io: fix fill_buf by not calling poll_fill_buf twice (#4084)

tokio/src/io/util/fill_buf.rs

@@ -34,15 +34,16 @@ impl<'a, R: AsyncBufRead + ?Sized + Unpin> Future for FillBuf<'a, R> {
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let me = self.project();
 
-        // Due to a limitation in the borrow-checker, we cannot return the value
-        // directly on Ready. Once Rust starts using the polonius borrow checker,
-        // this can be simplified.
         let reader = me.reader.take().expect("Polled after completion.");
         match Pin::new(&mut *reader).poll_fill_buf(cx) {
-            Poll::Ready(_) => match Pin::new(reader).poll_fill_buf(cx) {
-                Poll::Ready(slice) => Poll::Ready(slice),
-                Poll::Pending => panic!("poll_fill_buf returned Pending while having data"),
+            Poll::Ready(Ok(slice)) => unsafe {
+                // Safety: This is necessary only due to a limitation in the
+                // borrow checker. Once Rust starts using the polonius borrow
+                // checker, this can be simplified.
+                let slice = std::mem::transmute::<&[u8], &'a [u8]>(slice);
+                Poll::Ready(Ok(slice))
             },
+            Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
             Poll::Pending => {
                 *me.reader = Some(reader);
                 Poll::Pending

tokio/tests/io_fill_buf.rs

@@ -0,0 +1,34 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tempfile::NamedTempFile;
+use tokio::fs::File;
+use tokio::io::{AsyncBufReadExt, BufReader};
+use tokio_test::assert_ok;
+
+#[tokio::test]
+async fn fill_buf_file() {
+    let file = NamedTempFile::new().unwrap();
+
+    assert_ok!(std::fs::write(file.path(), b"hello"));
+
+    let file = assert_ok!(File::open(file.path()).await);
+    let mut file = BufReader::new(file);
+
+    let mut contents = Vec::new();
+
+    loop {
+        let consumed = {
+            let buffer = assert_ok!(file.fill_buf().await);
+            if buffer.is_empty() {
+                break;
+            }
+            contents.extend_from_slice(buffer);
+            buffer.len()
+        };
+
+        file.consume(consumed);
+    }
+
+    assert_eq!(contents, b"hello");
+}