before_after.rs

// before
fn parse_args<Iter>(mut args: Iter) -> Result<Args, &'static str>
    where Iter: Iterator<Item = String>
{
    let hostname = match args.next() {
        Some(s) => s,
        None => return Err("argument 'hostname' missing"),
    };

    let port: u16 = match args.next() {
        Some(s) => s.parse().map_err(|_| "cannot parse port")?,
        None => return Err("argument 'port' missing"),
    };

    let username = match args.next() {
        Some(s) => s,
        None => return Err("argument 'username' missing"),
    };

    let priv_key_path = match args.next() {
        Some(s) => s,
        None => return Err("path to private key is missing"),
    };

    Ok(Args {
        hostname: hostname,
        port: port,
        username: username,
        priv_key_path: PathBuf::from(priv_key_path),
    })
}

// after
fn parse_args<Iter>(mut args: Iter) -> Result<Args, &'static str>
    where Iter: Iterator<Item = String>
{
    let hostname = args.next().ok_or("argument 'hostname' missing")?;
    let port = args.next().ok_or("argument 'port' missing")?;
    let port: u16 = port.parse().map_err(|_| "cannot parse port")?;
    let username = args.next().ok_or("argument 'username' missing")?;
    let priv_key_path = args.next().ok_or("path to private key is missing")?;

    Ok(Args {
        hostname: hostname,
        port: port,
        username: username,
        priv_key_path: PathBuf::from(priv_key_path),
    })
}