Specialize dropless arena for common alignment

[Zoxc]

This adds a dropless arena (AlignedDroplessArena) which is specialized for a specific alignment and AlignedDroplessArenas which dispatches to an appropriate arena based on the required alignment. This saves instructions for alignment and memory for padding, with the drawback of more active arenas.

This PR is LLM written.

[Zoxc]

Local benchmarks:

Benchmark	Before	After		Before	After		Before	After
	Time	Time	%	Physical Memory	Physical Memory	%	Committed Memory	Committed Memory	%
🟣 clap:check	1.3760s	1.3628s	-0.96%	150.90 MiB	150.80 MiB	-0.07%	197.79 MiB	198.06 MiB	0.14%
🟣 hyper:check	0.2528s	0.2534s	0.24%	80.71 MiB	80.70 MiB	-0.01%	128.66 MiB	128.77 MiB	0.09%
🟣 regex:check	0.7874s	0.7837s	-0.47%	113.43 MiB	113.53 MiB	0.09%	156.50 MiB	156.81 MiB	0.20%
🟣 syn:check	1.3187s	1.3124s	-0.48%	144.15 MiB	144.33 MiB	0.13%	186.56 MiB	187.12 MiB	0.30%
Total	3.7350s	3.7123s	-0.61%	489.19 MiB	489.36 MiB	0.04%	669.51 MiB	670.76 MiB	0.19%
Summary	1.0000s	0.9958s	-0.42%	1 byte	1.00 bytes	0.04%	1 byte	1.00 bytes	0.18%

[Zoxc]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e63fa69 (e63fa6952cf86b7a54e2bbe26d894851adeb716b)
Base parent: 8a3308e (8a3308ed506be56b8157dfc9242a145153331296)

[Zoxc]

@rust-timer build e63fa69

[rust-timer]

Finished benchmarking commit (e63fa69): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.3%	[0.3%, 0.3%]	2
Regressions ❌ (secondary)	0.7%	[0.2%, 1.0%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	8
All ❌✅ (primary)	0.3%	[0.3%, 0.3%]	2

Max RSS (memory usage)

Results (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)	5.0%	[1.9%, 7.0%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-4.9%, -2.0%]	12
All ❌✅ (primary)	-	-	0

Cycles

This perf run didn't have relevant results for this metric.

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 485.656s -> 484.454s (-0.25%)
Artifact size: 390.03 MiB -> 390.08 MiB (0.01%)