{"ast_node_type":"variable","tokens":[{"end":67,"kind":"keyword","start":0,"text":"let"}]}
            let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
            infcx.sub_regions(SubregionOrigin::RelateObjectBound(span), r_b, r_a);
            let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };