Alloc String::retain optimization

[fereidani]

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]

r? @Mark-Simulacrum

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[ZuseZ4]

@bors try @rust-timer queue

[rust-bors]

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

[rust-timer]

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%)