also remap RPITITs nested in other types back to their opaques

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -4,7 +4,7 @@ use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
 use rustc_infer::traits::util;
 use rustc_middle::ty::GenericArgs;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeFolder};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::Span;
 
@@ -214,7 +214,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTyToOpaque<'tcx> {
         {
             self.tcx.type_of(projection_ty.def_id).instantiate(self.tcx, projection_ty.args)
         } else {
-            ty
+            ty.super_fold_with(self)
         }
     }
 }

tests/ui/impl-trait/in-trait/nested-rpitit-bounds.rs

@@ -6,6 +6,10 @@ trait Foo {
     fn foo() -> impl Deref<Target = impl Deref<Target = impl Sized>> {
         &&()
     }
+
+    fn bar() -> impl Deref<Target = Option<impl Sized>> {
+        &Some(())
+    }
 }
 
 fn main() {}