1 files changed, 89 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/sink/unfold.rs b/third_party/rust/futures-util/src/sink/unfold.rs
new file mode 100644
index 0000000000..dea1307b66
--- /dev/null
+++ b/third_party/rust/futures-util/src/sink/unfold.rs
@@ -0,0 +1,89 @@
+use super::assert_sink;
+use crate::unfold_state::UnfoldState;
+use core::{future::Future, pin::Pin};
+use futures_core::ready;
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Sink for the [`unfold`] function.
+    #[derive(Debug)]
+    #[must_use = "sinks do nothing unless polled"]
+    pub struct Unfold<T, F, R> {
+        function: F,
+        #[pin]
+        state: UnfoldState<T, R>,
+    }
+}
+
+/// Create a sink from a function which processes one item at a time.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::sink::{self, SinkExt};
+///
+/// let unfold = sink::unfold(0, |mut sum, i: i32| {
+///     async move {
+///         sum += i;
+///         eprintln!("{}", i);
+///         Ok::<_, futures::never::Never>(sum)
+///     }
+/// });
+/// futures::pin_mut!(unfold);
+/// unfold.send(5).await?;
+/// # Ok::<(), futures::never::Never>(()) }).unwrap();
+/// ```
+pub fn unfold<T, F, R, Item, E>(init: T, function: F) -> Unfold<T, F, R>
+where
+    F: FnMut(T, Item) -> R,
+    R: Future<Output = Result<T, E>>,
+{
+    assert_sink::<Item, E, _>(Unfold { function, state: UnfoldState::Value { value: init } })
+}
+
+impl<T, F, R, Item, E> Sink<Item> for Unfold<T, F, R>
+where
+    F: FnMut(T, Item) -> R,
+    R: Future<Output = Result<T, E>>,
+{
+    type Error = E;
+
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.poll_flush(cx)
+    }
+
+    fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
+        let mut this = self.project();
+        let future = match this.state.as_mut().take_value() {
+            Some(value) => (this.function)(value, item),
+            None => panic!("start_send called without poll_ready being called first"),
+        };
+        this.state.set(UnfoldState::Future { future });
+        Ok(())
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        let mut this = self.project();
+        Poll::Ready(if let Some(future) = this.state.as_mut().project_future() {
+            match ready!(future.poll(cx)) {
+                Ok(state) => {
+                    this.state.set(UnfoldState::Value { value: state });
+                    Ok(())
+                }
+                Err(err) => {
+                    this.state.set(UnfoldState::Empty);
+                    Err(err)
+                }
+            }
+        } else {
+            Ok(())
+        })
+    }
+
+    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.poll_flush(cx)
+    }
+}