1 files changed, 114 insertions, 0 deletions
diff --git a/library/core/src/task/ready.rs b/library/core/src/task/ready.rs
new file mode 100644
index 000000000..b1daf545f
--- /dev/null
+++ b/library/core/src/task/ready.rs
@@ -0,0 +1,114 @@
+use core::convert;
+use core::fmt;
+use core::ops::{ControlFlow, FromResidual, Try};
+use core::task::Poll;
+
+/// Extracts the successful type of a [`Poll<T>`].
+///
+/// This macro bakes in propagation of [`Pending`] signals by returning early.
+///
+/// [`Poll<T>`]: crate::task::Poll
+/// [`Pending`]: crate::task::Poll::Pending
+///
+/// # Examples
+///
+/// ```
+/// use std::task::{ready, Context, Poll};
+/// use std::future::{self, Future};
+/// use std::pin::Pin;
+///
+/// pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
+///     let mut fut = future::ready(42);
+///     let fut = Pin::new(&mut fut);
+///
+///     let num = ready!(fut.poll(cx));
+///     # drop(num);
+///     // ... use num
+///
+///     Poll::Ready(())
+/// }
+/// ```
+///
+/// The `ready!` call expands to:
+///
+/// ```
+/// # use std::task::{Context, Poll};
+/// # use std::future::{self, Future};
+/// # use std::pin::Pin;
+/// #
+/// # pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
+///     # let mut fut = future::ready(42);
+///     # let fut = Pin::new(&mut fut);
+///     #
+/// let num = match fut.poll(cx) {
+///     Poll::Ready(t) => t,
+///     Poll::Pending => return Poll::Pending,
+/// };
+///     # drop(num);
+///     # // ... use num
+///     #
+///     # Poll::Ready(())
+/// # }
+/// ```
+#[stable(feature = "ready_macro", since = "1.64.0")]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro ready($e:expr) {
+    match $e {
+        $crate::task::Poll::Ready(t) => t,
+        $crate::task::Poll::Pending => {
+            return $crate::task::Poll::Pending;
+        }
+    }
+}
+
+/// Extracts the successful type of a [`Poll<T>`].
+///
+/// See [`Poll::ready`] for details.
+#[unstable(feature = "poll_ready", issue = "89780")]
+pub struct Ready<T>(pub(crate) Poll<T>);
+
+#[unstable(feature = "poll_ready", issue = "89780")]
+impl<T> Try for Ready<T> {
+    type Output = T;
+    type Residual = Ready<convert::Infallible>;
+
+    #[inline]
+    fn from_output(output: Self::Output) -> Self {
+        Ready(Poll::Ready(output))
+    }
+
+    #[inline]
+    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
+        match self.0 {
+            Poll::Ready(v) => ControlFlow::Continue(v),
+            Poll::Pending => ControlFlow::Break(Ready(Poll::Pending)),
+        }
+    }
+}
+
+#[unstable(feature = "poll_ready", issue = "89780")]
+impl<T> FromResidual for Ready<T> {
+    #[inline]
+    fn from_residual(residual: Ready<convert::Infallible>) -> Self {
+        match residual.0 {
+            Poll::Pending => Ready(Poll::Pending),
+        }
+    }
+}
+
+#[unstable(feature = "poll_ready", issue = "89780")]
+impl<T> FromResidual<Ready<convert::Infallible>> for Poll<T> {
+    #[inline]
+    fn from_residual(residual: Ready<convert::Infallible>) -> Self {
+        match residual.0 {
+            Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+#[unstable(feature = "poll_ready", issue = "89780")]
+impl<T> fmt::Debug for Ready<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Ready").finish()
+    }
+}