1 files changed, 72 insertions, 0 deletions
diff --git a/tokio/src/io/util/read_buf.rs b/tokio/src/io/util/read_buf.rs
new file mode 100644
index 00000000..696deefd
--- /dev/null
+++ b/tokio/src/io/util/read_buf.rs
@@ -0,0 +1,72 @@
+use crate::io::AsyncRead;
+
+use bytes::BufMut;
+use pin_project_lite::pin_project;
+use std::future::Future;
+use std::io;
+use std::marker::PhantomPinned;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pub(crate) fn read_buf<'a, R, B>(reader: &'a mut R, buf: &'a mut B) -> ReadBuf<'a, R, B>
+where
+    R: AsyncRead + Unpin,
+    B: BufMut,
+{
+    ReadBuf {
+        reader,
+        buf,
+        _pin: PhantomPinned,
+    }
+}
+
+pin_project! {
+    /// Future returned by [`read_buf`](crate::io::AsyncReadExt::read_buf).
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct ReadBuf<'a, R, B> {
+        reader: &'a mut R,
+        buf: &'a mut B,
+        #[pin]
+        _pin: PhantomPinned,
+    }
+}
+
+impl<R, B> Future for ReadBuf<'_, R, B>
+where
+    R: AsyncRead + Unpin,
+    B: BufMut,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        use crate::io::ReadBuf;
+        use std::mem::MaybeUninit;
+
+        let me = self.project();
+
+        if !me.buf.has_remaining_mut() {
+            return Poll::Ready(Ok(0));
+        }
+
+        let n = {
+            let dst = me.buf.bytes_mut();
+            let dst = unsafe { &mut *(dst as *mut _ as *mut [MaybeUninit<u8>]) };
+            let mut buf = ReadBuf::uninit(dst);
+            let ptr = buf.filled().as_ptr();
+            ready!(Pin::new(me.reader).poll_read(cx, &mut buf)?);
+
+            // Ensure the pointer does not change from under us
+            assert_eq!(ptr, buf.filled().as_ptr());
+            buf.filled().len()
+        };
+
+        // Safety: This is guaranteed to be the number of initialized (and read)
+        // bytes due to the invariants provided by `ReadBuf::filled`.
+        unsafe {
+            me.buf.advance_mut(n);
+        }
+
+        Poll::Ready(Ok(n))
+    }
+}