fn drop_tys_helper<'tcx>(
tcx: TyCtxt<'tcx>,
ty: Ty<'tcx>,
param_env: ParamEnv<'tcx>,
adt_has_dtor: impl Fn(AdtDef<'tcx>) -> Option<DtorType>,
only_significant: bool,
) -> impl Iterator<Item = Result<Ty<'tcx>, AlwaysRequiresDrop>>