Lower RPITIT to ImplTraitPlaceholder item

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1465,6 +1465,11 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
     obligation: &ProjectionTyObligation<'tcx>,
     candidate_set: &mut ProjectionCandidateSet<'tcx>,
 ) {
+    // Can't assemble candidate from impl for RPITIT
+    if selcx.tcx().def_kind(obligation.predicate.item_def_id) == DefKind::ImplTraitPlaceholder {
+        return;
+    }
+
     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
     // start out by selecting the predicate `T as TraitRef<...>`:
     let poly_trait_ref = ty::Binder::dummy(obligation.predicate.trait_ref(selcx.tcx()));