You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: io_copy.rs
    path of file: ./repos/kani/tests/slow/tokio-proofs/src/tokio
    
    the code of the file until where you have to start completion: // SPDX-License-Identifier: Apache-2.0 OR MIT
//
// Modifications Copyright Kani Contributors
// See GitHub history for details.

// Original copyright tokio contributors.
// origin: tokio/tests/tok
    
async fn copy() {
    struct Rd(bool);

    impl AsyncRead for Rd {
        fn poll_read(
            mut self: Pin<&mut Self>,
            _cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            if self.0 {
                buf.put_slice(b"hello world");
                self.0 = false;
                Poll::Ready(Ok(()))
            } else {
                Poll::Ready(Ok(()))
            }
        }
    }

    let mut rd = Rd(true);
    let mut wr = Vec::new();

    let n = assert_ok!(io::copy(&mut rd, &mut wr).await);
    assert_eq!(n, 11);
    assert_eq!(wr, b"hello world");
}