fn poll_head<'p, S, T>(
stream: Pin<&mut S>,
peeked: &'p mut Option<T>,
cx: &mut Context<'_>,
) -> Poll<Option<&'p T>>where
S: Stream<Item = T>,Expand description
Lazily fill the cached head of stream into peeked and return a borrow.
On entry, Some(v) in peeked is a previously-buffered head that has not
yet been consumed; None means the head must be pulled from the stream.
Returns Ready(Some(&v)) when a head is available, Ready(None) once
stream is exhausted, or Pending when waiting on the stream.