Implement ExactSizeIterator for `Zip<A, Repeat<B>> where A: ExactSizeIterator, B: Clone`

libs
1

This is a proposal to add two instances:

impl<A, B> ExactSizeIterator for Zip<A, Repeat<B>>
where
    A: ExactSizeIterator,
    B: Clone,
{
}

impl<A, B> ExactSizeIterator for Zip<Repeat<A>, B>
where
    A: Clone,
    B: ExactSizeIterator,
{
}

This would appear to be completely safe. .size_hint() will take the minimum of usize::MAX (from Repeat), and the other lower bound.

I've added these instances to core, and added this coretest:

#[test]
fn zip_repeat_bounds() {
    let iter = [1, 2, 3].into_iter().zip(repeat(2));
    let (lower, upper) = iter.size_hint();
    assert_eq!(lower, 3);
    assert_eq!(upper, Some(3));
    assert_eq!(iter.len(), 3);

    let iter = repeat(2).zip([1, 2, 3].into_iter());
    let (lower, upper) = iter.size_hint();
    assert_eq!(lower, 3);
    assert_eq!(upper, Some(3));
    assert_eq!(iter.len(), 3);
}

Which passes.

There is currently only one implementation of ExactSizeIterator for Zip:

impl<A, B> ExactSizeIterator for Zip<A, B>
where
    A: ExactSizeIterator,
    B: ExactSizeIterator {}

Which doesn't overlap with the proposed instances, as Repeat does not implement ExactSizeIterator.

If this seems sensible, I'll add a tracking issue, and PR the changes upstream.

2

You don't need a tracking issue for adding a new trait impl, as they are insta-stable. Just PR it

1 Like
3

Thanks, I was hoping that was the case.

4

Why can't Repeat be exact size? It is known to always be infinite after all.

5

What usize would you have .len() return for Repeat?

6

PR is up: Implement ExactSizeIterator for `Zip<A, Repeat<B>>` by 414owen · Pull Request #146642 · rust-lang/rust · GitHub

7

Ah, that is indeed a fair point. I only thought about size hint.

Edit: would adding a ExactInfinite trait make sense? Because the same logic could apply to other infinite iterators. Such as RepeatWith and probably several in the itertools crate.

8

Could yo remove “[Pre-RFC]” from the title since this thread does not (and should not) contain an RFC draft?

1 Like
9

Trait impls can be unstable now, actually! I haven't tried it out but I'm almost positive that was merged.

2 Likes
10

Yes. But this would require a language feature that allows marking InfiniteIterator and ExactSizeIterator as mutually exclusive traits.

3 Likes