tee: improve stdout performance

[xtqqczze]

Alternative to #12131

[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/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/dd/no-allocate is now passing!

[codspeed-hq]

Merging this PR will improve performance by ×17

⚠️ 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
✅ 316 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	tee_stdin_file[10000000]	3,970.8 µs	233.4 µs	×17

---

_{Comparing xtqqczze:tee-throughput3 (f4d31a6) with main (9bf907d)}

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. ↩

[xtqqczze]

@oech3 I don't know if this is sound on Windows.

[oech3]

How about mem::forget to block drop() completely?
I don't like new unsafe on non-Windows and dummy flush anyway.

[xtqqczze]

We need to use ManuallyDrop instead of mem::forget to guarantee the file descriptor is never dropped, even in the presence of early returns or panics. This ensures the unsafe code remains sound.

[oech3]

This is really bad limitation of Rust std...

[oech3]

Also manual bench on Windows is needed since this removed flush from Win too.

[xtqqczze]

This is really bad limitation of Rust std...

• Tracking Issue for stdin/stdout/stderr file descriptor constants rust-lang/rust#150836

[oech3]

Another possibility is adding URL to nightly std's code to Windows's unsafe code and copy paste it. Then people can use it with minimized concern.

Also I'm considering to add .write() to AsFd instead of .write_all() for cat instead of #12131 (comment) . But it would conflict.