Easier splitting for types containing slice DSTs

[valaphee]

While working on file system drivers for a custom operating system, I'm using buffers, which basically consist of multiple entries. Each entry contains a slice DST, but it's not possible to store them as slice itself, which is why I need to split them up.

One entry might look like:

#[repr(C)]
#[derive(Debug, FromBytes, Immutable, KnownLayout)]
pub struct Entry {
    pub index: u64,
    pub name_length: u8,
    pub name: [u8],
}

where the first entry's name has the full length of the buffer.

And are split up by

let buffer_align = align_of_val(&mut buffer);
let Ok(next_buffer) = Entry::try_mut_from_bytes(
    &mut buffer.name[(buffer.name_length as usize)..].align(buffer_align),
) else {
    break;
};
buffer = next_buffer;

but requires some boilerplate. (and also a align method, which aligns the pointer)

The compiler will probably figure out the alignment at compile-time and optimize out the alignment check. But it's better to use the already known layout information.

[joshlf]

For the length, we already have this tracked in #1289. Can you confirm that that issue would support your use case?

Also, can you clarify what .align does here? I'm not sure what method that refers to.

    &mut buffer.name[(buffer.name_length as usize)..].align(buffer_align),

[valaphee]

In my example, Entry has to be aligned. I'm getting the end of the name, and align it to the next 8-byte, to read the next Entry. (The align method just takes the ptr, and aligns it, and corrects the length)

Had to use a custom method, as align_to doesn't work on slice DSTs.

#1289 would be helpful.

[joshlf]

Okay, in that case I'm going to close this as a duplicate of #1289. Feel free to continue the conversation if you have more questions!