use crate::{Iter, Stream}; use core::option; use core::pin::Pin; use core::task::{Context, Poll}; /// Stream for the [`once`](fn@once) function. #[derive(Debug)] #[must_use = "streams do nothing unless polled"] pub struct Once { iter: Iter>, } impl Unpin for Once {} /// Creates a stream that emits an element exactly once. /// /// The returned stream is immediately ready and emits the provided value once. /// /// # Examples /// /// ``` /// use tokio_stream::{self as stream, StreamExt}; /// /// #[tokio::main] /// async fn main() { /// // one is the loneliest number /// let mut one = stream::once(1); /// /// assert_eq!(Some(1), one.next().await); /// /// // just one, that's all we get /// assert_eq!(None, one.next().await); /// } /// ``` pub fn once(value: T) -> Once { Once { iter: crate::iter(Some(value).into_iter()), } } impl Stream for Once { type Item = T; fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll> { Pin::new(&mut self.iter).poll_next(cx) } fn size_hint(&self) -> (usize, Option) { self.iter.size_hint() } }