{"ast_node_type":"function","tokens":[{"end":81,"kind":"keyword","start":0,"text":"fn"}]}
    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
        assert_eq!(a, b); // we are misusing TypeRelation here; both LHS and RHS ought to be ==