Use get_unchecked in str::[r]split_once

[nickkuk]

This PR removes indices checking in str::split_once and str::rsplit_once methods.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

[nagisa]

This seems reasonable to me. Searcher trait is unsafe to implement with this exact invariant for implementers.

Do you have any examples of the improvements achieved by this change?

[nickkuk]

Yes, I noticed that current implementation adds panic handling, then I found the reason of this.

Godbolt comparison is here (link was edited for history).

[nickkuk]

I've removed redundant bounds in str::split_once and str::rsplit_once methods only.

@rustbot label -S-waiting-on-author +S-waiting-on-review

[Mark-Simulacrum]

Thanks. I don't think it makes sense to edit unrelated code in this PR anyway.

@bors r+

[bors]

📌 Commit a35aaa2 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit a35aaa2 with merge 0c87288...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 0c87288 to master...

[rust-timer]

Finished benchmarking commit (0c87288): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression