Alloc `String::retain` optimization by fereidani · Pull Request #150067 · rust-lang/rust

fereidani · 2025-12-16T18:33:20Z

Hi again!
This uses the exact same algorithm as my other PR: #149784

Technically it should improve performance for String::retain too, But let's see what bors thinks.

rustbot · 2025-12-16T18:33:24Z

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

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ZuseZ4 · 2025-12-16T20:11:44Z

@bors try @rust-timer queue

Alloc `String::retain` optimization

rust-bors · 2025-12-16T22:28:36Z

☀️ Try build successful (CI)
Build commit: 2621d01 (2621d0177c40d6c9165ad462e5826558b57ea69a, parent: 31010ca61c3ff019e1480dda0a7ef16bd2bd51c0)

rust-timer · 2025-12-16T23:09:44Z

Finished benchmarking commit (2621d01): comparison URL.

Overall result: no relevant changes - no action needed

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.

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

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary -2.1%, secondary 0.5%)

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)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	-2.1%	[-2.1%, -2.1%]	1

Cycles

Results (primary 68.5%, secondary -2.4%)

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

	mean	range	count
Regressions ❌ (primary)	68.5%	[68.5%, 68.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	68.5%	[68.5%, 68.5%]	1

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%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 479.038s -> 478.434s (-0.13%)
Artifact size: 390.32 MiB -> 390.32 MiB (-0.00%)

Mark-Simulacrum · 2025-12-28T15:11:01Z

r? joboet

Probably makes sense to have the same reviewer for both.

rustbot · 2025-12-28T15:11:04Z

joboet is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2026-01-04T02:17:54Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

joboet

I think this could be even faster by avoiding some UTF-8 reencoding...

View changes since this review

rustbot · 2026-01-05T10:47:27Z

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

Kobzol · 2026-01-07T13:04:00Z

@bors try @rust-timer queue

rust-timer · 2026-01-07T13:04:03Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

rust-bors · 2026-01-07T13:04:05Z

⌛ Trying commit 590ba93 with merge 886f85e…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/20782421026

Alloc `String::retain` optimization

joboet · 2026-01-07T14:03:18Z

+        let string_ptr = ptr::NonNull::from(&mut *self);
+        let data_ptr = self.vec.as_mut_ptr();
+        let mut chars = self.char_indices();


This is unsound – data_ptr and string_ptr are both invalidated by the call to char_indices, as that creates a mutable reference to self and the str.

joboet · 2026-01-07T14:04:00Z

+        // Critical section starts here, at least one character is going to be removed.
+        let mut g = PanicGuard { s: string_ptr, read, write };
+        // Slower write-path
+        while let Some((read, ch)) = chars.next() {


You'll thus need to create the CharIndices iterator on every iteration.

joboet · 2026-01-07T14:04:27Z

@bors try cancel
Since this will need a rerun once fixed anyway.

rust-bors · 2026-01-07T14:04:31Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/20782421026

SpriteOvO · 2026-05-03T17:05:04Z

@fereidani Hi, ping from triage team. This PR has been inactive for a while. There are still some review comments that need to be addressed. Would you like to proceed with this PR? Thanks.

fereidani · 2026-05-03T18:46:57Z

@SpriteOvO @joboet Please excuse the delay. My country is currently at war, and our government has cut off internet access for our people for over two months now. Now that I have a somewhat workable connection, I’ll try to complete the PR within the next few days. Thank you for your understanding.

SpriteOvO · 2026-05-04T02:09:52Z

@fereidani Oh, so sorry to hear that. No need to hurry at all here. Feel free to come back to this when you have time, staying safe first.

rustbot assigned Mark-Simulacrum Dec 16, 2025

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 Dec 16, 2025

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rust-bors Bot added a commit that referenced this pull request Dec 16, 2025

Auto merge of #150067 - fereidani:string_retain, r=<try>

2621d01

Alloc `String::retain` optimization

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 16, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 16, 2025

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rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 3, 2026

fereidani force-pushed the string_retain branch from bc23951 to 875c63d Compare January 4, 2026 02:17

rustbot removed has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 4, 2026

fereidani mentioned this pull request Jan 5, 2026

Improve alloc Vec::retain_mut performance #149784

Merged

fereidani added 2 commits January 5, 2026 13:25

Add feature flag likely/unlikely for alloc

c68f67c

Improve and optimize string::retain implementation

2af59e0

fereidani force-pushed the string_retain branch from 66f0f66 to 2af59e0 Compare January 5, 2026 09:56

joboet requested changes Jan 5, 2026

View reviewed changes

Comment thread library/alloc/src/string.rs Outdated

Comment thread library/alloc/src/string.rs Outdated

Comment thread library/alloc/src/string.rs Outdated

rustbot 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 Jan 5, 2026

Improve string::retain implementation

590ba93

fereidani requested a review from joboet January 7, 2026 11:46

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 7, 2026

rust-bors Bot added a commit that referenced this pull request Jan 7, 2026

Auto merge of #150067 - fereidani:string_retain, r=<try>

886f85e

Alloc `String::retain` optimization

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 7, 2026

joboet requested changes Jan 7, 2026

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rustbot 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 Jan 7, 2026

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Overall result: no relevant changes - no action needed

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