Assert that the trait ref does not need inference

src/librustc_ty/instance.rs

@@ -89,6 +89,7 @@ fn resolve_associated_item<'tcx>(
                 param_env, trait_item, rcvr_substs, impl_data
             );
             assert!(!rcvr_substs.needs_infer());
+            assert!(!trait_ref.needs_infer());
 
             let trait_def_id = tcx.trait_id_of_impl(impl_data.impl_def_id).unwrap();
             let trait_def = tcx.trait_def(trait_def_id);