Alloc `String::retain` optimization#150067

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.
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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

library/alloc/src/string.rs

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

library/alloc/src/string.rs

+        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

library/alloc/src/string.rs

+        // 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

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

2621d01

Alloc `String::retain` optimization

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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

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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 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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