perf: use `get_unchecked` for `TwoWaySearcher` ## What is this PR? *This is related to rust-lang/rust#27721.* This PR is a proposal for a performance improvement in `std::pattern`. Profiling of [https://github.com/quickwit-oss/quickwit](https://github.com/quickwit-oss/quickwit) in production shows that `TwoWaySearcher::next` is one of the most CPU-time-consuming functions, so I thought I would give it a look. I read the [contribution guide](https://std-dev-guide.rust-lang.org/development/perf-benchmarking.html) and this seems to be a fitting proposal. It seems like `TwoWaySearcher::next` and `TwoWaySearcher::next_back` could be made faster by using `get_unchecked` in the inner loop comparisons instead of regular indexing, which is safe in the conditions where it would be done (indices are within bounds by construction). I added some `SAFETY` comments in the code to explain why this is safe, as I believe is customary in those cases (and according to [this page as well](https://std-dev-guide.rust-lang.org/policy/safety-comments.html)). ### Benchmarks I ran the existing bencharmks before/after the changes (only on my laptop, I can run them in other places if that's necessary). ``` ./x.py bench library/coretests -- pattern:: ``` We seem to be getting a ~7.5-12% performance improvement at a very low cost, which sounds worthwhile to me. But this is the first time I'm proposing a change in Rust, so I'm looking forward to feedback on this. ``` BEFORE CHANGES pattern::ends_with_char 3398.91ns/iter +/- 526.28 pattern::ends_with_str 3545.04ns/iter +/- 1108.76 pattern::starts_with_char 3348.31ns/iter +/- 352.38 pattern::starts_with_str 3710.59ns/iter +/- 435.57 AFTER CHANGES pattern::ends_with_char 3125.99ns/iter +/- 567.09 (-8.03%) pattern::ends_with_str 3106.43ns/iter +/- 258.33 (-12.38%) pattern::starts_with_char 3094.55ns/iter +/- 595.42 (-7.59%) pattern::starts_with_str 3365.75ns/iter +/- 268.88 (-9.29%) ``` System info for the benchmarks run <details> ``` Based on commit 8317fef20409adedaa7c385fa6e954867bf626fc rustc 1.96.0-dev binary: rustc commit-hash: unknown commit-date: unknown host: aarch64-apple-darwin release: 1.96.0-dev LLVM version: 22.1.2 Apple M4 Max 16 64 GB ProductName: macOS ProductVersion: 26.3 BuildVersion: 25D125 (this was run on AC and without any heavy load from other apps or whatnot) ``` </details>