pub fn once<Fut: Future>(future: Fut) -> Once<Fut> {
    assert_stream::<Fut::Output, _>(Once::new(future))
}
Creates a stream of a single element.

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

let stream = stream::once(async { 17 });
let collected = stream.collect::<Vec<i32>>().await;
assert_eq!(collected, vec![17]);
# });
```
futures-util/src/stream/once.rs