speed up String::push and String::insert

[lincot]

Addresses the concerns described in #116235.

The performance gain comes mainly from avoiding temporary buffers.

Complex pattern matching in encode_utf8 (introduced in #67569) has been simplified to a comparison and an exhaustive match in the encode_utf8_raw_unchecked helper function. It takes a slice of MaybeUninit<u8> because otherwise we'd have to construct a normal slice to uninitialized data, which is not desirable, I guess.

Several functions still have that unneeded zeroing, but a single instruction is not that important, I guess.

@rustbot label T-libs C-optimization A-str

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @scottmcm (or someone else) some time within the next two weeks.

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[scottmcm]

I had a variety of thoughts; let me know what you think.

Also, is there anything here for which it would make sense to have a codegen test to confirm what's happening? Or some other test to help confirm it's better?

[lincot]

A codegen check for the absence of memcpy would be nice, since the original String::push has one.

[rust-timer]

Insufficient permissions to issue commands to rust-timer.

[bors]

@lincot: 🔑 Insufficient privileges: not in try users

[rust-timer]

Insufficient permissions to issue commands to rust-timer.

[bors]

☔ The latest upstream changes (presumably #116113) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust.git master
$ git push --force-with-lease

The following commits are merge commits:

• 9511918

[bors]

☔ The latest upstream changes (presumably #127840) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #130511) made this pull request unmergeable. Please resolve the merge conflicts.

[lincot]

The proposed implementation uses get_unchecked_mut, which cannot be used in char::encode_utf8, which is now const. Also, get_unchecked_mut actually has a cost when running with debug assertions. So I am reverting to using pointers. The codegen of String::push seems to be unchanged: godbolt.