Rust: Tracking issue for DoubleEndedIterator::nth_back

[clarfonthey]

PR: #56802

Currently, none of the standard adapters specialise this except for Rev, which trivially swaps nth and nth_back. These should probably be added before this feature is stabilised, and it's probably a good idea to check if nth should be specialised for any of these adapters too.

Additionally, the standard collection iterators should specialise this where appropriate.

[clarfonthey]

Ping @alexcrichton (or whomever) to add the appropriate labels to this.

[SimonSapin]

https://github.com/rust-lang/rust/issues/54054 tracks adding more impls of nth_back, but I think this doesn’t need to block stabilization.

@rfcbot fcp merge

[rfcbot]

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

• [x] @Amanieu
• [ ] @Kimundi
• [x] @SimonSapin
• [x] @alexcrichton
• [x] @dtolnay
• [x] @sfackler
• [ ] @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), 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.

[rfcbot]

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

[rfcbot]

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

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

[sallyyu0]

Hello, I'm getting this error suddenly which points to this issue. Any clue how to fix it? Thanks!

Compiling lock_api v0.3.1
error[E0658]: use of unstable library feature 'iter_nth_back'
--> /home/zzz/.cargo/registry/src/github.com-1ecc6299db9ec823/ipnet-2.1.0/src/ipext.rs:638:5
|
638 | / fn nth_back(&mut self, n: usize) -> Option {
639 | | match *self {
640 | | IpAddrRange::V4(ref mut a) => a.nth_back(n).map(IpAddr::V4),
641 | | IpAddrRange::V6(ref mut a) => a.nth_back(n).map(IpAddr::V6),
642 | | }
643 | | }
| |_____^
|
= note: for more information, see https://github.com/rust-lang/rust/issues/56995

error[E0658]: use of unstable library feature 'iter_nth_back'

[clarfonthey]

Are you using an up to date nightly?

[koalatux]

error[E0658]: use of unstable library feature 'iter_nth_back'

nth_back() has been stabilized in Rust 1.37.0, have you tried updating to a recent version of Rust?