Rust for C-Programmers

21.8 Ignoring Parts of a Value: _ and ..

Often, you only care about certain parts of a value. Rust provides ways to ignore the rest:

• _: Ignores a single element or field. Can be used multiple times.
• _name: A variable name starting with _ still binds the value but signals intent to potentially not use it, suppressing the “unused variable” warning.
• ..: Ignores all remaining elements in a tuple, struct, slice, or array pattern. Can appear at most once per pattern.

struct Config {
    hostname: String,
    port: u16,
    retries: u8,
}

fn check_port(config: &Config) {
    match config {
        // Match only standard web ports, ignore other fields with `..`
        Config { port: 80 | 443, .. } => {
            println!("Using standard web port: {}", config.port);
        }
        // Match specific hostname, ignore port using `_`, ignore retries with `..`
        Config { hostname: h, port: _, .. } if h == "localhost" => {
             println!("Connecting to localhost on some port.");
        }
        // Ignore the entire struct content
        _ => {
            println!("Using non-standard configuration on host: {}", config.hostname);
        }
    }
}

fn main() {
    let cfg1 = Config { hostname: "example.com".to_string(), port: 80, retries: 3 };
    let cfg2 = Config { hostname: "localhost".to_string(), port: 8080, retries: 5 };
    let cfg3 = Config { hostname: "internal.net".to_string(), port: 9000, retries: 1 };

    check_port(&cfg1); // Output: Using standard web port: 80
    check_port(&cfg2); // Output: Connecting to localhost on some port.
    check_port(&cfg3); // Output: Using non-standard configuration on host: internal.net
}

Using .. is more concise than listing all ignored fields with _, e.g., Config { port: 80, hostname: _, retries: _ }.