{"ast_node_type":"variable","tokens":[{"end":85,"kind":"keyword","start":0,"text":"let"}]}
            let arg_idx = args.iter().position(|a| a.hir_id == expr.hir_id).unwrap();
                                   kind: CallableKind| {
            let fn_ty = self.tcx.type_of(def_id).skip_binder();