Keyword match

Control flow based on pattern matching.

match can be used to run code conditionally. Every pattern must be handled exhaustively either explicitly or by using wildcards like _ in the match. Since match is an expression, values can also be returned.

let opt = Option::None::<usize>;
let x = match opt {
    Some(int) => int,
    None => 10,
};
assert_eq!(x, 10);

let a_number = Option::Some(10);
match a_number {
    Some(x) if x <= 5 => println!("0 to 5 num = {x}"),
    Some(x @ 6..=10) => println!("6 to 10 num = {x}"),
    None => panic!(),
    // all other numbers
    _ => panic!(),
}

match can be used to gain access to the inner members of an enum and use them directly.

enum Outer {
    Double(Option<u8>, Option<String>),
    Single(Option<u8>),
    Empty
}

let get_inner = Outer::Double(None, Some(String::new()));
match get_inner {
    Outer::Double(None, Some(st)) => println!("{st}"),
    Outer::Single(opt) => println!("{opt:?}"),
    _ => panic!(),
}

For more information on match and matching in general, see the Reference.