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Auto merge of #114231 - ttsugriy:binary_search_slice, r=cjgillot

[rustc_data_structures] Use partition_point to find slice range end. This PR uses approach introduced in #114152 to find the end of the range. It's much easier to understand and reason about invariants of such implementation. Technically it's possible to make it even shorter by returning `&[start..end]` unconditionally because even if searched item is not present in the slice, `start` and `end` would point at the same index, so the range would be empty. The reason I decided not to use this shorter implementation is because it would involve more comparisons in case there are no elements in the slice with key equal to `key`. Also, not that it matters much, but this implementation also improves perf according to the benchmark below: https://gist.github.com/ttsugriy/63c0ed39ae132b131931fa1f8a3dea55 The results on my M1 macbook air are: ``` Running benches/bin_search_slice_benchmark.rs (target/release/deps/bin_search_slice_benchmark-90fa6d68c3bd1298) Benchmarking multiply add/binary_search_slice: Collecting 100 samples in estimated 5.0002 s (1 multiply add/binary_search_slice time: [44.719 ns 44.918 ns 45.158 ns] No change in performance detected. Found 3 outliers among 100 measurements (3.00%) 1 (1.00%) high mild 2 (2.00%) high severe Benchmarking multiply add/binary_search_slice_new: Collecting 100 samples in estimated 5.0001 multiply add/binary_search_slice_new time: [36.955 ns 37.060 ns 37.221 ns] No change in performance detected. Found 7 outliers among 100 measurements (7.00%) 3 (3.00%) high mild 4 (4.00%) high severe ```

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compiler/rustc_data_structures/src/binary_search_util
- mod.rs

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`‎compiler/rustc_data_structures/src/binary_search_util/mod.rs`

Lines changed: 4 additions & 21 deletions

Original file line number	Diff line number	Diff line change
`@@ -18,27 +18,10 @@ where`
`18`	`18`	`return &[];`
`19`	`19`	`};`
`20`	`20`
`21`		`- // Now search forward to find the last one.`
`22`		`- let mut end = start;`
`23`		`- let mut previous = start;`
`24`		`- let mut step = 1;`
`25`		`- loop {`
`26`		`- end = end.saturating_add(step).min(size);`
`27`		`- if end == size \|\| key_fn(&data[end]) != *key {`
`28`		`- break;`
`29`		`- }`
`30`		`- previous = end;`
`31`		`- step *= 2;`
`32`		`- }`
`33`		`- step = end - previous;`
`34`		`- while step > 1 {`
`35`		`- let half = step / 2;`
`36`		`- let mid = end - half;`
`37`		`- if key_fn(&data[mid]) != *key {`
`38`		`- end = mid;`
`39`		`- }`
`40`		`- step -= half;`
`41`		`- }`
	`21`	+ // Find the first entry with key > `key`. Skip `start` entries since
	`22`	`+ // key_fn(&data[start]) == *key`
	`23`	`+ let offset = start + 1;`
	`24`	`+ let end = data[offset..].partition_point(\|x\| key_fn(x) <= *key) + offset;`
`42`	`25`
`43`	`26`	`&data[start..end]`
`44`	`27`	`}`

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`‎compiler/rustc_data_structures/src/binary_search_util/mod.rs`

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