Implement Write trait for MaybeUninit<u8>

[SUPERCILEX]

This PR makes it possible to write something like the following:

let mut buf = [MaybeUninit::<u8>::uninit(); 20];
let written = {
    let slice = buf.as_mut_slice();
    write!(slice, "{}", u64_int).unwrap();
    buf.len() - slice.len()
};
let buf: [u8] = unsafe { mem::transmute(buf[..written]) };

[rust-highfive]

r? @m-ou-se

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[SUPERCILEX]

Fixed the benchmark and added one for MaybeUninit:

test io::impls::tests::bench_write_maybeuninit                    ... bench:          24 ns/iter (+/- 0)
test io::impls::tests::bench_write_slice                          ... bench:          25 ns/iter (+/- 6)
test io::impls::tests::bench_write_vec                            ... bench:          28 ns/iter (+/- 0)

[m-ou-se]

I'm not sure if this is the right kind of API we want to provide for this. The guarantee that the written part is properly initialized and usable as &[u8] is not expressed through this API, and still requires the user to use unsafe code. An abstraction/type for partially initialized buffers is probably a lot nicer to use, and could be entirely safe to use.

[SUPERCILEX]

Would it make sense to implement Write for ReadBuf then?

[JohnCSimon]

@SUPERCILEX
Ping from triage: Can you please post the status of this PR? Maybe ping any commenters for additional feedback?
Thank you.

[SUPERCILEX]

I'm waiting on a reply to my above comment.

[JohnCSimon]

@m-ou-se ☝️ ?

[bors]

☔ The latest upstream changes (presumably #97910) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

I'm waiting on a reply to my above comment.

@m-ou-se - can you comment on this?

[SUPERCILEX]

This has been implemented in #97015. You can now do this:

let mut buf = [MaybeUninit::<u8>::uninit(); 20];
let mut buf: BorrowedBuf = buf.as_mut_slice().into();
write!(buf.unfilled(), "{}", 42).unwrap();
let buf: &[u8] = buf.filled();