.
+    /// This method should be preferred over `fn enter_forall_and_leak_universe`.
     ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
     #[instrument(level = "debug", skip(self, f))]
     pub fn enter_forall<T, U>(&self, forall: ty::Binder<'tcx, T>, f: impl FnOnce(T) -> U) -> U
     where
         T: TypeFoldable<TyCtxt<'tcx>> + Copy,
     {
+        // FIXME: currently we do nothing to prevent placeholders with the new universe being
+        // used after exiting `f`. For example region subtyping can result in outlive constraints
+        // that name placeholders created in this function. Nested goals from type relations can
+        // also contain placeholders created by this function.
         let value = self.enter_forall_and_leak_universe(forall);
         debug!("?value");
         f(value)

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -480,7 +480,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             self.infcx.enter_forall(kind, |kind| {
                 let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
                 self.add_goal(GoalSource::Misc, goal);
-                // FIXME(tree_universes): leaking universes
                 self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             })
         }

compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs

@@ -31,7 +31,6 @@ impl<'tcx> InferCtxtSelectExt<'tcx> for InferCtxt<'tcx> {
     ) -> SelectionResult<'tcx, Selection<'tcx>> {
         assert!(self.next_trait_solver());
 
-        // FIXME(tree_universes): leaking universes?
         self.enter_forall(obligation.predicate, |pred| {
             let trait_goal = Goal::new(self.tcx, obligation.param_env, pred);
 

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -999,7 +999,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
                 constituent_tys(ecx, goal.predicate.self_ty())?
                     .into_iter()
                     .map(|ty| {
-                        // FIXME(tree_universes): leaking universes
                         ecx.enter_forall(ty, |ty| {
                             goal.with(ecx.tcx(), goal.predicate.with_self_ty(ecx.tcx(), ty))
                         })

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -4621,7 +4621,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 let ocx = ObligationCtxt::new(self);
                 self.enter_forall(pred, |pred| {
                     let pred = ocx.normalize(&ObligationCause::dummy(), param_env, pred);
-                    // FIXME(tree_universes): universe leakage
                     ocx.register_obligation(Obligation::new(
                         self.tcx,
                         ObligationCause::dummy(),

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -2042,7 +2042,6 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     for (obligation_arg, impl_arg) in
                         std::iter::zip(obligation_trait_ref.args, impl_trait_ref.args)
                     {
-                        // FIXME(tree_universes): universe leakage
                         if let Err(terr) =
                             ocx.eq(&ObligationCause::dummy(), param_env, impl_arg, obligation_arg)
                         {

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -665,64 +665,63 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         self.infcx.probe(|_snapshot| {
             let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
-            let placeholder_trait_predicate =
-                self.infcx.enter_forall_and_leak_universe(poly_trait_predicate);
-
-            let self_ty = placeholder_trait_predicate.self_ty();
-            let principal_trait_ref = match self_ty.kind() {
-                ty::Dynamic(data, ..) => {
-                    if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
-                        debug!(
-                            "assemble_candidates_from_object_ty: matched builtin bound, \
+            self.infcx.enter_forall(poly_trait_predicate, |placeholder_trait_predicate| {
+                let self_ty = placeholder_trait_predicate.self_ty();
+                let principal_trait_ref = match self_ty.kind() {
+                    ty::Dynamic(data, ..) => {
+                        if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
+                            debug!(
+                                "assemble_candidates_from_object_ty: matched builtin bound, \
                              pushing candidate"
-                        );
-                        candidates.vec.push(BuiltinObjectCandidate);
-                        return;
-                    }
+                            );
+                            candidates.vec.push(BuiltinObjectCandidate);
+                            return;
+                        }
 
-                    if let Some(principal) = data.principal() {
-                        if !self.infcx.tcx.features().object_safe_for_dispatch {
-                            principal.with_self_ty(self.tcx(), self_ty)
-                        } else if self.tcx().check_is_object_safe(principal.def_id()) {
-                            principal.with_self_ty(self.tcx(), self_ty)
+                        if let Some(principal) = data.principal() {
+                            if !self.infcx.tcx.features().object_safe_for_dispatch {
+                                principal.with_self_ty(self.tcx(), self_ty)
+                            } else if self.tcx().check_is_object_safe(principal.def_id()) {
+                                principal.with_self_ty(self.tcx(), self_ty)
+                            } else {
+                                return;
+                            }
                         } else {
+                            // Only auto trait bounds exist.
                             return;
                         }
-                    } else {
-                        // Only auto trait bounds exist.
+                    }
+                    ty::Infer(ty::TyVar(_)) => {
+                        debug!("assemble_candidates_from_object_ty: ambiguous");
+                        candidates.ambiguous = true; // could wind up being an object type
                         return;
                     }
-                }
-                ty::Infer(ty::TyVar(_)) => {
-                    debug!("assemble_candidates_from_object_ty: ambiguous");
-                    candidates.ambiguous = true; // could wind up being an object type
-                    return;
-                }
-                _ => return,
-            };
-
-            debug!(?principal_trait_ref, "assemble_candidates_from_object_ty");
-
-            // Count only those upcast versions that match the trait-ref
-            // we are looking for. Specifically, do not only check for the
-            // correct trait, but also the correct type parameters.
-            // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
-            // but `Foo` is declared as `trait Foo: Bar<u32>`.
-            let candidate_supertraits = util::supertraits(self.tcx(), principal_trait_ref)
-                .enumerate()
-                .filter(|&(_, upcast_trait_ref)| {
-                    self.infcx.probe(|_| {
-                        self.match_normalize_trait_ref(
-                            obligation,
-                            upcast_trait_ref,
-                            placeholder_trait_predicate.trait_ref,
-                        )
-                        .is_ok()
+                    _ => return,
+                };
+
+                debug!(?principal_trait_ref, "assemble_candidates_from_object_ty");
+
+                // Count only those upcast versions that match the trait-ref
+                // we are looking for. Specifically, do not only check for the
+                // correct trait, but also the correct type parameters.
+                // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
+                // but `Foo` is declared as `trait Foo: Bar<u32>`.
+                let candidate_supertraits = util::supertraits(self.tcx(), principal_trait_ref)
+                    .enumerate()
+                    .filter(|&(_, upcast_trait_ref)| {
+                        self.infcx.probe(|_| {
+                            self.match_normalize_trait_ref(
+                                obligation,
+                                upcast_trait_ref,
+                                placeholder_trait_predicate.trait_ref,
+                            )
+                            .is_ok()
+                        })
                     })
-                })
-                .map(|(idx, _)| ObjectCandidate(idx));
+                    .map(|(idx, _)| ObjectCandidate(idx));
 
-            candidates.vec.extend(candidate_supertraits);
+                candidates.vec.extend(candidate_supertraits);
+            })
         })
     }
 

compiler/rustc_type_ir/src/region_kind.rs

@@ -163,8 +163,7 @@ pub enum RegionKind<I: Interner> {
     /// A placeholder region -- the higher-ranked version of `ReLateParam`.
     /// Should not exist outside of type inference.
     ///
-    /// Used when instantiating a `forall` binder via
-    /// `infcx.enter_forall` and `infcx.enter_forall_and_leak_universe`.
+    /// Used when instantiating a `forall` binder via `infcx.enter_forall`.
     RePlaceholder(I::PlaceholderRegion),
 
     /// Erased region, used by trait selection, in MIR and during codegen.