break: pass len into spare_capacity_mut

[joseph-isaacs]

This is a common perf bottleneck since the capacity can be larger than the requested cap.

If the slice has a known length a compile can avoid bounds checks when writing to it

[AdamGS]

spare_capacity_mut is a common API in the std, I think staying familiar is a much better API.

[joseph-isaacs]

What do you propose avoid this common footgun all over the codebase

[codspeed-hq]

Merging this PR will degrade performance by 5.68%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ 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
❌ 4 regressed benchmarks
✅ 166 untouched benchmarks
⏩ 2244 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	filtered_owned_i64_avx512[OneNullInEight]	22.3 µs	26.3 µs	-15.09%
❌	dict_canonicalize_gt_u8_neon[1000000]	487.3 µs	559.3 µs	-12.87%
❌	mul_i32_nonnull_avx512	7.1 µs	7.9 µs	-10.8%
❌	dict_canonicalize_gt_u8_neon[16000000]	8.4 ms	9.3 ms	-10.12%
⚡	dict_canonicalize_gt_u8_avx2[16000000]	11.1 ms	8.8 ms	+25.89%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ji/buffer-spare-capacity-len (8aa5cef) with develop (f9093df)}

Footnotes

1. 2244 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

Dismissing my own review.