library/core/src/task/ready.rs

   1 /// Extracts the successful type of a [`Poll<T>`].
   2 ///
   3 /// This macro bakes in propagation of [`Pending`] signals by returning early.
   4 ///
   5 /// [`Poll<T>`]: crate::task::Poll
   6 /// [`Pending`]: crate::task::Poll::Pending
   7 ///
   8 /// # Examples
   9 ///
  10 /// ```
  11 /// #![feature(ready_macro)]
  12 ///
  13 /// use std::task::{ready, Context, Poll};
  14 /// use std::future::{self, Future};
  15 /// use std::pin::Pin;
  16 ///
  17 /// pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
  18 ///     let mut fut = future::ready(42);
  19 ///     let fut = Pin::new(&mut fut);
  20 ///
  21 ///     let num = ready!(fut.poll(cx));
  22 ///     # drop(num);
  23 ///     // ... use num
  24 ///
  25 ///     Poll::Ready(())
  26 /// }
  27 /// ```
  28 ///
  29 /// The `ready!` call expands to:
  30 ///
  31 /// ```
  32 /// # #![feature(ready_macro)]
  33 /// # use std::task::{Context, Poll};
  34 /// # use std::future::{self, Future};
  35 /// # use std::pin::Pin;
  36 /// #
  37 /// # pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
  38 ///     # let mut fut = future::ready(42);
  39 ///     # let fut = Pin::new(&mut fut);
  40 ///     #
  41 /// let num = match fut.poll(cx) {
  42 ///     Poll::Ready(t) => t,
  43 ///     Poll::Pending => return Poll::Pending,
  44 /// };
  45 ///     # drop(num);
  46 ///     # // ... use num
  47 ///     #
  48 ///     # Poll::Ready(())
  49 /// # }
  50 /// ```
  51 #[unstable(feature = "ready_macro", issue = "70922")]
  52 #[rustc_macro_transparency = "semitransparent"]
  53 pub macro ready($e:expr) {
  54     match $e {
  55         $crate::task::Poll::Ready(t) => t,
  56         $crate::task::Poll::Pending => {
  57             return $crate::task::Poll::Pending;
  58         }
  59     }
  60 }