alloc: specialize String::extend for slices of str

[lolbinarycat]

Here's a small optimization via specialization that can potentially eliminate a fair few reallocations when building strings using specific patterns, unsure if this justifies the use of unsafe, but I had the code implemented from #148604, so I thought it was worth submitting to see.

[rustbot]

r? @Mark-Simulacrum

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.

Use r? to explicitly pick a reviewer

[Kobzol]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 281de2c (281de2c09f393bab648058208a34357b68b660d1, parent: 66428d92bec337ed4785d695d0127276a482278c)

[lolbinarycat]

Fixed CI failure and used saturating arithmetic

[rust-timer]

Finished benchmarking commit (281de2c): 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 (secondary -1.0%)

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)	1.7%	[0.9%, 2.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-6.2%, -1.2%]	2
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -6.4%, 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)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-6.4%	[-6.4%, -6.4%]	1
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-6.4%	[-6.4%, -6.4%]	1

Binary size

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

Bootstrap: 470.249s -> 470.338s (0.02%)
Artifact size: 386.85 MiB -> 386.82 MiB (-0.01%)

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 38ab135 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 38ab135 with merge fc4be63...

[bors]

💔 Test failed - checks-actions

[lolbinarycat]

looks like a spurious failure due to network disruption with the x86_64-apple builder.

@bors retry

[bors]

⌛ Testing commit 38ab135 with merge 67944d6...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 67944d6 to main...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 56f24e0 (parent) -> 67944d6 (this PR)

Test differences

Show 363 test diffs

363 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 67944d6bec6b8fbbc3870e5365ab73cf88a9de62 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. dist-apple-various: 4389.2s -> 3546.0s (-19.2%)
2. dist-aarch64-apple: 8699.4s -> 7144.5s (-17.9%)
3. x86_64-msvc-ext3: 5919.9s -> 6429.0s (+8.6%)
4. dist-sparcv9-solaris: 5356.8s -> 4912.4s (-8.3%)
5. dist-aarch64-msvc: 5829.8s -> 5357.5s (-8.1%)
6. dist-various-2: 2321.6s -> 2142.8s (-7.7%)
7. dist-x86_64-solaris: 5004.7s -> 5360.7s (+7.1%)
8. x86_64-gnu: 8235.9s -> 7705.5s (-6.4%)
9. dist-x86_64-llvm-mingw: 6624.8s -> 7024.0s (+6.0%)
10. dist-armv7-linux: 5176.4s -> 4884.5s (-5.6%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (67944d6): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	1

Max RSS (memory usage)

Results (secondary -1.0%)

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
Improvements ✅ (secondary)	-1.0%	[-1.0%, -1.0%]	1
All ❌✅ (primary)	-	-	0

Cycles

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)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 482.457s -> 484.762s (0.48%)
Artifact size: 390.83 MiB -> 390.83 MiB (0.00%)