Function futures::stream::once

pub fn once<Fut>(future: Fut) -> Once<Fut> where
    Fut: Future, 

Creates a stream of single element

use futures::future;
use futures::executor::block_on;
use futures::stream::{self, StreamExt};

let mut stream = stream::once(future::ready(17));
let collected = block_on(stream.collect::<Vec<i32>>());
assert_eq!(collected, vec![17]);