Improve btree's unwrap_unchecked

[ssomers]

The seemingly optimized function unwrap_unchecked used in btree code appears to be slower than standard unwrap. Redirecting to unwrap as in this PR, we get some juicy improvements:

cargo-benchcmp.exe benchcmp old unwrap --threshold 10 name old ns/iter unwrap ns/iter diff ns/iter diff % speedup btree::map::iter_0 1,509 1,762 253 16.77% x 0.86 btree::map::iteration_1000 4,079 951 -3,128 -76.69% x 4.29 btree::map::iteration_100000 497,565 275,210 -222,355 -44.69% x 1.81 btree::map::iteration_20 81 20 -61 -75.31% x 4.05 btree::map::iteration_mut_1000 3,913 955 -2,958 -75.59% x 4.10 btree::map::iteration_mut_100000 480,135 277,160 -202,975 -42.27% x 1.73 btree::map::iteration_mut_20 77 19 -58 -75.32% x 4.05 btree::set::difference_random_100_vs_100 736 647 -89 -12.09% x 1.14 btree::set::difference_random_10k_vs_10k 187,103 161,630 -25,473 -13.61% x 1.16 btree::set::difference_staggered_100_vs_100 737 658 -79 -10.72% x 1.12 btree::set::difference_staggered_10k_vs_10k 71,737 64,181 -7,556 -10.53% x 1.12 btree::set::intersection_random_100_vs_100 624 334 -290 -46.47% x 1.87 btree::set::intersection_random_10k_vs_10k 162,862 120,976 -41,886 -25.72% x 1.35 btree::set::intersection_staggered_100_vs_100 636 354 -282 -44.34% x 1.80 btree::set::intersection_staggered_10k_vs_10k 60,853 33,245 -27,608 -45.37% x 1.83 btree::set::is_subset_100_vs_100 589 301 -288 -48.90% x 1.96 btree::set::is_subset_10k_vs_10k 59,345 29,873 -29,472 -49.66% x 1.99 

But this is also an opener for the obvious question: why?

• Changing the inline(always) to inline makes no difference.
• Removing the inline altogether has a terribly negative impact on many tests (up to factor 5) except a 40% improvement on clone tests.
• It doesn't happen in the laboratory. A standalone microbenchmark says that unwrap takes twice as long as unwrap_or_else(|| unreachable_unchecked())) for small and big option contents.
• There's one unwrap_unchecked call in a DropGuard that is averse to panic, but changing that alone doesn't change performance.

Putting on draft because it's weird and because it interferes with #73971.

[rust-highfive]

r? @withoutboats

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[bors]

☔ The latest upstream changes (presumably #73265) made this pull request unmergeable. Please resolve the merge conflicts.

[nbdd0121]

Link to the likely root cause #74615.

[LeSeulArtichaut]

Does someone from T-compiler want to review or help with this? Maybe r? @Mark-Simulacrum

[Mark-Simulacrum]

I am inclined to not try and change unwrap_unchecked since it seems likely that the improvements are optimizer noise (e.g., will be regressed/improved for various changes just because of slightly different inlining decisions and such), so I am going to close this, but if people feel differently feel free to reopen/let me know.

[ssomers]

Ralf seemed to like the added doc comment, when I split this off of some other PR.

[Mark-Simulacrum]

I am happy to accept the doc comment (either in this PR, reopened and adjusted, or a new one). That would be a quick review too :)

[ssomers]

Can't reopen so another 5 digit issue number sacrificed.