Function write_pending 

pub fn write_pending<E: Encoder>(
    enc: &mut E,
    w: &mut impl Write,
) -> Result<usize>

Write every byte enc has pending to w, then consume it. Returns how many bytes were written.

The drain step of a streaming encode, without the footgun of forgetting consume_output and sending the same bytes twice. On an I/O error nothing is consumed, and the bytes w accepted before the error are unknown: abort the transport.