1 files changed, 134 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/sink/with.rs b/third_party/rust/futures-util/src/sink/with.rs
new file mode 100644
index 0000000000..86d3dcc7b8
--- /dev/null
+++ b/third_party/rust/futures-util/src/sink/with.rs
@@ -0,0 +1,134 @@
+use core::fmt;
+use core::marker::PhantomData;
+use core::pin::Pin;
+use futures_core::future::Future;
+use futures_core::ready;
+use futures_core::stream::Stream;
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Sink for the [`with`](super::SinkExt::with) method.
+    #[must_use = "sinks do nothing unless polled"]
+    pub struct With<Si, Item, U, Fut, F> {
+        #[pin]
+        sink: Si,
+        f: F,
+        #[pin]
+        state: Option<Fut>,
+        _phantom: PhantomData<fn(U) -> Item>,
+    }
+}
+
+impl<Si, Item, U, Fut, F> fmt::Debug for With<Si, Item, U, Fut, F>
+where
+    Si: fmt::Debug,
+    Fut: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("With").field("sink", &self.sink).field("state", &self.state).finish()
+    }
+}
+
+impl<Si, Item, U, Fut, F> With<Si, Item, U, Fut, F>
+where
+    Si: Sink<Item>,
+    F: FnMut(U) -> Fut,
+    Fut: Future,
+{
+    pub(super) fn new<E>(sink: Si, f: F) -> Self
+    where
+        Fut: Future<Output = Result<Item, E>>,
+        E: From<Si::Error>,
+    {
+        Self { state: None, sink, f, _phantom: PhantomData }
+    }
+}
+
+impl<Si, Item, U, Fut, F> Clone for With<Si, Item, U, Fut, F>
+where
+    Si: Clone,
+    F: Clone,
+    Fut: Clone,
+{
+    fn clone(&self) -> Self {
+        Self {
+            state: self.state.clone(),
+            sink: self.sink.clone(),
+            f: self.f.clone(),
+            _phantom: PhantomData,
+        }
+    }
+}
+
+// Forwarding impl of Stream from the underlying sink
+impl<S, Item, U, Fut, F> Stream for With<S, Item, U, Fut, F>
+where
+    S: Stream + Sink<Item>,
+    F: FnMut(U) -> Fut,
+    Fut: Future,
+{
+    type Item = S::Item;
+
+    delegate_stream!(sink);
+}
+
+impl<Si, Item, U, Fut, F, E> With<Si, Item, U, Fut, F>
+where
+    Si: Sink<Item>,
+    F: FnMut(U) -> Fut,
+    Fut: Future<Output = Result<Item, E>>,
+    E: From<Si::Error>,
+{
+    delegate_access_inner!(sink, Si, ());
+
+    /// Completes the processing of previous item if any.
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), E>> {
+        let mut this = self.project();
+
+        let item = match this.state.as_mut().as_pin_mut() {
+            None => return Poll::Ready(Ok(())),
+            Some(fut) => ready!(fut.poll(cx))?,
+        };
+        this.state.set(None);
+        this.sink.start_send(item)?;
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl<Si, Item, U, Fut, F, E> Sink<U> for With<Si, Item, U, Fut, F>
+where
+    Si: Sink<Item>,
+    F: FnMut(U) -> Fut,
+    Fut: Future<Output = Result<Item, E>>,
+    E: From<Si::Error>,
+{
+    type Error = E;
+
+    fn poll_ready(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        ready!(self.as_mut().poll(cx))?;
+        ready!(self.project().sink.poll_ready(cx)?);
+        Poll::Ready(Ok(()))
+    }
+
+    fn start_send(self: Pin<&mut Self>, item: U) -> Result<(), Self::Error> {
+        let mut this = self.project();
+
+        assert!(this.state.is_none());
+        this.state.set(Some((this.f)(item)));
+        Ok(())
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        ready!(self.as_mut().poll(cx))?;
+        ready!(self.project().sink.poll_flush(cx)?);
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        ready!(self.as_mut().poll(cx))?;
+        ready!(self.project().sink.poll_close(cx)?);
+        Poll::Ready(Ok(()))
+    }
+}