1 files changed, 51 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/io/fill_buf.rs b/third_party/rust/futures-util/src/io/fill_buf.rs
new file mode 100644
index 0000000000..a1484c0322
--- /dev/null
+++ b/third_party/rust/futures-util/src/io/fill_buf.rs
@@ -0,0 +1,51 @@
+use futures_core::future::Future;
+use futures_core::task::{Context, Poll};
+use futures_io::AsyncBufRead;
+use std::io;
+use std::pin::Pin;
+
+/// Future for the [`fill_buf`](super::AsyncBufReadExt::fill_buf) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct FillBuf<'a, R: ?Sized> {
+    reader: Option<&'a mut R>,
+}
+
+impl<R: ?Sized> Unpin for FillBuf<'_, R> {}
+
+impl<'a, R: AsyncBufRead + ?Sized + Unpin> FillBuf<'a, R> {
+    pub(super) fn new(reader: &'a mut R) -> Self {
+        Self { reader: Some(reader) }
+    }
+}
+
+impl<'a, R> Future for FillBuf<'a, R>
+where
+    R: AsyncBufRead + ?Sized + Unpin,
+{
+    type Output = io::Result<&'a [u8]>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = &mut *self;
+        let reader = this.reader.take().expect("Polled FillBuf after completion");
+
+        match Pin::new(&mut *reader).poll_fill_buf(cx) {
+            // With polonius it is possible to remove this inner match and just have the correct
+            // lifetime of the reference inferred based on which branch is taken
+            Poll::Ready(Ok(_)) => match Pin::new(reader).poll_fill_buf(cx) {
+                Poll::Ready(Ok(slice)) => Poll::Ready(Ok(slice)),
+                Poll::Ready(Err(err)) => {
+                    unreachable!("reader indicated readiness but then returned an error: {:?}", err)
+                }
+                Poll::Pending => {
+                    unreachable!("reader indicated readiness but then returned pending")
+                }
+            },
+            Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
+            Poll::Pending => {
+                this.reader = Some(reader);
+                Poll::Pending
+            }
+        }
+    }
+}