implement core::future::join

library/core/src/future/join.rs

@@ -0,0 +1,147 @@
+#![allow(unused_imports)] // items are used by the macro
+
+use crate::cell::UnsafeCell;
+use crate::future::{poll_fn, Future};
+use crate::pin::Pin;
+use crate::task::Poll;
+/// Polls multiple futures simultaneously, returning a tuple
+/// of all results once complete.
+///
+/// While `join!(a, b)` is similar to `(a.await, b.await)`,
+/// `join!` polls both futures concurrently and is therefore more efficient.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(future_join, future_poll_fn)]
+///
+/// use std::future::join;
+///
+/// async fn one() -> usize { 1 }
+/// async fn two() -> usize { 2 }
+///
+/// # let _ =  async {
+/// let x = join!(one(), two());
+/// assert_eq!(x, (1, 2));
+/// # };
+/// ```
+///
+/// `join!` is variadic, so you can pass any number of futures:
+///
+/// ```
+/// #![feature(future_join, future_poll_fn)]
+///
+/// use std::future::join;
+///
+/// async fn one() -> usize { 1 }
+/// async fn two() -> usize { 2 }
+/// async fn three() -> usize { 3 }
+///
+/// # let _ = async {
+/// let x = join!(one(), two(), three());
+/// assert_eq!(x, (1, 2, 3));
+/// # };
+/// ```
+#[unstable(feature = "future_join", issue = "91642")]
+pub macro join {
+    ( $($fut:expr),* $(,)?) => {
+        join! { @count: (), @futures: {}, @rest: ($($fut,)*) }
+    },
+    // Recurse until we have the position of each future in the tuple
+    (
+        // A token for each future that has been expanded: "_ _ _"
+        @count: ($($count:tt)*),
+        // Futures and their positions in the tuple: "{ a => (_), b => (_ _)) }"
+        @futures: { $($fut:tt)* },
+        // The future currently being expanded, and the rest
+        @rest: ($current:expr, $($rest:tt)*)
+    ) => {
+        join! {
+            @count: ($($count)* _), // Add to the count
+            @futures: { $($fut)* $current => ($($count)*), }, // Add the future from @rest with it's position
+            @rest: ($($rest)*) // And leave the rest
+        }
+    },
+    // Now generate the output future
+    (
+        @count: ($($count:tt)*),
+        @futures: {
+            $( $fut:expr => ( $($pos:tt)* ), )*
+        },
+        @rest: ()
+    ) => {{
+        let mut futures = ( $( MaybeDone::Future($fut), )* );
+
+        poll_fn(move |cx| {
+            let mut done = true;
+
+            $(
+                // Extract the future from the tuple
+                let ( $($pos,)* fut, .. ) = &mut futures;
+
+                // SAFETY: the futures are never moved
+                done &= unsafe { Pin::new_unchecked(fut).poll(cx).is_ready() };
+            )*
+
+            if done {
+                Poll::Ready(($({
+                    let ( $($pos,)* fut, .. ) = &mut futures;
+
+                    // SAFETY: the futures are never moved
+                    unsafe { Pin::new_unchecked(fut).take_output().unwrap() }
+                }),*))
+            } else {
+                Poll::Pending
+            }
+        }).await
+    }}
+}
+
+/// Future used by `join!` that stores it's output to
+/// be later taken and doesn't panic when polled after ready.
+#[allow(dead_code)]
+#[unstable(feature = "future_join", issue = "none")]
+enum MaybeDone<F: Future> {
+    Future(F),
+    Done(F::Output),
+    Took,
+}
+
+#[unstable(feature = "future_join", issue = "none")]
+impl<F: Future + Unpin> Unpin for MaybeDone<F> {}
+
+#[unstable(feature = "future_join", issue = "none")]
+impl<F: Future> MaybeDone<F> {
+    #[allow(dead_code)]
+    fn take_output(self: Pin<&mut Self>) -> Option<F::Output> {
+        unsafe {
+            match &*self {
+                MaybeDone::Done(_) => match mem::replace(self.get_unchecked_mut(), Self::Took) {
+                    MaybeDone::Done(val) => Some(val),
+                    _ => unreachable!(),
+                },
+                _ => None,
+            }
+        }
+    }
+}
+
+#[unstable(feature = "future_join", issue = "none")]
+impl<F: Future> Future for MaybeDone<F> {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        unsafe {
+            match self.as_mut().get_unchecked_mut() {
+                MaybeDone::Future(f) => match Pin::new_unchecked(f).poll(cx) {
+                    Poll::Ready(val) => self.set(Self::Done(val)),
+                    Poll::Pending => return Poll::Pending,
+                },
+                MaybeDone::Done(_) => {}
+                MaybeDone::Took => unreachable!(),
+            }
+        }
+
+        Poll::Ready(())
+    }
+}

library/core/src/future/mod.rs

@@ -11,6 +11,7 @@ use crate::{
 
 mod future;
 mod into_future;
+mod join;
 mod pending;
 mod poll_fn;
 mod ready;
@@ -18,6 +19,9 @@ mod ready;
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub use self::future::Future;
 
+#[unstable(feature = "future_join", issue = "91642")]
+pub use self::join::join;
+
 #[unstable(feature = "into_future", issue = "67644")]
 pub use into_future::IntoFuture;