Support extracting underlying filled buffer from BorrowedBuf

[SUPERCILEX]

Proposal

Problem statement

I'd like to be able to modify a MaybeUninit buffer and return the contents to the caller.

Motivating examples or use cases

Currently, if a function wishes to safely modified a MaybeUninit buffer passed in by reference by the caller, they cannot return the filled buffer to the caller.

Fixing this would let the following code compile:

pub fn direct_file_name(
    buf: &mut [MaybeUninit<u8>; DIRECT_FILE_NAME_LEN + 1],
    to: RingKind,
    index: u32,
) -> DirectFileNameToken<()> {
    let mut buf = BorrowedBuf::from(buf.as_mut_slice());
    write!(buf.unfilled(), "{:0>13}\0", composite_id(to, index)).unwrap();
    DirectFileNameToken(buf.filled_mut(), PhantomData) // Doesn't compile!
}

Solution sketch

Add two methods:

fn into_filled(self) -> &'data [u8]
fn into_filled_mut(self) -> &'data mut [u8]

Alternatives

Transmute to get lifetime extension or manually convert the MaybeUninits to u8s by using the filled length. Both of these solutions are very error prone and annoying to write.

[Amanieu]

We discussed this in the libs-api meeting and we're happy with these methods.

[joshtriplett]

Quick correction: into_filled_mut has the wrong signature, it should take self.

[joshtriplett]

Looks like rust-lang/rust#132533 is correct and this was just a typo in the ACP.

[SUPERCILEX]

Oops, yeah sorry. Fixed.