Ensure that new_self_ty has no escaping bound vars

Otherwise inserting it to the `Binder` used by `trait_ref` would cause
problems. This is just to be extra carefult: we aren't going to
start recommending that the user start using HKTs anytime soon.

src/librustc_trait_selection/traits/error_reporting/mod.rs

@@ -1385,6 +1385,8 @@ fn mk_trait_obligation_with_new_self_ty(
         trait_ref: &ty::PolyTraitRef<'tcx>,
         new_self_ty: Ty<'tcx>,
     ) -> PredicateObligation<'tcx> {
+        assert!(!new_self_ty.has_escaping_bound_vars());
+
         let trait_ref = trait_ref.map_bound_ref(|tr| ty::TraitRef {
             substs: self.tcx.mk_substs_trait(new_self_ty, &tr.substs[1..]),
             ..*tr