Tracking issue for std::iter::repeat_with

[Centril]

This is the tracking issue for std::iter::repeat_with.
Currently in nightly, PR = #48156.

[Centril]

@SimonSapin Would stabilizing this be appropriate now?

[SimonSapin]

The iterator_repeat_with feature covers these APIs in std::iter:

pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> { /*…*/ }
pub struct RepeatWith<F> { /*…*/ }
impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> { type Item = A; /* … */ }
impl<A, F: FnMut() -> A> DoubleEndedIterator for RepeatWith<F> { /*…*/ }
impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {}
unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {}

They’ve been in Nightly since February 15 and seem fine to me to stabilize.

@rfcbot fcp merge

[rfcbot]

Team member @SimonSapin has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Kimundi
• @SimonSapin
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler
• @withoutboats

Concerns:

• DEI resolved by Tracking issue for std::iter::repeat_with #48169 (comment)

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[dtolnay]

@rfcbot concern DEI

The DoubleEndedIterator impl is somewhat surprising. I see this was brought up in #48156 (review) as well. While this has been in nightly has anyone observed use cases that benefit from having the DoubleEndedIterator impl?

[Centril]

@dtolnay I haven't; DEI for RepeatWith doesn't seem useful on its own; and given
repeat_with(|| ..).zip(my_dei).rev() you can reorder as repeat_with(|| ..).zip(my_dei.rev()). This also applies to .chain(..). However, I think forcing the user to re-order might not be such a good thing from an ergonomics perspective. But... We can always get rid of the DEI impl now and re-add it if anyone asks for it.

[dtolnay]

I would accept this without a DoubleEndedIterator impl and then follow up when someone can explain their concrete use case for it.

[Centril]

@dtolnay Fair enough, sounds like a plan 👍.

[dtolnay]

@rfcbot resolved DEI

[Kimundi]

Removing the DEI impl seems like the best course of action, yeah.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.