Add a helper for always ready futures (#2825)

futures-util/src/future/always_ready.rs

@@ -0,0 +1,62 @@
+use super::assert_future;
+use core::pin::Pin;
+use futures_core::future::{FusedFuture, Future};
+use futures_core::task::{Context, Poll};
+
+/// Future for the [`always_ready`](always_ready()) function.
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct AlwaysReady<T, F: Fn() -> T>(F);
+
+impl<T, F: Fn() -> T> core::fmt::Debug for AlwaysReady<T, F> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_tuple("AlwaysReady").finish()
+    }
+}
+
+impl<T, F: Fn() -> T + Clone> Clone for AlwaysReady<T, F> {
+    fn clone(&self) -> Self {
+        Self(self.0.clone())
+    }
+}
+
+impl<T, F: Fn() -> T + Copy> Copy for AlwaysReady<T, F> {}
+
+impl<T, F: Fn() -> T> Unpin for AlwaysReady<T, F> {}
+
+impl<T, F: Fn() -> T> FusedFuture for AlwaysReady<T, F> {
+    fn is_terminated(&self) -> bool {
+        false
+    }
+}
+
+impl<T, F: Fn() -> T> Future for AlwaysReady<T, F> {
+    type Output = T;
+
+    #[inline]
+    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<T> {
+        Poll::Ready(self.0())
+    }
+}
+
+/// Creates a future that is always immediately ready with a value.
+///
+/// This is particularly useful in avoiding a heap allocation when an API needs [`Box<dyn Future<Output = T>>`],
+/// as [`AlwaysReady`] does not have to store a boolean for `is_finished`.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use std::mem::size_of_val;
+///
+/// use futures::future;
+///
+/// let a = future::always_ready(|| 1);
+/// assert_eq!(size_of_val(&a), 0);
+/// assert_eq!(a.await, 1);
+/// assert_eq!(a.await, 1);
+/// # });
+/// ```
+pub fn always_ready<T, F: Fn() -> T>(prod: F) -> AlwaysReady<T, F> {
+    assert_future::<T, _>(AlwaysReady(prod))
+}

futures-util/src/future/mod.rs

@@ -74,6 +74,9 @@ pub use self::poll_immediate::{poll_immediate, PollImmediate};
 mod ready;
 pub use self::ready::{err, ok, ready, Ready};
 
+mod always_ready;
+pub use self::always_ready::{always_ready, AlwaysReady};
+
 mod join;
 pub use self::join::{join, Join};