Tracking issue for `Read::initializer`

[sfackler]

Implemented in #42002.

Open questions / blockers

• The code currently creates references to uninitialized integers (grep for 42788), can we avoid that?

[alexcrichton]

I'm personally no longer sold on the infrastructure that we've got here I think. I was initially hoping that we'd have some safe way of working with these APIs and such, but in the end it all turned up unsafe. In light of all that I'd personally advocate for much simpler API:

impl Read {
    #[inline]
    unsafe fn may_read_buffer(&self) -> bool { true }
}

I don't think the complexity we have today is worth the cost currently, and I'd imagine that we could start making abstractions like:

fn initialize_buffer_for(r: &Read, buf: &mut [u8]) {
    // ...
}

[tbu-]

I wrote a tiny library for reading into uninitialized buffers: https://crates.io/crates/buffer/

Maybe its ideas could be adapted for the standard library. The following is safe code using the library:

let mut vec = Vec::with_capacity(1024);
if reader.read_buffer(&mut vec)?.len() != 0 {
    if vec[0] == 0 {
        // ...
    }
}

Adoption of something like this in the standard library would probably also help with adoption by third party crates, see e.g. tokio-rs/mio#628.

[seanmonstar]

An alternative to the current API is to go along the path that TrustedLen did, which is just to have an unsafe trait.

unsafe trait ReadUninit: Read {}

With specialization landing, anyone who cares to can take advantage of ReadUinit:

impl<T: Read> ReadBuf for T {
    fn read_buf<B: BufMut>(&mut self, buf: &mut B) {
        self.read(buf.zeroed())
    }
}

impl<T: ReadUninit> ReadBuf for T {
    fn read_buf<B: BufMut>(&mut self, buf: &mut B) {
        self.read(buf.bytes())
    }
}

[sfackler]

That's incompatible with trait objects which are used pretty heavily with Read in particular.

[seanmonstar]

Specialization is incompatible with trait objects?

[sfackler]

If I have a &mut Read, I have no way of checking if it implements ReadUninit or not.

[jethrogb]

@RalfJung suggested a function that takes &mut [MaybeUninit<u8>]