{"ast_node_type":"function","tokens":[{"end":82,"kind":"keyword","start":0,"text":"fn"}]}
    fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
    #[instrument(level = "debug", skip(self, t2), ret)]
        assert_eq!(t, t2); // we are misusing TypeRelation here; both LHS and RHS ought to be ==