let mut buf = [0u8; 16]; // we want to fill this
let mut window = buf.as_mut_slice();

while !window.is_empty() {
    match input.read(window) {
        Ok(n) => {
            let (_old, now) = window.split_at_mut(n);
            window = now;
        }
        Err(e) => ...,
    }
}