{"ast_node_type":"variable","tokens":[{"end":57,"kind":"keyword","start":0,"text":"let"}]}
    let polarity = match tcx.impl_polarity(impl_def_id) {
        .item_name(tcx.trait_id_of_impl(impl_def_id.to_def_id()).expect("expected impl of trait"));
        ty::ImplPolarity::Positive | ty::ImplPolarity::Reservation => "",