Function poll_dispatch_then 

pub fn poll_dispatch_then<T>(
    dispatch: &mut DispatchState,
    rx: &mut Receiver<Result<T, Error>>,
    cx: &mut Context<'_>,
) -> Poll<Result<T, Error>>

Drives dispatch then polls rx, short-circuiting on a dispatch error.

This is the standard body of every lazy-future poll implementation: reserve a channel slot, send the command, then wait for the oneshot reply.