pub fn copy_buf<R, W>(reader: R, writer: &mut W) -> CopyBuf<'_, R, W>
where
    R: AsyncBufRead,
    W: AsyncWrite + Unpin + ?Sized,
{
    CopyBuf { reader, writer, amt: 0 }
}
Creates a future which copies all the bytes from one object to another.

The returned future will copy all the bytes read from this `AsyncBufRead` into the
`writer` specified. This future will only complete once the `reader` has hit
EOF and all bytes have been written to and flushed from the `writer`
provided.

On success the number of bytes is returned.

# Examples

```
# futures::executor::block_on(async {
use futures::io::{self, AsyncWriteExt, Cursor};

let reader = Cursor::new([1, 2, 3, 4]);
let mut writer = Cursor::new(vec![0u8; 5]);

let bytes = io::copy_buf(reader, &mut writer).await?;
writer.close().await?;

assert_eq!(bytes, 4);
assert_eq!(writer.into_inner(), [1, 2, 3, 4, 0]);
# Ok::<(), Box<dyn std::error::Error>>(()) }).unwrap();
```
futures-util/src/io/copy_buf.rs