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_chain.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-Ide
    
async fn chain() {
    let mut buf = Vec::new();
    let rd1: &[u8] = b"hello ";
    let rd2: &[u8] = b"world";

    let mut rd = rd1.chain(rd2);
    assert_ok!(rd.read_to_end(&mut buf).await);
    assert_eq!(buf, b"hello world");
}