fn equate<'tcx>(
    infcx: &InferCtxt<'tcx>,
    impl_env: ParamEnv<'tcx>,
    subject1: ImplSubject<'tcx>,
    subject2: ImplSubject<'tcx>,
    obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
    body_def_id: DefId
) -> bool