+ let sig = tcx.liberate_late_bound_regions(id.to_def_id(), tcx.fn_sig(id));
+ let sig = tcx.normalize_erasing_regions(ParamEnv::empty(), sig);
+
+ // We don't currently require that the function signature is equal to
+ // `fn(TokenStream) -> TokenStream`, but instead monomorphizes to
+ // `fn(TokenStream) -> TokenStream` after some substitution of generic arguments.
+ //
+ // Properly checking this means pulling in additional `rustc` crates, so we don't.
+ let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsInfer };