src/librustc_error_codes/error_codes/E0029.md

   1 Something other than numbers and characters has been used for a range.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0029
   6 let string = "salutations !";
   7
   8 // The ordering relation for strings cannot be evaluated at compile time,
   9 // so this doesn't work:
  10 match string {
  11     "hello" ..= "world" => {}
  12     _ => {}
  13 }
  14
  15 // This is a more general version, using a guard:
  16 match string {
  17     s if s >= "hello" && s <= "world" => {}
  18     _ => {}
  19 }
  20 ```
  21
  22 In a match expression, only numbers and characters can be matched against a
  23 range. This is because the compiler checks that the range is non-empty at
  24 compile-time, and is unable to evaluate arbitrary comparison functions. If you
  25 want to capture values of an orderable type between two end-points, you can use
  26 a guard.