pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut>
where
    F: FnMut(T) -> Fut,
    Fut: Future<Output = Option<(Item, T)>>,
{
    assert_stream::<Item, _>(Unfold { f, state: UnfoldState::Value { value: init } })
}
Creates a `Stream` from a seed and a closure returning a `Future`.

This function is the dual for the `Stream::fold()` adapter: while
`Stream::fold()` reduces a `Stream` to one single value, `unfold()` creates a
`Stream` from a seed value.

`unfold()` will call the provided closure with the provided seed, then wait
for the returned `Future` to complete with `(a, b)`. It will then yield the
value `a`, and use `b` as the next internal state.

If the closure returns `None` instead of `Some(Future)`, then the `unfold()`
will stop producing items and return `Poll::Ready(None)` in future
calls to `poll()`.

This function can typically be used when wanting to go from the "world of
futures" to the "world of streams": the provided closure can build a
`Future` using other library functions working on futures, and `unfold()`
will turn it into a `Stream` by repeating the operation.

# Example

```
# futures::executor::block_on(async {
use futures::stream::{self, StreamExt};

let stream = stream::unfold(0, |state| async move {
if state <= 2 {
let next_state = state + 1;
let yielded = state * 2;
Some((yielded, next_state))
} else {
None
}
});

let result = stream.collect::<Vec<i32>>().await;
assert_eq!(result, vec![0, 2, 4]);
# });
```
futures-util/src/stream/unfold.rs