src/librustc_error_codes/error_codes/E0510.md

   1 The matched value was assigned in a match guard.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0510
   6 let mut x = Some(0);
   7 match x {
   8     None => {}
   9     Some(_) if { x = None; false } => {} // error!
  10     Some(_) => {}
  11 }
  12 ```
  13
  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
  17 Here executing `x = None` would modify the value being matched and require us
  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 ```