util: add links to tokio-util + example to BytesCodec (#2207)

tokio-util/src/codec/bytes_codec.rs

@@ -4,7 +4,43 @@ use crate::codec::encoder::Encoder;
 use bytes::{BufMut, Bytes, BytesMut};
 use std::io;
 
-/// A simple `Codec` implementation that just ships bytes around.
+/// A simple [`Decoder`] and [`Encoder`] implementation that just ships bytes around.
+///
+/// [`Decoder`]: crate::codec::Decoder
+/// [`Encoder`]: crate::codec::Encoder
+///
+/// # Example
+///
+/// Turn an [`AsyncRead`] into a stream of `Result<`[`BytesMut`]`, `[`Error`]`>`.
+///
+/// [`AsyncRead`]: tokio::io::AsyncRead
+/// [`BytesMut`]: bytes::BytesMut
+/// [`Error`]: std::io::Error
+///
+/// ```
+/// # mod hidden {
+/// # #[allow(unused_imports)]
+/// use tokio::fs::File;
+/// # }
+/// use tokio::io::AsyncRead;
+/// use tokio_util::codec::{FramedRead, BytesCodec};
+///
+/// # enum File {}
+/// # impl File {
+/// #     async fn open(_name: &str) -> Result<impl AsyncRead, std::io::Error> {
+/// #         use std::io::Cursor;
+/// #         Ok(Cursor::new(vec![0, 1, 2, 3, 4, 5]))
+/// #     }
+/// # }
+/// #
+/// # #[tokio::main(core_threads = 1)]
+/// # async fn main() -> Result<(), std::io::Error> {
+/// let my_async_read = File::open("filename.txt").await?;
+/// let my_stream_of_bytes = FramedRead::new(my_async_read, BytesCodec::new());
+/// # Ok(())
+/// # }
+/// ```
+///
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Default)]
 pub struct BytesCodec(());