io: add tokio_util::io::{CopyToBytes, SinkWriter} (#5070)

Co-authored-by: Alice Ryhl <[email protected]>
Co-authored-by: Simon Farnsworth <[email protected]>

tokio-util/src/io/copy_to_bytes.rs

@@ -0,0 +1,68 @@
+use bytes::Bytes;
+use futures_sink::Sink;
+use pin_project_lite::pin_project;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// A helper that wraps a [`Sink`]`<`[`Bytes`]`>` and converts it into a
+    /// [`Sink`]`<&'a [u8]>` by copying each byte slice into an owned [`Bytes`].
+    ///
+    /// See the documentation for [`SinkWriter`] for an example.
+    ///
+    /// [`Bytes`]: bytes::Bytes
+    /// [`SinkWriter`]: crate::io::SinkWriter
+    /// [`Sink`]: futures_sink::Sink
+    #[derive(Debug)]
+    pub struct CopyToBytes<S> {
+        #[pin]
+        inner: S,
+    }
+}
+
+impl<S> CopyToBytes<S> {
+    /// Creates a new [`CopyToBytes`].
+    pub fn new(inner: S) -> Self {
+        Self { inner }
+    }
+
+    /// Gets a reference to the underlying sink.
+    pub fn get_ref(&self) -> &S {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying sink.
+    pub fn get_mut(&mut self) -> &mut S {
+        &mut self.inner
+    }
+
+    /// Consumes this [`CopyToBytes`], returning the underlying sink.
+    pub fn into_inner(self) -> S {
+        self.inner
+    }
+}
+
+impl<'a, S> Sink<&'a [u8]> for CopyToBytes<S>
+where
+    S: Sink<Bytes>,
+{
+    type Error = S::Error;
+
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.project().inner.poll_ready(cx)
+    }
+
+    fn start_send(self: Pin<&mut Self>, item: &'a [u8]) -> Result<(), Self::Error> {
+        self.project()
+            .inner
+            .start_send(Bytes::copy_from_slice(item))
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.project().inner.poll_flush(cx)
+    }
+
+    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.project().inner.poll_close(cx)
+    }
+}