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: z_pong.rs
    path of file: ./repos/zenoh/examples/examples
    
    the code of the file until where you have to start completion: use std::io::{stdin, Read};

//
// C
    
fn main() {
    // initiate logging
    env_logger::init();

    let config = parse_args();

    let session = zenoh::open(config).res().unwrap().into_arc();

    // The key expression to read the data from
    let key_expr_ping = keyexpr::new("test/ping").unwrap();

    // The key expression to echo the data back
    let key_expr_pong = keyexpr::new("test/pong").unwrap();

    let publisher = session
        .declare_publisher(key_expr_pong)
        .congestion_control(CongestionControl::Block)
        .res()
        .unwrap();

    let _sub = session
        .declare_subscriber(key_expr_ping)
        .callback(move |sample| publisher.put(sample.value).res().unwrap())
        .res()
        .unwrap();
    for _ in stdin().bytes().take_while(|b| !matches!(b, Ok(b'q'))) {}
}