io: add ReadBuf::inner_mut

[sunshowers]

This returns the entire buffer as a &mut [MaybeUninit<u8>], which may
be useful for building abstractions on top.

Motivation

It looks like ReadBuf doesn't have a way to get the entire buffer, only various sections of it. Being able to work with the entire buffer is useful in some situations, such as while building testing tools that limit the size of the buffer (similar to BufMut::limit in tokio 0.2).

Solution

Add ReadBuf::inner_mut which returns the entire inner buffer.

This is unsafe for the same reason that unfilled_mut is: the caller must not deinitialize portions of the buffer that have already been initialized.

I've used this method in partial-io: facebookarchive/rust-partial-io#33 (comment) and it looks like it does the job.

I'm not sure if any tests for this make sense, given the fact that the implementation is one line long.

Thank you!

[sunshowers]

There's an upstream Rust RFC for ReadBuf, and i guess this should go there too?

[Darksonn]

This seems good to me.

[sunshowers]

Updated with a bit more documentation about the invariants calling code is expected to uphold.

[Darksonn]

Prefer to push more commits instead of force-pushing.

[sunshowers]

Prefer to push more commits instead of force-pushing.

Hmm, I was going by the contributor guide, which recommends using logical commits and not checkpoints, and which is also my preferred style of contributions from personal experience. (Though later I do see that the guide talks about not rebasing/force pushing, which seems to contradict this?)

[Darksonn]

It could be written clearer, yes. The point is that, once I have already looked at the PR, it is a lot easier to see what changed since I last looked if all changes are done as additional commits.