Is your feature request related to a problem? Please describe.
Consider this code:

let mut stream = FramedWrite::new(stream, LengthDelimitedCodec::new());
let mut buf = vec![0_u8; max_message_size];
loop {
    let message = gen_message(&mut buf);
    // an allocation here:
    stream.feed(Bytes::copy_from_slice(&message)).await?;
}

This allocation is unneeded. impl Encoder<Bytes> for LengthDelimitedCodec just reslices src, so impl<'a> Encoder<&'a [u8]> for LengthDelimitedCodec is just as possible.

Describe the solution you'd like
I first thought that just adding the missing impl would be enough, but unfortunately having two Encoder<_> impls breaks type inference, so this is a backwards incompatible change, oh well.

Some possible options:

1. Make it opt-in via some kind of type tag, i.e.

   pub struct LengthDelimitedCodec<Item = Bytes> { ... } 

   All the existing methods would have to be implemented on the <Bytes> variant. <&[u8]> variant would need new methods like ::new_borrowed() or something.

2. Make a wrapper type.

3. Just wait for the next breaking release and break it then.

Describe alternatives you've considered

Don’t do it. Cases like the above one will continue to require unneccesary allocation, which is kind of sad, but not world-ending.

Additional context

I’m willing to send a PR if some possible path of solution is accepted.