main.rs

use futures::sink::Sink;
use bytes::{Buf, Bytes, IntoBuf};

// I want to have SinkItem = IntoBuf so it accepts anything that I can turn into a Buf
trait Backend: Sink<SinkItem = Bytes, SinkError = u8> {}



impl<W: Write> Backend for WriterBackend<W> {}

impl<W: Write> Sink for WriterBackend<W> {
    type SinkItem = Bytes; // <--- how would I change the associated type to accept IntoBuf ?
    type SinkError = u8;

    fn start_send(&mut self, item: Self::SinkItem) -> StartSend<Self::SinkItem, Self::SinkError> {
        // use it! item.into_buf().bytes()
    }

    fn poll_complete(&mut self) -> Poll<(), Self::SinkError> {
       // omitted...
    }
}