[rustc.git] / src / librustc_error_codes / error_codes / E0510.md

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!
    }
}
```
Commit	Line	Data
ba9703b0	1	The matched value was assigned in a match guard.
60c5eb7d	2
ba9703b0	3	Erroneous code example:
60c5eb7d XL	4
	5	```compile_fail,E0510
	6	let mut x = Some(0);
	7	match x {
ba9703b0 XL	8	None => {}
	9	Some(_) if { x = None; false } => {} // error!
	10	Some(_) => {}
60c5eb7d XL	11	}
	12	```
	13
ba9703b0 XL	14	When matching on a variable it cannot be mutated in the match guards, as this
	15	could cause the match to be non-exhaustive.
	16
60c5eb7d	17	Here executing `x = None` would modify the value being matched and require us
ba9703b0 XL	18	to go "back in time" to the `None` arm. To fix it, change the value in the match
	19	arm:
	20
	21	```
	22	let mut x = Some(0);
	23	match x {
	24	None => {}
	25	Some(_) => {
	26	x = None; // ok!
	27	}
	28	}
	29	```