Specialize `advance_back_by` like the forward version by cuviper · Pull Request #163361 · rust-lang/rust

cuviper · 2026-09-25T21:56:28Z

We specialized advance_by in #141086, so it makes sense to have
advance_back_by use the same trick in reverse, i.e. try_rfold.

This also updates nth_back to mirror nth, and the rest of the
DoubleEndedIterator methods already match Iterator.

We specialized `advance_by` in rust-lang#141086, so it makes sense to have `advance_back_by` use the same trick in reverse, i.e. `try_rfold`. This also updates `nth_back` to mirror `nth`, and the rest of the `DoubleEndedIterator` methods already match `Iterator`.

rustbot · 2026-09-25T21:56:33Z

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rustbot · 2026-09-25T21:56:34Z

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- cbc8109

cuviper · 2026-09-25T21:58:33Z

The previous PR used a Cycle<slice::Iter> to demonstrate the benefit, but that doesn't work in reverse. Here's a different example with Flatten: https://iter.godbolt.org/z/ojWT3n9xz

Darksonn

@bors r+

View changes since this review

rust-bors · 2026-09-29T12:09:00Z

📌 Commit cbc8109 has been approved by Darksonn

It is now in the queue for this repository.

Kobzol · 2026-09-29T13:08:33Z

@bors try @rust-timer queue

Specialize `advance_back_by` like the forward version

…rksonn Specialize `advance_back_by` like the forward version We specialized `advance_by` in rust-lang#141086, so it makes sense to have `advance_back_by` use the same trick in reverse, i.e. `try_rfold`. This also updates `nth_back` to mirror `nth`, and the rest of the `DoubleEndedIterator` methods already match `Iterator`.

…uwer Rollup of 3 pull requests Successful merges: - #163024 (Add `Dir` equivalents of `fs::metadata` & `fs::symlink_metadata`) - #163279 (const and NonZero impl for clamp_magnitude()) - #163361 (Specialize `advance_back_by` like the forward version)

rust-bors · 2026-09-29T14:08:28Z

☀️ Try build successful (CI)
Build commit: 12cc55c (12cc55c134530eab53fe88d9f828b527840a7ba2)
Base parent: b46c90c (b46c90c5619a16a0c7e4d0f4e9e1ad4db4cf106e)

rust-timer · 2026-09-29T15:30:35Z

Finished benchmarking commit (12cc55c): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.3%, 0.4%]	3
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.3%, 0.4%]	3

Max RSS (memory usage)

Results (primary 1.9%, secondary 2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.2%, 2.7%]	2
Regressions ❌ (secondary)	2.9%	[2.1%, 4.2%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	1.9%	[1.2%, 2.7%]	2

Cycles

Results (primary 4.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.5%	[2.0%, 7.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.5%	[2.0%, 7.0%]	2

Binary size

Results (primary 0.4%, secondary 0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.6%]	9
Regressions ❌ (secondary)	0.4%	[0.3%, 0.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.2%, 0.6%]	9

Bootstrap: 491.711s -> 492.163s (0.09%)
Artifact size: 406.38 MiB -> 406.43 MiB (0.01%)

cuviper · 2026-09-29T17:44:02Z

IMO (though biased as author) that perf change is not significant enough to worry about.

Kobzol · 2026-09-29T18:18:28Z

Yeah, it's fine. I just wanted to see if we'd see any improvements.

rustbot label: +perf-regression-triaged

rust-bors · 2026-09-29T18:49:57Z

☀️ Test successful - CI
Approved by: Darksonn
Duration: 3h 5m 6s
Pushing 4853db6 to main...

github-actions · 2026-09-29T18:53:33Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing ea6bb45 (parent) -> 4853db6 (this PR)

Test differences

Show 42 test diffs

42 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 4853db6f65c9edf8e52fdf3f4eb711d188d15fc6 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

optional-test-x86_64-gnu-autodiff: 33m 5s -> 57m 47s (+74.7%)
dist-powerpc64-linux-gnu: 1h 2m -> 1h 34m (+52.4%)
test-x86_64-gnu-miri: 55m -> 1h 23m (+51.3%)
test-i686-gnu-nopt-2: 1h 34m -> 2h 22m (+50.3%)
test-x86_64-msvc-ext3: 1h 28m -> 2h 2m (+38.8%)
test-pr-check-1: 33m 26s -> 20m 34s (-38.5%)
dist-armhf-linux: 1h 7m -> 1h 32m (+37.8%)
dist-s390x-linux: 1h 9m -> 1h 35m (+37.3%)
test-x86_64-gnu-distcheck: 1h 44m -> 2h 22m (+35.9%)
dist-i686-linux: 1h 19m -> 1h 46m (+34.7%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-09-29T19:35:03Z

Finished benchmarking commit (4853db6): comparison URL.

Overall result: ❌ regressions - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.3%, 0.4%]	3
Regressions ❌ (secondary)	0.3%	[0.3%, 0.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.3%, 0.4%]	3

Max RSS (memory usage)

Results (primary 0.7%, secondary 2.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.0%, 3.0%]	2
Regressions ❌ (secondary)	2.9%	[2.1%, 4.9%]	20
Improvements ✅ (primary)	-2.8%	[-2.8%, -2.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[-2.8%, 3.0%]	3

Cycles

Results (secondary 1.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.5%	[1.5%, 1.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.4%, secondary 0.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.6%]	15
Regressions ❌ (secondary)	0.3%	[0.3%, 0.4%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.2%, 0.6%]	15

Bootstrap: 489.575s -> 489.608s (0.01%)
Artifact size: 406.37 MiB -> 406.35 MiB (-0.00%)

rustbot assigned Darksonn Sep 25, 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 Sep 25, 2026

Darksonn approved these changes Sep 29, 2026

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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 Sep 29, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 29, 2026

Auto merge of #163361 - cuviper:sync-advance_back_by, r=<try>

12cc55c

Specialize `advance_back_by` like the forward version

JonathanBrouwer mentioned this pull request Sep 29, 2026

Rollup of 3 pull requests #163486

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Sep 29, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 29, 2026

rust-bors Bot merged commit 4853db6 into rust-lang:main Sep 29, 2026
15 checks passed

rustbot added this to the 1.101.0 milestone Sep 29, 2026

github-actions Bot mentioned this pull request Oct 3, 2026

Toolchain upgrade to nightly-2026-10-09 flux-rs/flux#1811

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rust-timer commented Sep 29, 2026

Overall result: ❌ regressions - please read:

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github-actions Bot commented Sep 29, 2026

Test differences

Job duration changes

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rust-timer commented Sep 29, 2026

Overall result: ❌ regressions - please read:

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