{"ast_node_type":"variable","tokens":[{"end":40,"kind":"keyword","start":0,"text":"let"}]}
    let caller_sig = tcx.fn_sig(sig_id);
    let sig_id = tcx.hir_opt_delegation_sig_id(def_id).unwrap();
    if let Err(err) = check_constraints(tcx, def_id, sig_id) {