compiler/rustc_error_codes/src/error_codes/E0158.md

   1 An associated const has been referenced in a pattern.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0158
   6 enum EFoo { A, B, C, D }
   7
   8 trait Foo {
   9     const X: EFoo;
  10 }
  11
  12 fn test<A: Foo>(arg: EFoo) {
  13     match arg {
  14         A::X => { // error!
  15             println!("A::X");
  16         }
  17     }
  18 }
  19 ```
  20
  21 `const` and `static` mean different things. A `const` is a compile-time
  22 constant, an alias for a literal value. This property means you can match it
  23 directly within a pattern.
  24
  25 The `static` keyword, on the other hand, guarantees a fixed location in memory.
  26 This does not always mean that the value is constant. For example, a global
  27 mutex can be declared `static` as well.
  28
  29 If you want to match against a `static`, consider using a guard instead:
  30
  31 ```
  32 static FORTY_TWO: i32 = 42;
  33
  34 match Some(42) {
  35     Some(x) if x == FORTY_TWO => {}
  36     _ => {}
  37 }
  38 ```