library/core/src/future/ready.rs

   1 use crate::future::Future;
   2 use crate::pin::Pin;
   3 use crate::task::{Context, Poll};
   4
   5 /// Creates a future that is immediately ready with a value.
   6 ///
   7 /// This `struct` is created by [`ready()`]. See its
   8 /// documentation for more.
   9 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
  10 #[derive(Debug, Clone)]
  11 #[must_use = "futures do nothing unless you `.await` or poll them"]
  12 pub struct Ready<T>(Option<T>);
  13
  14 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
  15 impl<T> Unpin for Ready<T> {}
  16
  17 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
  18 impl<T> Future for Ready<T> {
  19     type Output = T;
  20
  21     #[inline]
  22     fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<T> {
  23         Poll::Ready(self.0.take().expect("Ready polled after completion"))
  24     }
  25 }
  26
  27 /// Creates a future that is immediately ready with a value.
  28 ///
  29 /// Futures created through this function are functionally similar to those
  30 /// created through `async {}`. The main difference is that futures created
  31 /// through this function are named and implement `Unpin`.
  32 ///
  33 /// # Examples
  34 ///
  35 /// ```
  36 /// use core::future;
  37 ///
  38 /// # async fn run() {
  39 /// let a = future::ready(1);
  40 /// assert_eq!(a.await, 1);
  41 /// # }
  42 /// ```
  43 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
  44 pub fn ready<T>(t: T) -> Ready<T> {
  45     Ready(Some(t))
  46 }