give noalias back to refs in closures by RalfJung · Pull Request #162361 · rust-lang/rust

RalfJung · 2026-09-06T08:58:14Z

Reverts parts of #160745 to fix #163220.

Needs team approval; FCP is happening in #163220.

RalfJung · 2026-09-06T08:58:36Z

perf experiment: give noalias back to refs in closures

rust-bors · 2026-09-06T09:59:04Z

☀️ Try build successful (CI)
Build commit: 53adad8 (53adad86ea8dd1518c719893cfc8f19dc743b965)
Base parent: 46a266b (46a266b9fb0fb75e05fa4c9132f7355fcb62d4a4)

rust-timer · 2026-09-06T13:57:34Z

Finished benchmarking commit (53adad8): comparison URL.

Overall result: ❌✅ regressions and improvements - 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
Regressions ❌ (secondary)	0.3%	[0.3%, 0.4%]	4
Improvements ✅ (primary)	-0.3%	[-0.7%, -0.2%]	67
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.2%]	19
All ❌✅ (primary)	-0.3%	[-0.7%, -0.2%]	67

Max RSS (memory usage)

Results (primary -4.5%, secondary 1.2%)

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)	3.5%	[2.5%, 4.5%]	2
Improvements ✅ (primary)	-4.5%	[-4.5%, -4.5%]	1
Improvements ✅ (secondary)	-3.5%	[-3.5%, -3.5%]	1
All ❌✅ (primary)	-4.5%	[-4.5%, -4.5%]	1

Cycles

Results (primary 0.4%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[3.2%, 3.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-2.4%, 3.2%]	2

Binary size

Results (primary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	2

Bootstrap: 478.283s -> 476.002s (-0.48%)
Artifact size: 403.38 MiB -> 403.31 MiB (-0.02%)

shepmaster · 2026-09-23T13:17:20Z

I have a criterion benchmark of some AVX2 hashing code on x86_64 Windows which reports...

commit	speed (GiB/s)
`aa05936` (1.99.0-beta.7)	57.537
`1a82b5d` (main)	23.559
`1a82b5d` (main) + reverting the 4 commits from #160745	58.155
`1a82b5d` (main) + cherry-picking this commit	59.121

I am 👍 for the performance results from this PR 😉

(Investigation occurred on Zulip)

RalfJung · 2026-09-23T14:06:27Z

Is this a small piece of code you can share?

Either that code has a FnOnce closure that captures exactly 1 or 2 references and the noalias they get is somehow very important, or there's something going on we do not understand.

rustbot · 2026-09-23T16:21:24Z

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

rustbot · 2026-09-23T16:21:27Z

r? @Darksonn

rustbot has assigned @Darksonn.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: @ChrisDenton, libs
@ChrisDenton, libs expanded to 13 candidates
Random selection from 7 candidates

RalfJung · 2026-09-23T16:22:13Z

Let's make sure this still fixes the regression (the previous run didn't have the MaybeDangling for coroutines but we need that).
@bors try
@rust-timer queue

r? @saethlin

RalfJung · 2026-09-23T16:23:53Z

@bors try

give noalias back to refs in closures

RalfJung · 2026-09-23T16:27:37Z

                    || def.flags().contains(AdtFlags::IS_MANUALLY_DROP)
            }
-            ty::Closure(..) | ty::Coroutine(..) | ty::CoroutineClosure(..) => true,
+            ty::Coroutine(..) | ty::CoroutineClosure(..) => true,


I'm not entirely sure what a CoroutineClosure is / why it is a separate type. But I'll assume it contains a corotuine so we should have MaybeDangling there.

View changes since the review

rust-bors · 2026-09-23T17:28:37Z

☀️ Try build successful (CI)
Build commit: 47167be (47167bec2164397e62586212b9a876a49c2f8df7)
Base parent: 9a9d194 (9a9d1946d1e119676b7a6a8a26a264061e61027d)

rust-timer · 2026-09-23T18:38:43Z

Finished benchmarking commit (47167be): comparison URL.

Overall result: ❌✅ regressions and improvements - 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
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	4
Improvements ✅ (primary)	-0.3%	[-0.7%, -0.2%]	83
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.2%]	19
All ❌✅ (primary)	-0.3%	[-0.7%, -0.2%]	83

Max RSS (memory usage)

Results (primary -5.6%, secondary 0.8%)

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)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-5.6%	[-5.6%, -5.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-5.6%	[-5.6%, -5.6%]	1

Cycles

Results (secondary -11.8%)

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)	3.9%	[2.6%, 5.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-16.3%	[-32.3%, -3.3%]	7
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	3

Bootstrap: 489.957s -> 489.572s (-0.08%)
Artifact size: 407.04 MiB -> 406.27 MiB (-0.19%)

saethlin · 2026-09-23T21:32:54Z

@bors r+

rust-bors · 2026-09-23T21:32:57Z

📌 Commit a18393e has been approved by saethlin

It is now in the queue for this repository.

give noalias back to refs in closures Reverts parts of #160745 to fix #163220. Needs team approval; FCP is happening in #163220.

rust-bors · 2026-09-23T23:57:17Z

💔 Test for 7f1e616 failed: CI. Failed job:

auto - test-x86_64-gnu-distcheck (web logs, enhanced plaintext logs)

saethlin · 2026-09-24T00:16:31Z

GHA raw logs seem to be missing:

<Error>
<Code>BlobNotFound</Code>
<Message>
The specified blob does not exist. RequestId:73546f72-f01e-0013-25b9-4bf0fc000000 Time:2026-09-24T00:15:15.1139570Z
</Message>
</Error>

And in the web UI the logs look truncated. So a GitHub bug?

@bors retry

jhpratt · 2026-09-24T00:20:22Z

@bors try jobs=test-x86_64-gnu-distcheck

give noalias back to refs in closures try-job: test-x86_64-gnu-distcheck

rust-bors · 2026-09-24T02:52:36Z

☀️ Try build successful (CI)
Build commit: 4c92bf3 (4c92bf3119aad084e99e6e842500386e346dbab4)
Base parent: 6eeff9a (6eeff9a52c3e35c4c4cbf5651f342dcd2191866f)

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 6, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 6, 2026

Auto merge of #162361 - RalfJung:maybe-dangling-perf2, r=<try>

53adad8

perf experiment: give noalias back to refs in closures

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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 6, 2026

RalfJung mentioned this pull request Sep 6, 2026

make closures act like MaybeDangling #160745

Merged

shepmaster mentioned this pull request Sep 23, 2026

Performance regression in Criterion benchmark of AVX2 code on x86_64-pc-windows-msvc #163220

Open

RalfJung force-pushed the maybe-dangling-perf2 branch from d5af325 to 2d88470 Compare September 23, 2026 16:10

RalfJung marked this pull request as ready for review September 23, 2026 16:21

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 23, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 23, 2026

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rustbot assigned saethlin and unassigned Darksonn Sep 23, 2026

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 23, 2026

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RalfJung force-pushed the maybe-dangling-perf2 branch from cfdb944 to b0e171e Compare September 23, 2026 16:23

RalfJung changed the title ~~perf experiment: give noalias back to refs in closures~~ give noalias back to refs in closures Sep 23, 2026

rust-bors Bot pushed a commit that referenced this pull request Sep 23, 2026

Auto merge of #162361 - RalfJung:maybe-dangling-perf2, r=<try>

47167be

give noalias back to refs in closures

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RalfJung commented Sep 23, 2026

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update miri tests

a18393e

RalfJung force-pushed the maybe-dangling-perf2 branch from b0e171e to a18393e Compare September 23, 2026 17:02

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CAD97 approved these changes Sep 23, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 23, 2026

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 23, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 23, 2026

Auto merge of #162361 - RalfJung:maybe-dangling-perf2, r=saethlin

7f1e616

give noalias back to refs in closures Reverts parts of #160745 to fix #163220. Needs team approval; FCP is happening in #163220.

rust-bors Bot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 23, 2026

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 24, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 24, 2026

Auto merge of #162361 - RalfJung:maybe-dangling-perf2, r=<try>

4c92bf3

give noalias back to refs in closures try-job: test-x86_64-gnu-distcheck

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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