perf(bench): use NeverGrowInPlaceAllocator for stable benchmark results

[stormslowly]

Summary

• Add NeverGrowInPlaceAllocator as #[global_allocator] in benchmarks to eliminate non-deterministic variance from libc::realloc in-place growth
• Only alloc/dealloc are delegated to System; realloc falls back to the default alloc-copy-dealloc path, representing the worst-case scenario consistently

Test plan

• cargo check --benches passes
• Run cargo bench and verify reduced variance in results

[gemini-code-assist]

Summary of Changes

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This pull request aims to enhance the reliability and determinism of benchmark results by standardizing memory allocation behavior. By preventing non-deterministic in-place reallocations, the changes ensure that benchmark runs consistently reflect the worst-case scenario for memory operations, leading to more stable and comparable performance metrics.

Highlights

• Benchmark Stability: Introduced NeverGrowInPlaceAllocator as a global allocator for benchmarks to prevent non-deterministic variance caused by libc::realloc's in-place growth.
• Memory Allocation Control: Configured the custom allocator to delegate alloc and dealloc to System, while forcing realloc to use the less efficient but consistent alloc-copy-dealloc path.

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[gemini-code-assist]

Code Review

This pull request introduces a custom global allocator, NeverGrowInPlaceAllocator, to benches/resolver.rs. This allocator is designed to ensure deterministic memory allocation behavior during benchmarks by forcing a copy-on-reallocate strategy, thus preventing non-deterministic in-place growth. The review suggests an improvement to remove redundant unsafe blocks within the alloc and dealloc implementations for better readability, as the functions themselves are already marked unsafe.

[Copilot]

Pull request overview

This PR aims to make benchmark results more stable by installing a custom #[global_allocator] for the benchmark binary that prevents benefiting from non-deterministic in-place growth behavior during realloc.

Changes:

• Add a NeverGrowInPlaceAllocator global allocator to the resolver benchmark.
• Delegate alloc/dealloc to System while relying on the default GlobalAlloc fallback behavior for realloc.

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[codspeed-hq]

Merging this PR will improve performance by 20.97%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 11 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	resolver[resolve from symlinks multi thread]	680.1 KB	562.2 KB	+20.97%

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_{Comparing feat/bench-never-grow-in-place-allocator (04d1d6d) with main (7162b33)}