Optimize Extend for String

[paolobarbolini]

Implements the idea in #148604 (comment).
Enhancement to #149694.

Could we get a perf run?

[rustbot]

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[Urgau]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 3f3acdf (3f3acdfbe44242af2a9a5419227d7c20c118425e, parent: fbab541a7ad1c22fc51783d03c7d75fa577f5633)

[rust-timer]

Finished benchmarking commit (3f3acdf): 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
Regressions ❌ (secondary)	0.1%	[0.1%, 0.2%]	3
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-0.4%	[-1.3%, -0.1%]	4
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Max RSS (memory usage)

Results (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)	-	-	0
Regressions ❌ (secondary)	1.1%	[0.8%, 1.7%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-6.4%, -0.9%]	11
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -3.2%, secondary 1.4%)

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)	3.7%	[2.1%, 6.6%]	7
Improvements ✅ (primary)	-3.2%	[-3.2%, -3.2%]	1
Improvements ✅ (secondary)	-6.5%	[-6.8%, -6.2%]	2
All ❌✅ (primary)	-3.2%	[-3.2%, -3.2%]	1

Binary size

Results (primary -0.1%, secondary -0.1%)

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)	-0.1%	[-1.1%, -0.0%]	30
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	23
All ❌✅ (primary)	-0.1%	[-1.1%, -0.0%]	30

Bootstrap: 471.701s -> 471.964s (0.06%)
Artifact size: 388.90 MiB -> 388.53 MiB (-0.09%)

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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

fmt: checked 6594 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (library)]: /checkout/library/alloc/src/string.rs:2638: TODO is used for tasks that should be done before merging a PR; If you want to leave a message in the codebase use FIXME
tidy [style (library)]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (25.3)
linting python files
---
linting javascript files and applying suggestions
Running eslint on rustdoc JS files
info: ES-Check: there were no ES version matching errors!  🎉
typechecking javascript files
tidy: The following check failed: style (library)
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/yarn --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1314:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:49
  local time: Sun Dec  7 10:19:04 UTC 2025
  network time: Sun, 07 Dec 2025 10:19:04 GMT
##[error]Process completed with exit code 1.

[paolobarbolini]

Interesting perf results. Let's see what happens when we specialize on array and Vec iterators...

@Urgau: Could we do another perf run please?

I hope this is ok for you @lolbinarycat since it takes from #148604, #148604 (comment) and #149694

[Urgau]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[rust-bors]

⌛ Trying commit b6110e5 with merge 522da80…

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

Workflow: https://github.com/rust-lang/rust/actions/runs/20003300074

[rust-bors]

💥 Test timed out after 21600s

[lolbinarycat]

I would appreciate a Co-Authored-by field on the first commit if you could (git rebase -i then edit on that commit)

[bjorn3]

Suggested change

	/// SAFETY: `impl AsRef<str> for S` must be stable
	/// SAFETY: When calling `<S as AsRef<str>::as_ref()` multiple times, the same value must be returned.

at first I thought you were talking about a consistent trait impl needing to apply in the face of specialization.