{"ast_node_type":"function","tokens":[{"end":73,"kind":"keyword","start":0,"text":"fn"}]}
    fn is_fn_ref(ty: Ty<'tcx>) -> Option<(DefId, GenericArgsRef<'tcx>)> {
    /// returns that function's `DefId` and `GenericArgsRef`.
        let referent_ty = match ty.kind() {