Replacing bound vars is actually instantiating a binder

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -1139,7 +1139,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         if let ty::Adt(def, substs) = ty.kind()
                             && Some(def.did()) == tcx.lang_items().pin_type()
                             && let ty::Ref(_, _, hir::Mutability::Mut) = substs.type_at(0).kind()
-                            && let self_ty = infcx.replace_bound_vars_with_fresh_vars(
+                            && let self_ty = infcx.instantiate_binder_with_fresh_vars(
                                 fn_call_span,
                                 LateBoundRegionConversionTime::FnCall,
                                 tcx.fn_sig(method_did).subst(tcx, method_substs).input(0),

compiler/rustc_borrowck/src/type_check/input_output.rs

@@ -38,7 +38,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         // so that they represent the view from "inside" the closure.
         let user_provided_sig = self
             .instantiate_canonical_with_fresh_inference_vars(body.span, &user_provided_poly_sig);
-        let user_provided_sig = self.infcx.replace_bound_vars_with_fresh_vars(
+        let user_provided_sig = self.infcx.instantiate_binder_with_fresh_vars(
             body.span,
             LateBoundRegionConversionTime::FnCall,
             user_provided_sig,

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -246,7 +246,7 @@ fn compare_method_predicate_entailment<'tcx>(
 
     let mut wf_tys = FxIndexSet::default();
 
-    let unnormalized_impl_sig = infcx.replace_bound_vars_with_fresh_vars(
+    let unnormalized_impl_sig = infcx.instantiate_binder_with_fresh_vars(
         impl_m_span,
         infer::HigherRankedType,
         tcx.fn_sig(impl_m.def_id).subst_identity(),
@@ -640,7 +640,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
     let impl_sig = ocx.normalize(
         &norm_cause,
         param_env,
-        infcx.replace_bound_vars_with_fresh_vars(
+        infcx.instantiate_binder_with_fresh_vars(
             return_span,
             infer::HigherRankedType,
             tcx.fn_sig(impl_m.def_id).subst_identity(),

compiler/rustc_hir_typeck/src/callee.rs

@@ -156,7 +156,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // fnmut vs fnonce. If so, we have to defer further processing.
                 if self.closure_kind(substs).is_none() {
                     let closure_sig = substs.as_closure().sig();
-                    let closure_sig = self.replace_bound_vars_with_fresh_vars(
+                    let closure_sig = self.instantiate_binder_with_fresh_vars(
                         call_expr.span,
                         infer::FnCall,
                         closure_sig,
@@ -437,7 +437,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // renormalize the associated types at this point, since they
         // previously appeared within a `Binder<>` and hence would not
         // have been normalized before.
-        let fn_sig = self.replace_bound_vars_with_fresh_vars(call_expr.span, infer::FnCall, fn_sig);
+        let fn_sig = self.instantiate_binder_with_fresh_vars(call_expr.span, infer::FnCall, fn_sig);
         let fn_sig = self.normalize(call_expr.span, fn_sig);
 
         // Call the generic checker.

compiler/rustc_hir_typeck/src/closure.rs

@@ -544,7 +544,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             )
             .map(|(hir_ty, &supplied_ty)| {
                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
-                self.replace_bound_vars_with_fresh_vars(
+                self.instantiate_binder_with_fresh_vars(
                     hir_ty.span,
                     LateBoundRegionConversionTime::FnCall,
                     // (*) binder moved to here
@@ -566,7 +566,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 all_obligations.extend(obligations);
             }
 
-            let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
+            let supplied_output_ty = self.instantiate_binder_with_fresh_vars(
                 decl.output.span(),
                 LateBoundRegionConversionTime::FnCall,
                 supplied_sig.output(),

compiler/rustc_hir_typeck/src/expr.rs

@@ -568,7 +568,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // placeholder lifetimes with probing, we just replace higher lifetimes
                     // with fresh vars.
                     let span = args.get(i).map(|a| a.span).unwrap_or(expr.span);
-                    let input = self.replace_bound_vars_with_fresh_vars(
+                    let input = self.instantiate_binder_with_fresh_vars(
                         span,
                         infer::LateBoundRegionConversionTime::FnCall,
                         fn_sig.input(i),
@@ -586,7 +586,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // Also, as we just want to check sizedness, instead of introducing
             // placeholder lifetimes with probing, we just replace higher lifetimes
             // with fresh vars.
-            let output = self.replace_bound_vars_with_fresh_vars(
+            let output = self.instantiate_binder_with_fresh_vars(
                 expr.span,
                 infer::LateBoundRegionConversionTime::FnCall,
                 fn_sig.output(),

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -289,7 +289,7 @@ impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
         item_segment: &hir::PathSegment<'_>,
         poly_trait_ref: ty::PolyTraitRef<'tcx>,
     ) -> Ty<'tcx> {
-        let trait_ref = self.replace_bound_vars_with_fresh_vars(
+        let trait_ref = self.instantiate_binder_with_fresh_vars(
             span,
             infer::LateBoundRegionConversionTime::AssocTypeProjection(item_def_id),
             poly_trait_ref,

compiler/rustc_hir_typeck/src/method/confirm.rs

@@ -262,7 +262,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
                     let original_poly_trait_ref = principal.with_self_ty(this.tcx, object_ty);
                     let upcast_poly_trait_ref = this.upcast(original_poly_trait_ref, trait_def_id);
                     let upcast_trait_ref =
-                        this.replace_bound_vars_with_fresh_vars(upcast_poly_trait_ref);
+                        this.instantiate_binder_with_fresh_vars(upcast_poly_trait_ref);
                     debug!(
                         "original_poly_trait_ref={:?} upcast_trait_ref={:?} target_trait={:?}",
                         original_poly_trait_ref, upcast_trait_ref, trait_def_id
@@ -285,7 +285,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
             probe::WhereClausePick(poly_trait_ref) => {
                 // Where clauses can have bound regions in them. We need to instantiate
                 // those to convert from a poly-trait-ref to a trait-ref.
-                self.replace_bound_vars_with_fresh_vars(poly_trait_ref).substs
+                self.instantiate_binder_with_fresh_vars(poly_trait_ref).substs
             }
         }
     }
@@ -506,7 +506,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         let sig = self.tcx.fn_sig(def_id).subst(self.tcx, all_substs);
         debug!("type scheme substituted, sig={:?}", sig);
 
-        let sig = self.replace_bound_vars_with_fresh_vars(sig);
+        let sig = self.instantiate_binder_with_fresh_vars(sig);
         debug!("late-bound lifetimes from method instantiated, sig={:?}", sig);
 
         (sig, method_predicates)
@@ -625,10 +625,10 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         upcast_trait_refs.into_iter().next().unwrap()
     }
 
-    fn replace_bound_vars_with_fresh_vars<T>(&self, value: ty::Binder<'tcx, T>) -> T
+    fn instantiate_binder_with_fresh_vars<T>(&self, value: ty::Binder<'tcx, T>) -> T
     where
         T: TypeFoldable<'tcx> + Copy,
     {
-        self.fcx.replace_bound_vars_with_fresh_vars(self.span, infer::FnCall, value)
+        self.fcx.instantiate_binder_with_fresh_vars(self.span, infer::FnCall, value)
     }
 }

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -401,7 +401,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // with bound regions.
         let fn_sig = tcx.fn_sig(def_id).subst(self.tcx, substs);
         let fn_sig =
-            self.replace_bound_vars_with_fresh_vars(obligation.cause.span, infer::FnCall, fn_sig);
+            self.instantiate_binder_with_fresh_vars(obligation.cause.span, infer::FnCall, fn_sig);
 
         let InferOk { value, obligations: o } =
             self.at(&obligation.cause, self.param_env).normalize(fn_sig);

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -924,7 +924,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             ty::AssocKind::Fn => self.probe(|_| {
                 let substs = self.fresh_substs_for_item(self.span, method.def_id);
                 let fty = self.tcx.fn_sig(method.def_id).subst(self.tcx, substs);
-                let fty = self.replace_bound_vars_with_fresh_vars(self.span, infer::FnCall, fty);
+                let fty = self.instantiate_binder_with_fresh_vars(self.span, infer::FnCall, fty);
 
                 if let Some(self_ty) = self_ty {
                     if self

compiler/rustc_infer/src/infer/equate.rs

@@ -129,7 +129,7 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
                 let a_types = infcx.tcx.anonymize_bound_vars(a_types);
                 let b_types = infcx.tcx.anonymize_bound_vars(b_types);
                 if a_types.bound_vars() == b_types.bound_vars() {
-                    let (a_types, b_types) = infcx.replace_bound_vars_with_placeholders(
+                    let (a_types, b_types) = infcx.instantiate_binder_with_placeholders(
                         a_types.map_bound(|a_types| (a_types, b_types.skip_binder())),
                     );
                     for (a, b) in std::iter::zip(a_types, b_types) {

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

@@ -38,13 +38,13 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
         // First, we instantiate each bound region in the supertype with a
         // fresh placeholder region. Note that this automatically creates
         // a new universe if needed.
-        let sup_prime = self.infcx.replace_bound_vars_with_placeholders(sup);
+        let sup_prime = self.infcx.instantiate_binder_with_placeholders(sup);
 
         // Next, we instantiate each bound region in the subtype
         // with a fresh region variable. These region variables --
         // but no other pre-existing region variables -- can name
         // the placeholders.
-        let sub_prime = self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, sub);
+        let sub_prime = self.infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, sub);
 
         debug!("a_prime={:?}", sub_prime);
         debug!("b_prime={:?}", sup_prime);
@@ -70,7 +70,7 @@ impl<'tcx> InferCtxt<'tcx> {
     ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
     #[instrument(level = "debug", skip(self), ret)]
-    pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
+    pub fn instantiate_binder_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
     where
         T: TypeFoldable<'tcx> + Copy,
     {

compiler/rustc_infer/src/infer/mod.rs

@@ -995,7 +995,7 @@ impl<'tcx> InferCtxt<'tcx> {
 
         Ok(self.commit_if_ok(|_snapshot| {
             let ty::SubtypePredicate { a_is_expected, a, b } =
-                self.replace_bound_vars_with_placeholders(predicate);
+                self.instantiate_binder_with_placeholders(predicate);
 
             let ok = self.at(cause, param_env).sub_exp(a_is_expected, a, b)?;
 
@@ -1008,7 +1008,7 @@ impl<'tcx> InferCtxt<'tcx> {
         cause: &traits::ObligationCause<'tcx>,
         predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
     ) {
-        let ty::OutlivesPredicate(r_a, r_b) = self.replace_bound_vars_with_placeholders(predicate);
+        let ty::OutlivesPredicate(r_a, r_b) = self.instantiate_binder_with_placeholders(predicate);
         let origin =
             SubregionOrigin::from_obligation_cause(cause, || RelateRegionParamBound(cause.span));
         self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
@@ -1447,7 +1447,14 @@ impl<'tcx> InferCtxt<'tcx> {
         value
     }
 
-    pub fn replace_bound_vars_with_fresh_vars<T>(
+    // Instantiates the bound variables in a given binder with fresh inference
+    // variables in the current universe.
+    //
+    // Use this method if you'd like to find some substitution of the binder's
+    // variables (e.g. during a method call). If there isn't a [`LateBoundRegionConversionTime`]
+    // that corresponds to your use case, consider whether or not you should
+    // use [`InferCtxt::instantiate_binder_with_placeholders`] instead.
+    pub fn instantiate_binder_with_fresh_vars<T>(
         &self,
         span: Span,
         lbrct: LateBoundRegionConversionTime,

compiler/rustc_infer/src/infer/sub.rs

@@ -161,7 +161,7 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
                 let a_types = infcx.tcx.anonymize_bound_vars(a_types);
                 let b_types = infcx.tcx.anonymize_bound_vars(b_types);
                 if a_types.bound_vars() == b_types.bound_vars() {
-                    let (a_types, b_types) = infcx.replace_bound_vars_with_placeholders(
+                    let (a_types, b_types) = infcx.instantiate_binder_with_placeholders(
                         a_types.map_bound(|a_types| (a_types, b_types.skip_binder())),
                     );
                     for (a, b) in std::iter::zip(a_types, b_types) {

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -74,7 +74,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                     )
                                 }
                                 ty::PredicateKind::Subtype(pred) => {
-                                    let (a, b) = infcx.replace_bound_vars_with_placeholders(
+                                    let (a, b) = infcx.instantiate_binder_with_placeholders(
                                         goal.predicate.kind().rebind((pred.a, pred.b)),
                                     );
                                     let expected_found = ExpectedFound::new(true, a, b);
@@ -84,7 +84,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                     )
                                 }
                                 ty::PredicateKind::Coerce(pred) => {
-                                    let (a, b) = infcx.replace_bound_vars_with_placeholders(
+                                    let (a, b) = infcx.instantiate_binder_with_placeholders(
                                         goal.predicate.kind().rebind((pred.a, pred.b)),
                                     );
                                     let expected_found = ExpectedFound::new(false, a, b);
@@ -94,7 +94,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                     )
                                 }
                                 ty::PredicateKind::ConstEquate(a, b) => {
-                                    let (a, b) = infcx.replace_bound_vars_with_placeholders(
+                                    let (a, b) = infcx.instantiate_binder_with_placeholders(
                                         goal.predicate.kind().rebind((a, b)),
                                     );
                                     let expected_found = ExpectedFound::new(true, a, b);

compiler/rustc_trait_selection/src/solve/infcx_ext.rs

@@ -26,7 +26,7 @@ pub(super) trait InferCtxtExt<'tcx> {
         rhs: T,
     ) -> Result<Vec<Goal<'tcx, ty::Predicate<'tcx>>>, NoSolution>;
 
-    fn instantiate_bound_vars_with_infer<T: TypeFoldable<'tcx> + Copy>(
+    fn instantiate_binder_with_infer<T: TypeFoldable<'tcx> + Copy>(
         &self,
         value: ty::Binder<'tcx, T>,
     ) -> T;
@@ -65,11 +65,11 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
             })
     }
 
-    fn instantiate_bound_vars_with_infer<T: TypeFoldable<'tcx> + Copy>(
+    fn instantiate_binder_with_infer<T: TypeFoldable<'tcx> + Copy>(
         &self,
         value: ty::Binder<'tcx, T>,
     ) -> T {
-        self.replace_bound_vars_with_fresh_vars(
+        self.instantiate_binder_with_fresh_vars(
             DUMMY_SP,
             LateBoundRegionConversionTime::HigherRankedType,
             value,

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -304,7 +304,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 }
             }
         } else {
-            let kind = self.infcx.replace_bound_vars_with_placeholders(kind);
+            let kind = self.infcx.instantiate_binder_with_placeholders(kind);
             let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
             let (_, certainty) = self.evaluate_goal(goal)?;
             self.make_canonical_response(certainty)

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -323,7 +323,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         {
             ecx.infcx.probe(|_| {
                 let assumption_projection_pred =
-                    ecx.infcx.instantiate_bound_vars_with_infer(poly_projection_pred);
+                    ecx.infcx.instantiate_binder_with_infer(poly_projection_pred);
                 let nested_goals = ecx.infcx.eq(
                     goal.param_env,
                     goal.predicate.projection_ty,

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -72,7 +72,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             // FIXME: Constness and polarity
             ecx.infcx.probe(|_| {
                 let assumption_trait_pred =
-                    ecx.infcx.instantiate_bound_vars_with_infer(poly_trait_pred);
+                    ecx.infcx.instantiate_binder_with_infer(poly_trait_pred);
                 let nested_goals = ecx.infcx.eq(
                     goal.param_env,
                     goal.predicate.trait_ref,

compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs

@@ -54,7 +54,7 @@ pub(super) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
         }
 
         ty::GeneratorWitness(types) => {
-            Ok(infcx.replace_bound_vars_with_placeholders(types).to_vec())
+            Ok(infcx.instantiate_binder_with_placeholders(types).to_vec())
         }
 
         ty::GeneratorWitnessMIR(..) => todo!(),
@@ -174,7 +174,7 @@ pub(super) fn instantiate_constituent_tys_for_copy_clone_trait<'tcx>(
         }
 
         ty::GeneratorWitness(types) => {
-            Ok(infcx.replace_bound_vars_with_placeholders(types).to_vec())
+            Ok(infcx.instantiate_binder_with_placeholders(types).to_vec())
         }
 
         ty::GeneratorWitnessMIR(..) => todo!(),

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

@@ -22,7 +22,7 @@ pub fn recompute_applicable_impls<'tcx>(
     let impl_may_apply = |impl_def_id| {
         let ocx = ObligationCtxt::new_in_snapshot(infcx);
         let placeholder_obligation =
-            infcx.replace_bound_vars_with_placeholders(obligation.predicate);
+            infcx.instantiate_binder_with_placeholders(obligation.predicate);
         let obligation_trait_ref =
             ocx.normalize(&ObligationCause::dummy(), param_env, placeholder_obligation.trait_ref);
 
@@ -47,11 +47,11 @@ pub fn recompute_applicable_impls<'tcx>(
     let param_env_candidate_may_apply = |poly_trait_predicate: ty::PolyTraitPredicate<'tcx>| {
         let ocx = ObligationCtxt::new_in_snapshot(infcx);
         let placeholder_obligation =
-            infcx.replace_bound_vars_with_placeholders(obligation.predicate);
+            infcx.instantiate_binder_with_placeholders(obligation.predicate);
         let obligation_trait_ref =
             ocx.normalize(&ObligationCause::dummy(), param_env, placeholder_obligation.trait_ref);
 
-        let param_env_predicate = infcx.replace_bound_vars_with_fresh_vars(
+        let param_env_predicate = infcx.instantiate_binder_with_fresh_vars(
             DUMMY_SP,
             LateBoundRegionConversionTime::HigherRankedType,
             poly_trait_predicate,

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

@@ -1716,7 +1716,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             let (values, err) = if let ty::PredicateKind::Clause(ty::Clause::Projection(data)) =
                 bound_predicate.skip_binder()
             {
-                let data = self.replace_bound_vars_with_fresh_vars(
+                let data = self.instantiate_binder_with_fresh_vars(
                     obligation.cause.span,
                     infer::LateBoundRegionConversionTime::HigherRankedType,
                     bound_predicate.rebind(data),