bd33b83cfd
Improve alloc `Vec::retain_mut` performance
Hi,
While reading the rustc source code, I noticed it uses `smallvec` and `thin-vec` in many places. I started reviewing those crates, optimized their `retain_mut` implementation, and then realized they were using the exact same algorithm as `alloc::vec::Vec` with less unsafe So now I’m back here with a PR for the standard library 😂.
In my benchmarks, this version is noticeably faster when `retain_mut` actually removes elements (thanks to fewer pointer operations, it just advances `write_index`), while performing identically to the current implementation when nothing is removed.
Let’s see if bors likes this change or not.
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