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An `enum` with a discriminant must specify a `#[repr(inttype)]`.
Erroneous code example:
```compile_fail,E0732
#![feature(arbitrary_enum_discriminant)]
enum Enum { // error!
Unit = 1,
Tuple() = 2,
Struct{} = 3,
}
# fn main() {}
```
A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit
variant with a discriminant, or where there are both unit variants with
discriminants and non-unit variants. This restriction ensures that there
is a well-defined way to extract a variant's discriminant from a value;
for instance:
```
#![feature(arbitrary_enum_discriminant)]
#[repr(u8)]
enum Enum {
Unit = 3,
Tuple(u16) = 2,
Struct {
a: u8,
b: u16,
} = 1,
}
fn discriminant(v : &Enum) -> u8 {
unsafe { *(v as *const Enum as *const u8) }
}
fn main() {
assert_eq!(3, discriminant(&Enum::Unit));
assert_eq!(2, discriminant(&Enum::Tuple(5)));
assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
}
```
|
