Just pass `Layout` directly to `box_new_uninit` by scottmcm · Pull Request #152737 · rust-lang/rust

scottmcm · 2026-02-17T00:42:12Z

We have a constant for it already (used in RawVec for basically the same polymorphization) so let's use it.

This is a simple follow-up to #148190 from one of the comments.

rustbot · 2026-02-17T00:42:17Z

rustbot has assigned @Mark-Simulacrum.
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scottmcm · 2026-02-17T00:49:22Z

Since we saw this code is perf-sensitive, to make sure it doesn't blow up...
@bors try @rust-timer queue

Just pass `Layout` directly to `box_new_uninit`

rust-bors · 2026-02-17T03:01:38Z

☀️ Try build successful (CI)
Build commit: 8dd6c97 (8dd6c970aade6a83f0d68d1c80913bde8aa79b32, parent: 3c9faa0d037b9eecda4a440cc482ff7f960fb8a5)

rust-timer · 2026-02-17T03:41:45Z

Finished benchmarking commit (8dd6c97): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	0.7%	[0.4%, 1.0%]	3
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	2
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	2
Improvements ✅ (secondary)	-1.0%	[-3.2%, -0.4%]	8
All ❌✅ (primary)	0.3%	[-0.4%, 1.0%]	5

Max RSS (memory usage)

Results (primary -2.3%, secondary 3.4%)

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)	3.4%	[2.4%, 4.2%]	5
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Cycles

Results (secondary -3.2%)

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)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.2%, 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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.0%]	19
Improvements ✅ (secondary)	-2.0%	[-4.2%, -0.7%]	15
All ❌✅ (primary)	-0.2%	[-0.5%, 0.3%]	20

Bootstrap: 480.798s -> 483.051s (0.47%)
Artifact size: 395.81 MiB -> 395.83 MiB (0.00%)

scottmcm · 2026-02-17T08:32:03Z

@bors try @rust-timer queue

Just pass `Layout` directly to `box_new_uninit`

rust-bors · 2026-02-17T10:43:28Z

☀️ Try build successful (CI)
Build commit: b517e4f (b517e4fd131ad3e507891d12ba6980349893b354, parent: d1a11b670b617f1370f7b1cdf86e225a4e070f15)

rust-timer · 2026-02-17T12:30:45Z

Finished benchmarking commit (b517e4f): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	0.8%	[0.6%, 0.9%]	5
Improvements ✅ (primary)	-0.7%	[-1.2%, -0.3%]	13
Improvements ✅ (secondary)	-1.8%	[-4.1%, -0.8%]	11
All ❌✅ (primary)	-0.6%	[-1.2%, 0.8%]	14

Max RSS (memory usage)

Results (primary 1.3%, secondary 2.1%)

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

	mean	range	count
Regressions ❌ (primary)	4.1%	[2.8%, 8.5%]	5
Regressions ❌ (secondary)	3.4%	[1.3%, 5.7%]	13
Improvements ✅ (primary)	-3.4%	[-3.5%, -3.2%]	3
Improvements ✅ (secondary)	-3.8%	[-4.1%, -3.4%]	3
All ❌✅ (primary)	1.3%	[-3.5%, 8.5%]	8

Cycles

Results (primary 2.3%, secondary -3.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-3.6%, -3.2%]	3
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

Results (primary -0.6%, secondary -2.1%)

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)	-0.6%	[-1.2%, -0.0%]	21
Improvements ✅ (secondary)	-2.1%	[-3.6%, -0.1%]	18
All ❌✅ (primary)	-0.6%	[-1.2%, -0.0%]	21

Bootstrap: 483.207s -> 482.56s (-0.13%)
Artifact size: 397.79 MiB -> 397.80 MiB (0.00%)

We have a constant for it already (used in `RawVec` for basically the same polymorphization) so let's use it. Conveniently, it can even be safe that way!

scottmcm · 2026-02-17T18:34:40Z

@bors r=RalfJung

rust-bors · 2026-02-17T18:34:42Z

📌 Commit 4b03b97 has been approved by RalfJung

It is now in the queue for this repository.

Just pass `Layout` directly to `box_new_uninit` We have a constant for it already (used in `RawVec` for basically the same polymorphization) so let's use it. This is a simple follow-up to #148190 from one of the comments.

rust-bors · 2026-02-22T00:41:41Z

💔 Test for c39eac6 failed: CI. Failed job:

auto - dist-aarch64-apple (web logs, enhanced plaintext logs)

scottmcm · 2026-02-22T00:48:50Z

@bors retry (Could not resolve host: static.crates.io)

rust-log-analyzer · 2026-02-22T00:54:36Z

The job dist-aarch64-apple failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

/dev/disk1s2     500Mi    20Ki   495Mi     1%       0  5.1M    0%   /System/Volumes/xarts
/dev/disk1s1     500Mi   116Ki   495Mi     1%      25  5.1M    0%   /System/Volumes/iSCPreboot
/dev/disk1s3     500Mi   224Ki   495Mi     1%      18  5.1M    0%   /System/Volumes/Hardware
/dev/disk3s5     320Gi   256Gi    46Gi    85%    2.2M  477M    0%   /System/Volumes/Data
/dev/disk5s1     4.0Mi   676Ki   3.0Mi    19%      18   31k    0%   /System/Library/AssetsV2/com_apple_MobileAsset_PKITrustStore/purpose_auto/6dd55b0d06633a00de6f57ccb910a66a5ba2409a.asset/.AssetData
map auto_home      0Bi     0Bi     0Bi   100%       0     0     -   /System/Volumes/Data/home
##[group]Run src/ci/scripts/setup-environment.sh
src/ci/scripts/setup-environment.sh
shell: /bin/bash --noprofile --norc -e -o pipefail {0}
---
##[endgroup]
==> Fetching downloads for: tidy-html5
✔︎ Bottle Manifest tidy-html5 (5.8.0)
✔︎ Bottle tidy-html5 (5.8.0)
==> Pouring tidy-html5--5.8.0.arm64_sequoia.bottle.tar.gz
🍺  /opt/homebrew/Cellar/tidy-html5/5.8.0: 15 files, 3.2MB
##[group]Run src/ci/scripts/install-wix.sh
src/ci/scripts/install-wix.sh
shell: /bin/bash --noprofile --norc -e -o pipefail {0}
---
hw.optional.arm.SME_I16I32: 0
hw.optional.arm.FEAT_SME_F64F64: 0
hw.optional.arm.FEAT_SME_I16I64: 0
hw.optional.arm.FP_SyncExceptions: 1
hw.optional.arm.caps: 292171059125284863
hw.optional.arm.sme_max_svl_b: 0
hw.optional.floatingpoint: 1
hw.optional.neon: 1
hw.optional.neon_hpfp: 1
hw.optional.neon_fp16: 1
hw.optional.armv8_crc32: 1
---
/dev/disk1s2     500Mi    20Ki   495Mi     1%       0  5.1M    0%   /System/Volumes/xarts
/dev/disk1s1     500Mi   116Ki   495Mi     1%      25  5.1M    0%   /System/Volumes/iSCPreboot
/dev/disk1s3     500Mi   224Ki   495Mi     1%      18  5.1M    0%   /System/Volumes/Hardware
/dev/disk3s5     320Gi   262Gi    39Gi    87%    2.6M  412M    1%   /System/Volumes/Data
/dev/disk5s1     4.0Mi   676Ki   3.0Mi    19%      18   31k    0%   /System/Library/AssetsV2/com_apple_MobileAsset_PKITrustStore/purpose_auto/6dd55b0d06633a00de6f57ccb910a66a5ba2409a.asset/.AssetData
map auto_home      0Bi     0Bi     0Bi   100%       0     0     -   /System/Volumes/Data/home
Post job cleanup.
[command]/opt/homebrew/bin/git version
git version 2.53.0
Copying '/Users/runner/.gitconfig' to '/Users/runner/work/_temp/353fb001-1286-4413-be62-d1e411de7ee7/.gitconfig'

rust-bors · 2026-02-22T05:54:55Z

☀️ Test successful - CI
Approved by: RalfJung
Duration: 4h 32m 53s
Pushing d8b2222 to main...

github-actions · 2026-02-22T05:57:28Z

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 5fb2ff8 (parent) -> d8b2222 (this PR)

Test differences

Show 222 test diffs

222 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 d8b2222b11fc03a31d43707c8c9006f025809ece --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

aarch64-apple: 3h 4m -> 4h 25m (+44.1%)
i686-gnu-2: 1h 28m -> 1h 59m (+35.4%)
dist-apple-various: 1h 57m -> 1h 27m (-25.7%)
pr-check-1: 27m 18s -> 33m 10s (+21.5%)
i686-msvc-1: 2h 59m -> 3h 33m (+18.9%)
dist-aarch64-llvm-mingw: 1h 38m -> 1h 55m (+17.2%)
x86_64-rust-for-linux: 44m 46s -> 51m 49s (+15.7%)
x86_64-gnu-miri: 1h 10m -> 1h 21m (+15.5%)
x86_64-gnu-tools: 55m 44s -> 1h 3m (+14.6%)
aarch64-gnu-llvm-20-2: 45m 14s -> 51m 24s (+13.6%)

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 · 2026-02-22T06:35:35Z

Finished benchmarking commit (d8b2222): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	0.7%	[0.5%, 0.8%]	6
Improvements ✅ (primary)	-0.8%	[-1.1%, -0.5%]	7
Improvements ✅ (secondary)	-1.8%	[-3.8%, -0.4%]	9
All ❌✅ (primary)	-0.5%	[-1.1%, 1.3%]	8

Max RSS (memory usage)

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)	5.5%	[3.8%, 7.2%]	2
Regressions ❌ (secondary)	3.3%	[2.6%, 4.1%]	5
Improvements ✅ (primary)	-2.8%	[-4.5%, -1.9%]	5
Improvements ✅ (secondary)	-3.3%	[-5.6%, -1.8%]	4
All ❌✅ (primary)	-0.4%	[-4.5%, 7.2%]	7

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)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-3.7%, -3.4%]	3
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.7%, secondary -2.1%)

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)	-0.7%	[-1.5%, -0.0%]	27
Improvements ✅ (secondary)	-2.1%	[-3.7%, -0.1%]	18
All ❌✅ (primary)	-0.7%	[-1.5%, -0.0%]	27

Bootstrap: 480.585s -> 481.835s (0.26%)
Artifact size: 395.91 MiB -> 397.84 MiB (0.49%)

scottmcm · 2026-02-22T22:06:03Z

Clearly more green than red overall.

The one primary regression in icounts (1.27%) looks like it's probably just codegen unit shuffling:

That case, while red in cycles and wall time, didn't reach the significance threshold for either.

So overall don't think there's anything needed here. The monomorphization structure changing is unsurprising to not be a pure win, but I think it's clearly an overall improvement. The binary size changes are particularly interesting: purely green, with even some opt-profile size improvements, not just debug ones.

@rustbot label: +perf-regression-triaged

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 17, 2026

rustbot assigned Mark-Simulacrum Feb 17, 2026

rustbot added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Feb 17, 2026

scottmcm mentioned this pull request Feb 17, 2026

Update exchange_malloc to take Alignment too #152685

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rust-bors bot pushed a commit that referenced this pull request Feb 17, 2026

Auto merge of #152737 - scottmcm:box_new_uninit_layout, r=<try>

8dd6c97

Just pass `Layout` directly to `box_new_uninit`

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scottmcm force-pushed the box_new_uninit_layout branch from f6af841 to d413781 Compare February 17, 2026 03:27

rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Feb 17, 2026

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scottmcm force-pushed the box_new_uninit_layout branch 2 times, most recently from 035fa58 to 9ed30a0 Compare February 17, 2026 08:31

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rust-bors bot pushed a commit that referenced this pull request Feb 17, 2026

Auto merge of #152737 - scottmcm:box_new_uninit_layout, r=<try>

b517e4f

Just pass `Layout` directly to `box_new_uninit`

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scottmcm added 2 commits February 17, 2026 09:09

Just pass Layout directly to box_new_uninit

1b767bf

We have a constant for it already (used in `RawVec` for basically the same polymorphization) so let's use it. Conveniently, it can even be safe that way!

What if we discourage mir-inlining of box_new_uninit?

4b03b97

scottmcm force-pushed the box_new_uninit_layout branch from 9ed30a0 to 4b03b97 Compare February 17, 2026 17:10

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 Feb 17, 2026

scottmcm mentioned this pull request Feb 17, 2026

replace box_new with lower-level intrinsics #148190

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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 Feb 22, 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 Feb 22, 2026

rust-bors bot merged commit d8b2222 into rust-lang:main Feb 22, 2026
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rustbot added this to the 1.95.0 milestone Feb 22, 2026

rust-bors bot mentioned this pull request Feb 22, 2026

Prepare NonNull for pattern types #152702

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rust-bors bot mentioned this pull request Feb 22, 2026

Fix ICE when ABI computation fails for unreachable/no-codegen instances #152306

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scottmcm deleted the box_new_uninit_layout branch February 22, 2026 21:58

rustbot added the perf-regression-triaged The performance regression has been triaged. label Feb 22, 2026

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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Test differences

Job duration changes

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Overall result: ❌✅ regressions and improvements - please read the text below

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