Function futures::prelude::stream::once

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

Creates a stream of a single element.

use futures::stream::{self, StreamExt};

let stream = stream::once(async { 17 });
let collected = stream.collect::<Vec<i32>>().await;
assert_eq!(collected, vec![17]);