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# Error code E0510

The matched value was assigned in a match guard.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    let mut x = Some(0);
    match x {
        None => {}
        Some(_) if { x = None; false } => {} // error!
        Some(_) => {}
    }
    }

When matching on a variable it cannot be mutated in the match guards, as this could cause the match to be non-exhaustive.

Here executing `x = None` would modify the value being matched and require us to go "back in time" to the `None` arm. To fix it, change the value in the match arm:

    #![allow(unused)]
    fn main() {
    let mut x = Some(0);
    match x {
        None => {}
        Some(_) => {
            x = None; // ok!
        }
    }
    }