1 files changed, 60 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/future/lazy.rs b/third_party/rust/futures-util/src/future/lazy.rs
new file mode 100644
index 0000000000..e9a8cf2fa9
--- /dev/null
+++ b/third_party/rust/futures-util/src/future/lazy.rs
@@ -0,0 +1,60 @@
+use super::assert_future;
+use core::pin::Pin;
+use futures_core::future::{FusedFuture, Future};
+use futures_core::task::{Context, Poll};
+
+/// Future for the [`lazy`] function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Lazy<F> {
+    f: Option<F>,
+}
+
+// safe because we never generate `Pin<&mut F>`
+impl<F> Unpin for Lazy<F> {}
+
+/// Creates a new future that allows delayed execution of a closure.
+///
+/// The provided closure is only run once the future is polled.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::future;
+///
+/// let a = future::lazy(|_| 1);
+/// assert_eq!(a.await, 1);
+///
+/// let b = future::lazy(|_| -> i32 {
+///     panic!("oh no!")
+/// });
+/// drop(b); // closure is never run
+/// # });
+/// ```
+pub fn lazy<F, R>(f: F) -> Lazy<F>
+where
+    F: FnOnce(&mut Context<'_>) -> R,
+{
+    assert_future::<R, _>(Lazy { f: Some(f) })
+}
+
+impl<F, R> FusedFuture for Lazy<F>
+where
+    F: FnOnce(&mut Context<'_>) -> R,
+{
+    fn is_terminated(&self) -> bool {
+        self.f.is_none()
+    }
+}
+
+impl<F, R> Future for Lazy<F>
+where
+    F: FnOnce(&mut Context<'_>) -> R,
+{
+    type Output = R;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<R> {
+        Poll::Ready((self.f.take().expect("Lazy polled after completion"))(cx))
+    }
+}