uucore: build without default features on Unix

[limadog9]

Closes #15339

[oech3]

What is the usecase for this?

[limadog9]

This preserves the implicit nix feature previously exposed by optional = true. Removing it would reject downstream manifests explicitly enabling features = ["nix"]. There’s no known in-tree consumer; it’s precautionary compatibility and adds no runtime behavior.

[oech3]

Downstream should just remove it. No?

[limadog9]

@oech3 Agreed. Removed in 70c67f2; downstream manifests can drop the feature. Both build configurations still pass.

[oech3]

Please squash commits.

[limadog9]

@oech3 squashed into one , good to go?

[oech3]

Thanks. Wait maintainer's review.

[limadog9]

@oech3 Updated in 91ed852: CI now uses cargo check, and both configurations pass on stable Rust and Rust 1.88. The empty nix feature preserves compatibility with downstream manifests that explicitly enable it.

This addresses #15339 and is ready for another look.

[codspeed]

Merging this PR will degrade performance by 4.63%

⚠️ 120 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• sort_unique_utf8_locale
• sort_numeric_utf8_locale
• cksum_default
• df_deep_directory
• cksum_multiple_files
• cksum_crc32b
• factor_multiple_u64s[2]
• du_deep_tree[(100, 3)]
• du_wide_tree[(5000, 500)]
• factor_64bit_semiprime
• …and 110 more

ℹ️ 1 benchmark has no flame graph

The flamegraph could not be generated because too much time was spent in runtime internals, making the visualization not useful for performance analysis.

• sort_dictionary_order[500000]

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 399 untouched benchmarks
⏩ 70 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	factor_three_39_bit_primes	567 ms	644.4 ms	-12.01%
⚡	Simulation	factor_thirteen_39_bit_primes	9.5 s	9.2 s	+3.37%

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 limadog9:fix/uucore-no-default-features (ccb86ea) with main (7654e67)}

Footnotes

1. 70 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. ↩

[xtqqczze]

I've got a better fix for this in a branch somewhere...

[xtqqczze]

Superseded by #15372.