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std: implement clear via truncate#154095

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rust-lang:mainfrom
joboet:vec_clear_truncate
May 4, 2026
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std: implement clear via truncate#154095
rust-bors[bot] merged 1 commit into
rust-lang:mainfrom
joboet:vec_clear_truncate

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@joboet

@joboet joboet commented Mar 19, 2026

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This gets rid of some unsafe. truncate(0) is even documented to be equivalent to clear, this makes that equivalence even more obvious.

@rustbot 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 Mar 19, 2026
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rustbot commented Mar 19, 2026

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r? @Mark-Simulacrum

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@xtqqczze

xtqqczze commented Mar 19, 2026 •
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clear was intentionally implemented this way (rather than using truncate(0)) for performance reasons, as introduced in 9c59d04.

cc: @nnethercote

@Mark-Simulacrum

Mark-Simulacrum commented Mar 23, 2026

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r? @nnethercote

@nnethercote

nnethercote commented Mar 23, 2026

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I just wrote a microbenchmark and the difference between the two is still measurable. So I'd prefer to keep the code as is. It would be useful to add a comment explaining why truncate(0) is not used, if you are happy to do that? (I should have done that in #96002.)

@nnethercote nnethercote added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 26, 2026
@joboet joboet force-pushed the vec_clear_truncate branch from 9aa5814 to f8b83b2 Compare May 1, 2026 14:22
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rustbot commented May 1, 2026

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⚠️ Warning ⚠️

xtqqczze
xtqqczze reviewed May 1, 2026
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Comment thread library/alloc/src/vec/mod.rs
@@ -2992,6 +2985,10 @@ impl<T, A: Allocator> Vec<T, A> {
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn clear(&mut self) {
// Though this is equivalent to `truncate(0)`, the manual version
// optimizes better, justifying the additional complexity
// (see #96002 and #154095 for context).

@xtqqczze xtqqczze May 1, 2026 •
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maybe use full URLs for the GitHub issue links so they are navigable from IDEs?

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@nnethercote

nnethercote commented May 4, 2026

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@bors r+ rollup

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rust-bors Bot commented May 4, 2026

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📌 Commit f8b83b2 has been approved by nnethercote

It is now in the queue for this repository.

@rust-bors 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 4, 2026
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@rust-bors 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 May 4, 2026
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rust-bors Bot commented May 4, 2026

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☀️ Test successful - CI
Approved by: nnethercote
Duration: 3h 14m 22s
Pushing 783062d to main...

@rust-bors rust-bors Bot merged commit 783062d into rust-lang:main May 4, 2026
12 checks passed
@rustbot rustbot added this to the 1.97.0 milestone May 4, 2026
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github-actions Bot commented May 4, 2026

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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 a51f3a8 (parent) -> 783062d (this PR)

Test differences

Show 156 test diffs

156 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 783062d3c612d223e6841db4bbbc9bac4ab31ff4 --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

  1. dist-various-2: 34m 44s -> 44m 12s (+27.2%)
  2. i686-msvc-2: 1h 44m -> 2h 10m (+24.5%)
  3. dist-loongarch64-linux: 1h 28m -> 1h 49m (+23.1%)
  4. x86_64-gnu: 2h 12m -> 1h 48m (-18.2%)
  5. dist-sparcv9-solaris: 1h 28m -> 1h 14m (-16.0%)
  6. dist-x86_64-apple: 1h 53m -> 2h 10m (+15.1%)
  7. dist-aarch64-linux: 1h 59m -> 1h 45m (-11.7%)
  8. x86_64-msvc-ext2: 2h 5m -> 1h 51m (-11.2%)
  9. x86_64-msvc-ext1: 2h 20m -> 2h 5m (-10.5%)
  10. x86_64-gnu-stable: 2h 10m -> 2h 23m (+10.2%)
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

rust-timer commented May 4, 2026

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Finished benchmarking commit (783062d): comparison URL.

Overall result: ❌✅ regressions and improvements - 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
Regressions ❌
(secondary)		 0.2% [0.0%, 0.6%] 4
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -0.1% [-0.1%, -0.0%] 5
All ❌✅ (primary) - - 0

Max RSS (memory usage)

Results (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)		 - - 0
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -2.0% [-5.2%, -0.6%] 5
All ❌✅ (primary) - - 0

Cycles

Results (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)		 - - 0
Regressions ❌
(secondary)		 3.8% [1.7%, 6.0%] 11
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -0.9% [-3.1%, -0.4%] 7
All ❌✅ (primary) - - 0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 501.019s -> 495.891s (-1.02%)
Artifact size: 396.46 MiB -> 394.42 MiB (-0.51%)

@rustbot rustbot added the perf-regression Performance regression. label May 4, 2026
@panstromek

panstromek commented May 5, 2026

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perf triage:

These results are noise, this PR only adds comment.

@rustbot label: +perf-regression-triaged

@rustbot rustbot added the perf-regression-triaged The performance regression has been triaged. label May 5, 2026
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@joboet @rustbot @xtqqczze @Mark-Simulacrum @nnethercote @rust-timer @panstromek