Function derive_cause

fn derive_cause<'tcx>(
    tcx: TyCtxt<'tcx>,
    candidate_kind: ProbeKind<TyCtxt<'tcx>>,
    cause: ObligationCause<'tcx>,
    idx: usize,
    parent_trait_pred: PolyTraitPredicate<'tcx>,
) -> ObligationCause<'tcx>