compiler/rustc_error_codes/src/error_codes/E0302.md

#### Note: this error code is no longer emitted by the compiler.

Assignments are not allowed in pattern guards, because matching cannot have
side effects. Side effects could alter the matched object or the environment
on which the match depends in such a way, that the match would not be
exhaustive. For instance, the following would not match any arm if assignments
were allowed:

```compile_fail,E0594
match Some(()) {
    None => { },
    option if { option = None; false } => { },
    Some(_) => { } // When the previous match failed, the option became `None`.
}
```