Collect DefKind::AssocTy in new RPITITs on ImplTraitInTraitCollector

Author: spastorino

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -831,7 +831,14 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ImplTraitInTraitCollector<'_, 'tcx> {
 
     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
         if let ty::Alias(ty::Projection, proj) = ty.kind()
-            && self.interner().def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder
+            && (self.interner().def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder
+                || self
+                    .interner()
+                    .sess
+                    .opts
+                    .unstable_opts
+                    .lower_impl_trait_in_trait_to_assoc_ty
+                    && self.interner().def_kind(proj.def_id) == DefKind::AssocTy)
         {
             if let Some((ty, _)) = self.types.get(&proj.def_id) {
                 return *ty;