fn vars_since_snapshot<'tcx, T>(
    table: &mut UnificationTable<InPlace<T, &'_ mut UnificationStorage<T>, &'_ mut InferCtxtUndoLogs<'tcx>>>,
    snapshot_var_len: usize
) -> Range<T>where
    T: UnifyKey,
    UndoLog<'tcx>: From<UndoLog<Delegate<T>>>,