Match Guards

When matching, you can add a guard to a pattern. This is an arbitrary Boolean expression which will be executed if the pattern matches:

#[rustfmt::skip]
fn main() {
    let pair = (2, -2);
    println!("Tell me about {pair:?}");
    match pair {
        (x, y) if x == y     => println!("These are twins"),
        (x, y) if x + y == 0 => println!("Antimatter, kaboom!"),
        (x, _) if x % 2 == 1 => println!("The first one is odd"),
        _                    => println!("No correlation..."),
    }
}

Key Points:

  • Match guards as a separate syntax feature are important and necessary when we wish to concisely express more complex ideas than patterns alone would allow.
  • They are not the same as separate if expression inside of the match arm. An if expression inside of the branch block (after =>) happens after the match arm is selected. Failing the if condition inside of that block won’t result in other arms of the original match expression being considered.
  • You can use the variables defined in the pattern in your if expression.
  • The condition defined in the guard applies to every expression in a pattern with an |.