Add core::future::{pending,ready}

Author: yoshuawuyts

src/libcore/future/mod.rs

@@ -11,9 +11,17 @@ use crate::{
 };
 
 mod future;
+mod pending;
+mod ready;
+
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub use self::future::Future;
 
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+pub use pending::{pending, Pending};
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+pub use ready::{ready, Ready};
+
 /// This type is needed because:
 ///
 /// a) Generators cannot implement `for<'a, 'b> Generator<&'a mut Context<'b>>`, so we need to pass

src/libcore/future/pending.rs

@@ -0,0 +1,57 @@
+use crate::future::Future;
+use crate::marker;
+use crate::pin::Pin;
+use crate::task::{Context, Poll};
+
+/// Creates a future which never resolves, representing a computation that never
+/// finishes.
+///
+/// This `struct` is created by the [`pending`] function. See its
+/// documentation for more.
+///
+/// [`pending`]: fn.pending.html
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Pending<T> {
+    _data: marker::PhantomData<T>,
+}
+
+/// Creates a future which never resolves, representing a computation that never
+/// finishes.
+///
+/// # Examples
+///
+/// ```no_run
+/// #![feature(future_readiness_fns)]
+/// use core::future;
+///
+/// # async fn run() {
+/// let future = future::pending();
+/// let () = future.await;
+/// unreachable!();
+/// # }
+/// ```
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+pub fn pending<T>() -> Pending<T> {
+    Pending { _data: marker::PhantomData }
+}
+
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+impl<T> Future for Pending<T> {
+    type Output = T;
+
+    fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<T> {
+        Poll::Pending
+    }
+}
+
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+impl<T> Unpin for Pending<T> {}
+
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+impl<T> Clone for Pending<T> {
+    fn clone(&self) -> Self {
+        pending()
+    }
+}

src/libcore/future/ready.rs

@@ -0,0 +1,45 @@
+use crate::future::Future;
+use crate::pin::Pin;
+use crate::task::{Context, Poll};
+
+/// Creates a future that is immediately ready with a value.
+///
+/// This `struct` is created by the [`ready`] function. See its
+/// documentation for more.
+///
+/// [`ready`]: fn.ready.html
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+#[derive(Debug, Clone)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Ready<T>(Option<T>);
+
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+impl<T> Unpin for Ready<T> {}
+
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+impl<T> Future for Ready<T> {
+    type Output = T;
+
+    #[inline]
+    fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<T> {
+        Poll::Ready(self.0.take().expect("Ready polled after completion"))
+    }
+}
+
+/// Creates a future that is immediately ready with a value.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(future_readiness_fns)]
+/// use core::future;
+///
+/// # async fn run() {
+/// let a = future::ready(1);
+/// assert_eq!(a.await, 1);
+/// # }
+/// ```
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+pub fn ready<T>(t: T) -> Ready<T> {
+    Ready(Some(t))
+}