Implement `DoubleEndedIterator::next_chunk_back` by asder8215 · Pull Request #156737 · rust-lang/rust

asder8215 · 2026-05-19T03:10:05Z

This PR builds off on @vinDelphini's PR on introducing DoubleEndedIterator::next_chunk_back (which will be used in a follow up PR to optimize ArrayChunks::try_rfold once merged). This was in the works since late Jan and I thought to pick it up and get this over the finish line as vinDelphini has been away from the PR for a couple of months. I've made sure to pull from vinDelphini's branch to keep him authored on the commits he made.

Other things introduced from this PR is specialization like done for Iterator::next_chunk, constifying some stuff, making a custom DoubleEndedIterator::next_chunk_back for IntoIter (since it also has a custom impl of Iterator::next_chunk), and a bunch of tests mirroring that of Iterator::next_chunk.

There's no tracking issue for this yet, but the ACP for this introduction is here. I'll create the tracking issue for this soon.

Actually, can we have DoubleEndedIterator::next_chunk_back underneath Iterator::next_chunk tracking issue?

rustbot · 2026-05-19T03:10:11Z

r? @tgross35

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asder8215 · 2026-06-11T19:51:41Z

r? libs

aapoalas

Some minor nits to pick and one case of "I don't think that's what we want to indicate to the compiler".

I'm also a little worried about adding const stuff as this has recently been a topic of discussion - this is in line with the fn next_chunk and doesn't seem like a problem in that sense, but I figured it's best to mention it as well.

View changes since this review

rustbot · 2026-06-13T13:07:18Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

asder8215 · 2026-06-13T16:04:33Z

I'm also a little worried about adding const stuff as #155816 - this is in line with the fn next_chunk and doesn't seem like a problem in that sense, but I figured it's best to mention it as well.

Yeah, I'm aware that the Rust libs team are cautious with constifying functions/methods. I figured in this case it was fine for consistency with Iterator::next_chunk, which seemed to have been constified from this PR.

asder8215 · 2026-06-13T19:11:37Z

Did they change how tidy work on const impl trait blocks recently? My tidy --bless automatically puts the const right before the trait name rather than how it recommends me to put it here before the impl.

Clarify that GuardBack initializes arrays from the end. Restore performance note for iter_next_chunk_erased regarding optimization issues.

I copied the impl block from Guard without noticing that Destruct was only there for const contexts. Consumed the suggestion, thanks. Co-authored-by: Chayim Refael Friedman <chayimfr@gmail.com>

…troduced next_chunk_back implementation in IntoIter, updated documentation, and added test cases for DoubleEndedIterator::next_chunk_back

asder8215 · 2026-06-17T22:26:43Z

@aapoalas @nia-e actually, these functions need const implementations. I just noticed that the DoubleEndedIterator trait has been marked const from this PR 4 months ago.

When @vinDelphini started working on this, that DoubleEndedIterator wasn't marked as a const trait, which I can see why his implementation didn't have const markings.

I think there's no other changes I can make to this PR.

Edit: Actually nevermind. We can use the #[rustc_non_const_trait_method] to create a non-const trait method within a const trait. However, I also feel like making DoubleEndedIterator::next_chunk_back non-const defeats the purpose with the DoubleEndedIterator trait being marked const and the implementation for this method is already possible to be made const.

asder8215 · 2026-06-24T17:31:47Z

@aapoalas I non-constified the DoubleEndedIterator::next_chunk_back. I put it in a separate commit for commit history in case we want to come back to constifying this in a future commit (let me know if you want me to remove it from history/squash it out).

aapoalas · 2026-07-01T16:00:44Z

Looks good to me, thank you and great work <3

@bors r+

rust-bors · 2026-07-01T16:00:48Z

📌 Commit ae8f56d has been approved by aapoalas

It is now in the queue for this repository.

@vinDelphini

…xt_chunk_back, r=aapoalas Implement `DoubleEndedIterator::next_chunk_back` This PR builds off on @vinDelphini's [PR](rust-lang#151668) on introducing `DoubleEndedIterator::next_chunk_back` (which will be used in a follow up PR to optimize `ArrayChunks::try_rfold` once merged). This was in the works since late Jan and I thought to pick it up and get this over the finish line as vinDelphini has been away from the PR for a couple of months. I've made sure to pull from vinDelphini's branch to keep him authored on the commits he made. Other things introduced from this PR is specialization like done for `Iterator::next_chunk`, constifying some stuff, making a custom `DoubleEndedIterator::next_chunk_back` for `IntoIter` (since it also has a custom impl of `Iterator::next_chunk`), and a bunch of tests mirroring that of `Iterator::next_chunk`. There's no tracking issue for this yet, but the ACP for this introduction is [here](rust-lang/libs-team#734). I'll create the tracking issue for this soon. ---- Actually, can we have `DoubleEndedIterator::next_chunk_back` underneath [`Iterator::next_chunk` tracking issue](rust-lang#98326)?

…uwer Rollup of 6 pull requests Successful merges: - #156737 (Implement `DoubleEndedIterator::next_chunk_back`) - #158147 (std: fix stack buffer overflow in Windows junction_point) - #158180 (std: use `OnceLock` for SGX environment variable storage) - #158427 (Implement `ptr::{read,write}_unaligned` via `repr(packed)`) - #158531 (Change `adjust_ident_and_get_scope` arg to `LocalDefId`) - #158364 (rustc_target/asm: add LoongArch LSX/LASX inline asm register support)

rust-bors · 2026-07-01T21:26:48Z

⌛ Testing commit ae8f56d with merge 33e0f9b...

Workflow: https://github.com/rust-lang/rust/actions/runs/28548849803

@vinDelphini

…ck, r=aapoalas Implement `DoubleEndedIterator::next_chunk_back` This PR builds off on @vinDelphini's [PR](#151668) on introducing `DoubleEndedIterator::next_chunk_back` (which will be used in a follow up PR to optimize `ArrayChunks::try_rfold` once merged). This was in the works since late Jan and I thought to pick it up and get this over the finish line as vinDelphini has been away from the PR for a couple of months. I've made sure to pull from vinDelphini's branch to keep him authored on the commits he made. Other things introduced from this PR is specialization like done for `Iterator::next_chunk`, constifying some stuff, making a custom `DoubleEndedIterator::next_chunk_back` for `IntoIter` (since it also has a custom impl of `Iterator::next_chunk`), and a bunch of tests mirroring that of `Iterator::next_chunk`. There's no tracking issue for this yet, but the ACP for this introduction is [here](rust-lang/libs-team#734). I'll create the tracking issue for this soon. ---- Actually, can we have `DoubleEndedIterator::next_chunk_back` underneath [`Iterator::next_chunk` tracking issue](#98326)?

jhpratt · 2026-07-01T21:43:00Z

yielding to encompassing rollup

@bors yield

rust-bors · 2026-07-01T21:43:04Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/28548849803

The next pull request likely to be tested is #158676.

Rollup of 8 pull requests Successful merges: - #156737 (Implement `DoubleEndedIterator::next_chunk_back`) - #158180 (std: use `OnceLock` for SGX environment variable storage) - #158427 (Implement `ptr::{read,write}_unaligned` via `repr(packed)`) - #158531 (Change `adjust_ident_and_get_scope` arg to `LocalDefId`) - #158574 (Clarify ExitStatusExt documentation) - #158334 (rustdoc: Show use-site paths for unevaluated const array lengths) - #158364 (rustc_target/asm: add LoongArch LSX/LASX inline asm register support) - #158667 (rustc_sanitizers: use twox-hash without default features)

@vinDelphini

Rollup merge of #156737 - asder8215:double_ended_iterator_next_chunk_back, r=aapoalas Implement `DoubleEndedIterator::next_chunk_back` This PR builds off on @vinDelphini's [PR](#151668) on introducing `DoubleEndedIterator::next_chunk_back` (which will be used in a follow up PR to optimize `ArrayChunks::try_rfold` once merged). This was in the works since late Jan and I thought to pick it up and get this over the finish line as vinDelphini has been away from the PR for a couple of months. I've made sure to pull from vinDelphini's branch to keep him authored on the commits he made. Other things introduced from this PR is specialization like done for `Iterator::next_chunk`, constifying some stuff, making a custom `DoubleEndedIterator::next_chunk_back` for `IntoIter` (since it also has a custom impl of `Iterator::next_chunk`), and a bunch of tests mirroring that of `Iterator::next_chunk`. There's no tracking issue for this yet, but the ACP for this introduction is [here](rust-lang/libs-team#734). I'll create the tracking issue for this soon. ---- Actually, can we have `DoubleEndedIterator::next_chunk_back` underneath [`Iterator::next_chunk` tracking issue](#98326)?

tgross35 · 2026-07-02T21:15:21Z

Note that this could have used a squash before merge, the commit history has a lot of back-and-forth (for future reference).

aapoalas · 2026-07-03T05:46:52Z

Note that this could have used a squash before merge, the commit history has a lot of back-and-forth (for future reference).

Oof, my bad :(

tgross35 · 2026-07-03T05:53:26Z

No worries, just another thing for the checklist :)

@vinDelphini

…=aapoalas Optimize `ArrayChunks::try_rfold` with `DoubleEndedIterator::next_chunk_back` Since `DoubleEndedIterator::next_chunk_back` got merged in rust-lang#156737, we can apply this method to optimize `ArrayChunk::try_rfold`. I added @vinDelphini as a co-author because he was the one who originally made this change in his original PR on introducing [`DoubleEndedIterator::next_chunk_back`](rust-lang#151668).

@vinDelphini

…=aapoalas Optimize `ArrayChunks::try_rfold` with `DoubleEndedIterator::next_chunk_back` Since `DoubleEndedIterator::next_chunk_back` got merged in rust-lang#156737, we can apply this method to optimize `ArrayChunk::try_rfold`. I added @vinDelphini as a co-author because he was the one who originally made this change in his original PR on introducing [`DoubleEndedIterator::next_chunk_back`](rust-lang#151668).

@vinDelphini

…=aapoalas Optimize `ArrayChunks::try_rfold` with `DoubleEndedIterator::next_chunk_back` Since `DoubleEndedIterator::next_chunk_back` got merged in rust-lang#156737, we can apply this method to optimize `ArrayChunk::try_rfold`. I added @vinDelphini as a co-author because he was the one who originally made this change in his original PR on introducing [`DoubleEndedIterator::next_chunk_back`](rust-lang#151668).

rustbot assigned tgross35 May 19, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 19, 2026

asder8215 force-pushed the double_ended_iterator_next_chunk_back branch from 7cb34f0 to 210d0a1 Compare May 19, 2026 03:11

asder8215 mentioned this pull request May 19, 2026

Tracking Issue for Iterator::next_chunk #98326

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traviscross reviewed May 31, 2026

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Comment thread library/alloc/src/vec/into_iter.rs

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vinDelphini and others added 4 commits June 13, 2026 15:18

Optimize ArrayChunks by adding next_chunk_back to DoubleEndedIterator

ae080eb

library: Improve docs for GuardBack and iter_next_chunk_erased

f72b302

Clarify that GuardBack initializes arrays from the end. Restore performance note for iter_next_chunk_erased regarding optimization issues.

Update library/core/src/array/mod.rs

91e0e59

I copied the impl block from Guard without noticing that Destruct was only there for const contexts. Consumed the suggestion, thanks. Co-authored-by: Chayim Refael Friedman <chayimfr@gmail.com>

Introduced specialization to DoubleEndedIterator::next_chunk_back, in…

bfd1ba6

…troduced next_chunk_back implementation in IntoIter, updated documentation, and added test cases for DoubleEndedIterator::next_chunk_back

asder8215 force-pushed the double_ended_iterator_next_chunk_back branch from 2ea9ac3 to bfd1ba6 Compare June 13, 2026 19:21

nia-e removed the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Jun 23, 2026

Non-constify DoubleEndedIterator::next_chunk_back

ae8f56d

asder8215 requested a review from aapoalas June 24, 2026 19:09

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 1, 2026

JonathanBrouwer mentioned this pull request Jul 1, 2026

Rollup of 6 pull requests #158670

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jhpratt mentioned this pull request Jul 1, 2026

Rollup of 8 pull requests #158676

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rust-bors Bot merged commit 666e69e into rust-lang:main Jul 2, 2026
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This was referenced Jul 2, 2026

Optimize ArrayChunks::try_rfold with DoubleEndedIterator::next_chunk_back #158704

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Optimize array chunks #151668

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asder8215 mentioned this pull request Jul 2, 2026

Add next_chunk_back to DoubleEndedIterator rust-lang/libs-team#734

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