New upstream version 1.63.0+dfsg1

[rustc.git] / src / doc / rust-by-example / src / flow_control / match / guard.md

# Guards

A `match` *guard* can be added to filter the arm.

```rust,editable
enum Temperature {
    Celsius(i32),
    Farenheit(i32),
}

fn main() {
    let temperature = Temperature::Celsius(35);
    // ^ TODO try different values for `temperature`

    match temperature {
        Temperature::Celsius(t) if t > 30 => println!("{}C is above 30 Celsius", t),
        // The `if condition` part ^ is a guard
        Temperature::Celsius(t) => println!("{}C is below 30 Celsius", t),

        Temperature::Farenheit(t) if t > 86 => println!("{}F is above 86 Farenheit", t),
        Temperature::Farenheit(t) => println!("{}F is below 86 Farenheit", t),
    }
}
```

Note that the compiler won't take guard conditions into account when checking
if all patterns are covered by the match expression.

```rust,editable,ignore,mdbook-runnable
fn main() {
    let number: u8 = 4;

    match number {
        i if i == 0 => println!("Zero"),
        i if i > 0 => println!("Greater than zero"),
        // _ => unreachable!("Should never happen."),
        // TODO ^ uncomment to fix compilation
    }
}
```

### See also:

[Tuples](../../primitives/tuples.md)
[Enums](../../custom_types/enum.md)