compiler/rustc_error_codes/src/error_codes/E0301.md

   1 #### Note: this error code is no longer emitted by the compiler.
   2
   3 Mutable borrows 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 mutable
   7 borrows were allowed:
   8
   9 ```compile_fail,E0596
  10 match Some(()) {
  11     None => { },
  12     option if option.take().is_none() => {
  13         /* impossible, option is `Some` */
  14     },
  15     Some(_) => { } // When the previous match failed, the option became `None`.
  16 }
  17 ```