src/librustc_error_codes/error_codes/E0302.md

   1 #### Note: this error code is no longer emitted by the compiler.
   2
   3 Assignments are not allowed in pattern guards, because matching cannot have
   4 side effects. Side effects could alter the matched object or the environment
   5 on which the match depends in such a way, that the match would not be
   6 exhaustive. For instance, the following would not match any arm if assignments
   7 were allowed:
   8
   9 ```compile_fail,E0594
  10 match Some(()) {
  11     None => { },
  12     option if { option = None; false } => { },
  13     Some(_) => { } // When the previous match failed, the option became `None`.
  14 }
  15 ```