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Rollup merge of #70927 - GuillaumeGomez:cleanup-e0510, r=Dylan-DPC

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Clean up E0510 explanation

r? @Dylan-DPC
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Dylan DPC 2020-04-08 18:37:31 +02:00 committed by GitHub
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27
src/librustc_error_codes/error_codes/E0510.md
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@ -1,16 +1,29 @@
Cannot mutate place in this match guard.
The matched value was assigned in a match guard.
When matching on a variable it cannot be mutated in the match guards, as this
could cause the match to be non-exhaustive:
Erroneous code example:
```compile_fail,E0510
let mut x = Some(0);
match x {
None => (),
Some(_) if { x = None; false } => (),
Some(v) => (), // No longer matches
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 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!
}
}
```