Fix unreachable match arm in FillBuf being reachable

futures-util/src/io/fill_buf.rs

@@ -30,6 +30,11 @@ where
         let reader = this.reader.take().expect("Polled FillBuf after completion");
 
         match Pin::new(&mut *reader).poll_fill_buf(cx) {
+            // The reader can try to fill its buffer again if it returned no data, according to the
+            // AsyncBufRead contract. Don't poll again in that case, in order to make the
+            // Poll::Pending match arm below truly unreachable.
+            Poll::Ready(Ok([])) => Poll::Ready(Ok(&[])),
+
             // 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) {

futures/tests/io_buf_reader.rs

@@ -430,3 +430,16 @@ fn maybe_pending_seek() {
     Pin::new(&mut reader).consume(1);
     assert_eq!(run(reader.seek(SeekFrom::Current(-2))).ok(), Some(3));
 }
+
+#[test]
+fn fill_buf_pending_after_eof() {
+    let inner: &[u8] = &[1, 2];
+    let mut reader = BufReader::with_capacity(inner.len(), MaybePending::new(inner));
+
+    let buf = run(reader.fill_buf()).unwrap();
+    assert_eq!(buf, inner);
+    reader.consume_unpin(2);
+
+    let buf = run(reader.fill_buf()).unwrap();
+    assert_eq!(buf, []);
+}