O(1) count,nth,last for slice::{Windows,Chunks,ChunksMut}

[alex-ozdemir]

Provides a custom implementation of Iterator methods count, nth, and last for the structures slice::{Windows,Chunks,ChunksMut} in the core module.

These implementations run in constant time as opposed to the default implementations which run in linear time.

Addresses Issue #24214

r? @aturon

[rust-highfive]

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[bluss]

This addition may overflow. The methods for add with overflow might be appropriate, take the None case on overflow.

[alex-ozdemir]

I noticed that that said module is flagged as unstable - does that present an obstacle for using in in an impl which is stable?
Alternatively I could handle this manually.

[bluss]

Not an obstacle! There are several ways to do this, you can use .checked_add for example, but i thought overflowing_add would be the correct one for this particular line.

(saturating_add produces usize::MAX instead of overflowing, but that's not correct for the comparison end > self.len() — usize::MAX is not guaranteed to be larger).

[alex-ozdemir]

Addressed the well-raised concerns regarding overflow in the nth() methods.
Thanks @bluss!

I opted to use both checked and overflowing methods as I found convenient - is this seen as bad style?

[bluss]

Great! Handling overflow cases is of course an issue of correctness, not safety.

@bors r+

[bors]

📌 Commit b223abb has been approved by bluss

[bluss]

@bors r-

Actually, can you squash this together into just one commit? Since the commits partly change and change things back.

[bluss]

r=me with squash

[bluss]

Thank you! (No notifications go out from commit changes, need a message on the PR thread for that)

@bors r+

[bors]

📌 Commit e09f83e has been approved by bluss

[alex-ozdemir]

Cool - I'll remember to post after commits in the future.

[bors]

⌛ Testing commit e09f83e with merge 021389f...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt