{"ast_node_type":"variable","tokens":[{"end":87,"kind":"keyword","start":0,"text":"let"}]}
            let impl_trait_ref = ocx.normalize(&cause, self.param_env, impl_trait_ref);
            // we just skip this impl, since it's not particularly useful.
            ocx.eq(&cause, self.param_env, base_ty, impl_trait_ref.self_ty())?;