{"ast_node_type":"impl","tokens":[{"end":78,"kind":"keyword","start":0,"text":"variant:"}]}
        variant: impl FnOnce(DerivedCause<'tcx>) -> ObligationCauseCode<'tcx>,
        parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
    ) -> ObligationCause<'tcx> {