pub fn poll_fn<T, F>(f: F) -> PollFn<F>
where
    F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
{
    assert_stream::<T, _>(PollFn { f })
}
Creates a new stream wrapping a function returning `Poll<Option<T>>`.

Polling the returned stream calls the wrapped function.

# Examples

```
use futures::stream::poll_fn;
use futures::task::Poll;

let mut counter = 1usize;

let read_stream = poll_fn(move |_| -> Poll<Option<String>> {
if counter == 0 { return Poll::Ready(None); }
counter -= 1;
Poll::Ready(Some("Hello, World!".to_owned()))
});
```
futures-util/src/stream/poll_fn.rs