compiler/rustc_error_codes/src/error_codes/E0732.md

   1 An `enum` with a discriminant must specify a `#[repr(inttype)]`.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0732
   6 #![feature(arbitrary_enum_discriminant)]
   7
   8 enum Enum { // error!
   9     Unit = 1,
  10     Tuple() = 2,
  11     Struct{} = 3,
  12 }
  13 # fn main() {}
  14 ```
  15
  16 A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit
  17 variant with a discriminant, or where there are both unit variants with
  18 discriminants and non-unit variants. This restriction ensures that there
  19 is a well-defined way to extract a variant's discriminant from a value;
  20 for instance:
  21
  22 ```
  23 #![feature(arbitrary_enum_discriminant)]
  24
  25 #[repr(u8)]
  26 enum Enum {
  27     Unit = 3,
  28     Tuple(u16) = 2,
  29     Struct {
  30         a: u8,
  31         b: u16,
  32     } = 1,
  33 }
  34
  35 fn discriminant(v : &Enum) -> u8 {
  36     unsafe { *(v as *const Enum as *const u8) }
  37 }
  38
  39 fn main() {
  40     assert_eq!(3, discriminant(&Enum::Unit));
  41     assert_eq!(2, discriminant(&Enum::Tuple(5)));
  42     assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
  43 }
  44 ```