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The matched value was assigned in a match guard.
Erroneous code example:
```compile_fail,E0510
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:
```
let mut x = Some(0);
match x {
None => {}
Some(_) => {
x = None; // ok!
}
}
```
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