reviews + rebase

11 files changed, 101 insertions, 114 deletions

diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index f7969e31dee..2d87f01ff10 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -1032,7 +1032,6 @@ 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))
         })
@@ -1043,7 +1042,6 @@ 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)
diff --git a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs b/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
index ff01f620e66..90be80f67b4 100644
--- a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
+++ b/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)]
@@ -111,14 +110,14 @@ 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 created 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))]
@@ -126,6 +125,10 @@ impl<'tcx> InferCtxt<'tcx> {
     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 outlives 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)
diff --git a/compiler/rustc_infer/src/infer/relate/nll.rs b/compiler/rustc_infer/src/infer/relate/nll.rs
index 2760356bcdf..5e2d2af9b85 100644
--- a/compiler/rustc_infer/src/infer/relate/nll.rs
+++ b/compiler/rustc_infer/src/infer/relate/nll.rs
@@ -261,54 +261,6 @@ where
         Ok(a)
     }
 
-    #[instrument(skip(self), level = "debug")]
-    fn enter_forall_and_leak_universe<T>(&mut self, binder: ty::Binder<'tcx, T>) -> T
-    where
-        T: ty::TypeFoldable<TyCtxt<'tcx>> + Copy,
-    {
-        if let Some(inner) = binder.no_bound_vars() {
-            return inner;
-        }
-
-        let mut next_region = {
-            let nll_delegate = &mut self.delegate;
-            let mut lazy_universe = None;
-
-            move |br: ty::BoundRegion| {
-                // The first time this closure is called, create a
-                // new universe for the placeholders we will make
-                // from here out.
-                let universe = lazy_universe.unwrap_or_else(|| {
-                    let universe = nll_delegate.create_next_universe();
-                    lazy_universe = Some(universe);
-                    universe
-                });
-
-                let placeholder = ty::PlaceholderRegion { universe, bound: br };
-                debug!(?placeholder);
-                let placeholder_reg = nll_delegate.next_placeholder_region(placeholder);
-                debug!(?placeholder_reg);
-
-                placeholder_reg
-            }
-        };
-
-        let delegate = FnMutDelegate {
-            regions: &mut next_region,
-            types: &mut |_bound_ty: ty::BoundTy| {
-                unreachable!("we only replace regions in nll_relate, not types")
-            },
-            consts: &mut |_bound_var: ty::BoundVar, _ty| {
-                unreachable!("we only replace regions in nll_relate, not consts")
-            },
-        };
-
-        let replaced = self.infcx.tcx.replace_bound_vars_uncached(binder, delegate);
-        debug!(?replaced);
-
-        replaced
-    }
-
     fn enter_forall<T, U>(
         &mut self,
         binder: ty::Binder<'tcx, T>,
@@ -317,7 +269,46 @@ where
     where
         T: ty::TypeFoldable<TyCtxt<'tcx>> + Copy,
     {
-        let value = self.enter_forall_and_leak_universe(binder);
+        let value = if let Some(inner) = binder.no_bound_vars() {
+            inner
+        } else {
+            let mut next_region = {
+                let nll_delegate = &mut self.delegate;
+                let mut lazy_universe = None;
+
+                move |br: ty::BoundRegion| {
+                    // The first time this closure is called, create a
+                    // new universe for the placeholders we will make
+                    // from here out.
+                    let universe = lazy_universe.unwrap_or_else(|| {
+                        let universe = nll_delegate.create_next_universe();
+                        lazy_universe = Some(universe);
+                        universe
+                    });
+
+                    let placeholder = ty::PlaceholderRegion { universe, bound: br };
+                    debug!(?placeholder);
+                    let placeholder_reg = nll_delegate.next_placeholder_region(placeholder);
+                    debug!(?placeholder_reg);
+
+                    placeholder_reg
+                }
+            };
+
+            let delegate = FnMutDelegate {
+                regions: &mut next_region,
+                types: &mut |_bound_ty: ty::BoundTy| {
+                    unreachable!("we only replace regions in nll_relate, not types")
+                },
+                consts: &mut |_bound_var: ty::BoundVar, _ty| {
+                    unreachable!("we only replace regions in nll_relate, not consts")
+                },
+            };
+
+            self.infcx.tcx.replace_bound_vars_uncached(binder, delegate)
+        };
+
+        debug!(?value);
         f(self, value)
     }
 
diff --git a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs b/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
index daa4977a1e0..8dec04e2c4f 100644
--- a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
+++ b/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
@@ -59,7 +59,9 @@ pub(in crate::solve) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
 
         ty::Closure(_, args) => Ok(vec![ty::Binder::dummy(args.as_closure().tupled_upvars_ty())]),
 
-        ty::CoroutineClosure(_, args) => Ok(vec![args.as_coroutine_closure().tupled_upvars_ty()]),
+        ty::CoroutineClosure(_, args) => {
+            Ok(vec![ty::Binder::dummy(args.as_coroutine_closure().tupled_upvars_ty())])
+        }
 
         ty::Coroutine(_, args) => {
             let coroutine_args = args.as_coroutine();
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
index a1c55091c26..803379af005 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
+++ b/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)
             })
         }
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
index 2706b5c8e0d..69b69930432 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
+++ b/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);
 
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index 53307da1d9b..044832224e5 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -1060,7 +1060,6 @@ 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))
                         })
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 046b2718dba..5410a74199c 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/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(),
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index 0f253981930..6d787a0ea6e 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -2049,7 +2049,6 @@ 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)
                         {
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index a1894de311e..d056dd51f50 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -728,64 +728,63 @@ 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);
+            })
         })
     }
 
diff --git a/compiler/rustc_type_ir/src/region_kind.rs b/compiler/rustc_type_ir/src/region_kind.rs
index a85e86f41ac..777c649fabd 100644
--- a/compiler/rustc_type_ir/src/region_kind.rs
+++ b/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.