Implement advance_by, advance_back_by for iter::Chain by timvermeulen · Pull Request #77594 · rust-lang/rust (original) (raw)

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The code here all looks right to me, but I've become paranoid about early-exits and fusing in &mut self iterator methods.

Could you please add some tests for the edge cases of non-fused iterators? You're self.a = None;ing in the right places, AFAICT, but I don't think the tests right now would catch it if you weren't (since slice::Iter is always fused anyway).

If you need a non-fused iterator, you could use something like

pub fn make_silly_iterator(mut x: u8) -> impl Iterator<Item = u8> { std::iter::from_fn(move || { let i = x; x = x.wrapping_sub(1); if i == 0 { None } else { Some(i) } }) }