locale: honor LC_ALL and LC_MESSAGES precedence over LANG

[costajohnt]

Fixes #8922.

Locale detection only read LANG, so LC_ALL and LC_MESSAGES were ignored. GNU honors all three, so LC_ALL=fr_FR.UTF-8 sleep --help prints French there but English in uutils.

This applies the POSIX message-locale precedence LC_ALL > LC_MESSAGES > LANG: the first of those that is set and non-empty wins (a set-but-empty value is treated as unset), otherwise it falls back to the existing en-US default. The winning value keeps the same .<encoding> stripping as before (fr_FR.UTF-8 -> fr_FR), so the LANG-only path is unchanged.

The precedence lives in a small pure helper that takes the env lookup as an argument, so it can be unit-tested without mutating process-global env vars (which race under parallel test threads):

fn resolve_locale_string(lookup: impl Fn(&str) -> Option<String>) -> String

detect_system_locale just calls it with std::env::var.

Behavior change

Anyone who has LC_ALL or LC_MESSAGES set to something other than LANG will now get that locale for messages instead of LANG. That is the intended fix for #8922, but noting it since it is user-visible.

Tests

Seven unit tests on the helper: LC_ALL wins, LC_MESSAGES when LC_ALL is unset, LANG still works, empty treated as unset, LC_ALL over LANG, LC_ALL over LC_MESSAGES, and nothing-set fallback.

cargo test, cargo clippy --workspace --all-targets --all-features -- -D warnings, and cargo fmt --check all pass.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will degrade performance by 4.5%

⚠️ 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

❌ 2 regressed benchmarks
✅ 337 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	unexpand_large_file[10]	282.3 ms	295.6 ms	-4.5%
❌	Simulation	unexpand_many_lines[100000]	134.7 ms	141.1 ms	-4.5%

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 costajohnt:fix/8922-lc-all-locale-precedence (6390f88) with main (3aa5f8a)}

Footnotes

1. 46 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. ↩