Anonymize binders for refining_impl_trait check

compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs

@@ -5,7 +5,7 @@ use rustc_infer::infer::{outlives::env::OutlivesEnvironment, TyCtxtInferExt};
 use rustc_lint_defs::builtin::REFINING_IMPL_TRAIT;
 use rustc_middle::traits::{ObligationCause, Reveal};
 use rustc_middle::ty::{
-    self, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitor,
+    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperVisitable, TypeVisitable, TypeVisitor,
 };
 use rustc_span::{Span, DUMMY_SP};
 use rustc_trait_selection::traits::{
@@ -178,7 +178,8 @@ pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
 
     // For quicker lookup, use an `IndexSet`
     // (we don't use one earlier because it's not foldable..)
-    let trait_bounds = FxIndexSet::from_iter(trait_bounds);
+    let trait_bounds = FxIndexSet::from_iter(trait_bounds.fold_with(&mut Anonymize { tcx }));
+    let impl_bounds = impl_bounds.fold_with(&mut Anonymize { tcx });
 
     // Find any clauses that are present in the impl's RPITITs that are not
     // present in the trait's RPITITs. This will trigger on trivial predicates,
@@ -309,3 +310,20 @@ fn type_visibility<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<ty::Visibili
         _ => None,
     }
 }
+
+struct Anonymize<'tcx> {
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TypeFolder<TyCtxt<'tcx>> for Anonymize<'tcx> {
+    fn interner(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_binder<T>(&mut self, t: ty::Binder<'tcx, T>) -> ty::Binder<'tcx, T>
+    where
+        T: TypeFoldable<TyCtxt<'tcx>>,
+    {
+        self.tcx.anonymize_bound_vars(t)
+    }
+}

tests/ui/impl-trait/in-trait/anonymize-binders-for-refine.rs

@@ -0,0 +1,13 @@
+// compile-flags: --crate-type=lib
+// check-pass
+
+#![feature(return_position_impl_trait_in_trait)]
+#![deny(refining_impl_trait)]
+
+pub trait Tr<T> {
+    fn foo() -> impl for<'a> Tr<&'a Self>;
+}
+
+impl<T> Tr<T> for () {
+    fn foo() -> impl for<'a> Tr<&'a Self> {}
+}