rename instantiate_binder_with_placeholders

compiler/rustc_infer/src/infer/mod.rs

@@ -1031,7 +1031,7 @@ impl<'tcx> InferCtxt<'tcx> {
             _ => {}
         }
 
-        // FIXME(tree_universes): leaking universes
+        // 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 +1042,7 @@ impl<'tcx> InferCtxt<'tcx> {
         cause: &traits::ObligationCause<'tcx>,
         predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
     ) {
-        // FIXME(tree_universes): leaking universes
+        // 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

@@ -49,11 +49,13 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
             debug!("b_prime={:?}", sup_prime);
 
             // Compare types now that bound regions have been replaced.
-            // FIXME(tree_universes): leaked dead universes
+            // 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:?}");
             }
+            // NOTE: returning the result here would be dangerous as it contains
+            // placeholders which **must not** be named afterwards.
             result.map(|_| ())
         })
     }
@@ -68,9 +70,11 @@ 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.
+    ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
     #[instrument(level = "debug", skip(self), ret)]
-    pub fn instantiate_binder_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
+    pub fn enter_forall_and_leak_universe<T>(&self, binder: ty::Binder<'tcx, T>) -> T
     where
         T: TypeFoldable<TyCtxt<'tcx>> + Copy,
     {
@@ -106,11 +110,24 @@ impl<'tcx> InferCtxt<'tcx> {
         self.tcx.replace_bound_vars_uncached(binder, delegate)
     }
 
+    /// 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.
+    ///
+    /// 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`.
+    ///
+    /// [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,
     {
-        let value = self.instantiate_binder_with_placeholders(forall);
+        let value = self.enter_forall_and_leak_universe(forall);
+        debug!("?value");
         f(value)
     }
 

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

@@ -262,7 +262,7 @@ where
     }
 
     #[instrument(skip(self), level = "debug")]
-    fn instantiate_binder_with_placeholders<T>(&mut self, binder: ty::Binder<'tcx, T>) -> T
+    fn enter_forall_and_leak_universe<T>(&mut self, binder: ty::Binder<'tcx, T>) -> T
     where
         T: ty::TypeFoldable<TyCtxt<'tcx>> + Copy,
     {
@@ -317,7 +317,7 @@ where
     where
         T: ty::TypeFoldable<TyCtxt<'tcx>> + Copy,
     {
-        let value = self.instantiate_binder_with_placeholders(binder);
+        let value = self.enter_forall_and_leak_universe(binder);
         f(self, value)
     }
 

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -140,7 +140,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                     )
                                 }
                                 ty::PredicateKind::Subtype(pred) => {
-                                    let (a, b) = infcx.instantiate_binder_with_placeholders(
+                                    let (a, b) = infcx.enter_forall_and_leak_universe(
                                         goal.predicate.kind().rebind((pred.a, pred.b)),
                                     );
                                     let expected_found = ExpectedFound::new(true, a, b);
@@ -150,7 +150,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                     )
                                 }
                                 ty::PredicateKind::Coerce(pred) => {
-                                    let (a, b) = infcx.instantiate_binder_with_placeholders(
+                                    let (a, b) = infcx.enter_forall_and_leak_universe(
                                         goal.predicate.kind().rebind((pred.a, pred.b)),
                                     );
                                     let expected_found = ExpectedFound::new(false, a, b);

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -358,8 +358,7 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 | ty::PredicateKind::Coerce(_)
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
                 | ty::PredicateKind::ConstEquate(..) => {
-                    let pred =
-                        ty::Binder::dummy(infcx.instantiate_binder_with_placeholders(binder));
+                    let pred = ty::Binder::dummy(infcx.enter_forall_and_leak_universe(binder));
                     ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
                 }
                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -250,8 +250,7 @@ pub(super) fn poly_project_and_unify_type<'cx, 'tcx>(
     let infcx = selcx.infcx;
     let r = infcx.commit_if_ok(|_snapshot| {
         let old_universe = infcx.universe();
-        let placeholder_predicate =
-            infcx.instantiate_binder_with_placeholders(obligation.predicate);
+        let placeholder_predicate = infcx.enter_forall_and_leak_universe(obligation.predicate);
         let new_universe = infcx.universe();
 
         let placeholder_obligation = obligation.with(infcx.tcx, placeholder_predicate);

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

@@ -666,7 +666,7 @@ 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.instantiate_binder_with_placeholders(poly_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() {

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

@@ -150,7 +150,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
         let placeholder_trait_predicate =
-            self.infcx.instantiate_binder_with_placeholders(trait_predicate).trait_ref;
+            self.infcx.enter_forall_and_leak_universe(trait_predicate).trait_ref;
         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
         let (def_id, args) = match *placeholder_self_ty.kind() {
@@ -393,7 +393,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 
             let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
-            let trait_ref = self.infcx.instantiate_binder_with_placeholders(poly_trait_ref);
+            let trait_ref = self.infcx.enter_forall_and_leak_universe(poly_trait_ref);
             let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
                 &cause,
                 obligation.recursion_depth + 1,
@@ -484,7 +484,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let tcx = self.tcx();
         debug!(?obligation, ?index, "confirm_object_candidate");
 
-        let trait_predicate = self.infcx.instantiate_binder_with_placeholders(obligation.predicate);
+        let trait_predicate = self.infcx.enter_forall_and_leak_universe(obligation.predicate);
         let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
         let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
         let ty::Dynamic(data, ..) = *self_ty.kind() else {
@@ -682,7 +682,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 
         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
-        let output_ty = self.infcx.instantiate_binder_with_placeholders(sig.output());
+        let output_ty = self.infcx.enter_forall_and_leak_universe(sig.output());
         let output_ty = normalize_with_depth_to(
             self,
             obligation.param_env,
@@ -703,7 +703,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Vec<PredicateObligation<'tcx>> {
         debug!(?obligation, "confirm_trait_alias_candidate");
 
-        let predicate = self.infcx.instantiate_binder_with_placeholders(obligation.predicate);
+        let predicate = self.infcx.enter_forall_and_leak_universe(obligation.predicate);
         let trait_ref = predicate.trait_ref;
         let trait_def_id = trait_ref.def_id;
         let args = trait_ref.args;

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

@@ -1606,7 +1606,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> smallvec::SmallVec<[usize; 2]> {
         let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
         let placeholder_trait_predicate =
-            self.infcx.instantiate_binder_with_placeholders(poly_trait_predicate);
+            self.infcx.enter_forall_and_leak_universe(poly_trait_predicate);
         debug!(?placeholder_trait_predicate);
 
         let tcx = self.infcx.tcx;
@@ -2365,7 +2365,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             .flat_map(|ty| {
                 let ty: ty::Binder<'tcx, Ty<'tcx>> = types.rebind(*ty); // <----/
 
-                let placeholder_ty = self.infcx.instantiate_binder_with_placeholders(ty);
+                let placeholder_ty = self.infcx.enter_forall_and_leak_universe(ty);
                 let Normalized { value: normalized_ty, mut obligations } =
                     ensure_sufficient_stack(|| {
                         project::normalize_with_depth(
@@ -2451,7 +2451,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         obligation: &PolyTraitObligation<'tcx>,
     ) -> Result<Normalized<'tcx, GenericArgsRef<'tcx>>, ()> {
         let placeholder_obligation =
-            self.infcx.instantiate_binder_with_placeholders(obligation.predicate);
+            self.infcx.enter_forall_and_leak_universe(obligation.predicate);
         let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
 
         let impl_args = self.infcx.fresh_args_for_item(obligation.cause.span, impl_def_id);

compiler/rustc_type_ir/src/region_kind.rs

@@ -164,7 +164,7 @@ pub enum RegionKind<I: Interner> {
     /// Should not exist outside of type inference.
     ///
     /// Used when instantiating a `forall` binder via
-    /// `infcx.enter_forall` and `infcx.instantiate_binder_with_placeholders`.
+    /// `infcx.enter_forall` and `infcx.enter_forall_and_leak_universe`.
     RePlaceholder(I::PlaceholderRegion),
 
     /// Erased region, used by trait selection, in MIR and during codegen.