Optimize Extend for String

[paolobarbolini]

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

core::array::IntoIter::next_chunk currently doesn't generate great code, so this may bloat the binary size a bit.

Could we get a perf run?

[rustbot]

r? @joboet

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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 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 6590 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:2236: 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)]: /checkout/library/alloc/src/string.rs:2238: 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
tidy: The following check failed: style (library)
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 yarn` 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 `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:50
  local time: Sat Dec  6 09:12:47 UTC 2025
  network time: Sat, 06 Dec 2025 09:12:47 GMT
##[error]Process completed with exit code 1.

[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%)