util: add back public poll_read_buf() function (#3079)

This was accidentally removed in #3064.

tokio-util/CHANGELOG.md

@@ -1,11 +1,11 @@
+### Added
+- io: `poll_read_buf` util fn (#2972).
+
 # 0.5.0 (October 30, 2020)
 
 ### Changed
 - io: update `bytes` to 0.6 (#3071).
 
-### Added
-- io: `poll_read_buf` util fn (#2972).
-
 # 0.4.0 (October 15, 2020)
 
 ### Added

tokio-util/src/codec/framed_impl.rs

@@ -150,7 +150,7 @@ where
             // got room for at least one byte to read to ensure that we don't
             // get a spurious 0 that looks like EOF
             state.buffer.reserve(1);
-            let bytect = match poll_read_buf(cx, pinned.inner.as_mut(), &mut state.buffer)? {
+            let bytect = match poll_read_buf(pinned.inner.as_mut(), cx, &mut state.buffer)? {
                 Poll::Ready(ct) => ct,
                 Poll::Pending => return Poll::Pending,
             };

tokio-util/src/io/mod.rs

@@ -13,3 +13,4 @@ mod stream_reader;
 pub use self::read_buf::read_buf;
 pub use self::reader_stream::ReaderStream;
 pub use self::stream_reader::StreamReader;
+pub use crate::util::poll_read_buf;

tokio-util/src/io/read_buf.rs

@@ -59,7 +59,7 @@ where
 
         fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
             let this = &mut *self;
-            crate::util::poll_read_buf(cx, Pin::new(this.0), this.1)
+            crate::util::poll_read_buf(Pin::new(this.0), cx, this.1)
         }
     }
 }

tokio-util/src/io/reader_stream.rs

@@ -83,7 +83,7 @@ impl<R: AsyncRead> Stream for ReaderStream<R> {
             this.buf.reserve(CAPACITY);
         }
 
-        match poll_read_buf(cx, reader, &mut this.buf) {
+        match poll_read_buf(reader, cx, &mut this.buf) {
             Poll::Pending => Poll::Pending,
             Poll::Ready(Err(err)) => {
                 self.project().reader.set(None);

tokio-util/src/lib.rs

@@ -69,10 +69,49 @@ mod util {
     use std::pin::Pin;
     use std::task::{Context, Poll};
 
-    pub(crate) fn poll_read_buf<T: AsyncRead>(
-        cx: &mut Context<'_>,
+    /// Try to read data from an `AsyncRead` into an implementer of the [`Buf`] trait.
+    ///
+    /// [`Buf`]: bytes::Buf
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use bytes::{Bytes, BytesMut};
+    /// use tokio::stream;
+    /// use tokio::io::Result;
+    /// use tokio_util::io::{StreamReader, poll_read_buf};
+    /// use futures::future::poll_fn;
+    /// use std::pin::Pin;
+    /// # #[tokio::main]
+    /// # async fn main() -> std::io::Result<()> {
+    ///
+    /// // Create a reader from an iterator. This particular reader will always be
+    /// // ready.
+    /// let mut read = StreamReader::new(stream::iter(vec![Result::Ok(Bytes::from_static(&[0, 1, 2, 3]))]));
+    ///
+    /// let mut buf = BytesMut::new();
+    /// let mut reads = 0;
+    ///
+    /// loop {
+    ///     reads += 1;
+    ///     let n = poll_fn(|cx| poll_read_buf(Pin::new(&mut read), cx, &mut buf)).await?;
+    ///
+    ///     if n == 0 {
+    ///         break;
+    ///     }
+    /// }
+    ///
+    /// // one or more reads might be necessary.
+    /// assert!(reads >= 1);
+    /// assert_eq!(&buf[..], &[0, 1, 2, 3]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg_attr(not(feature = "io"), allow(unreachable_pub))]
+    pub fn poll_read_buf<T: AsyncRead, B: BufMut>(
         io: Pin<&mut T>,
-        buf: &mut impl BufMut,
+        cx: &mut Context<'_>,
+        buf: &mut B,
     ) -> Poll<io::Result<usize>> {
         if !buf.has_remaining_mut() {
             return Poll::Ready(Ok(0));