arbitrary_enum_discriminant - The Rust Unstable Book

# `arbitrary_enum_discriminant`

The tracking issue for this feature is: #60553

The `arbitrary_enum_discriminant` feature permits tuple-like and struct-like enum variants with `#[repr(<int-type>)]` to have explicit discriminants.

## Examples

    #![allow(unused)]
    #![feature(arbitrary_enum_discriminant)]

    fn main() {
    #[allow(dead_code)]
    #[repr(u8)]
    enum Enum {
        Unit = 3,
        Tuple(u16) = 2,
        Struct {
            a: u8,
            b: u16,
        } = 1,
    }

    impl Enum {
        fn tag(&self) -> u8 {
            unsafe { *(self as *const Self as *const u8) }
        }
    }

    assert_eq!(3, Enum::Unit.tag());
    assert_eq!(2, Enum::Tuple(5).tag());
    assert_eq!(1, Enum::Struct{a: 7, b: 11}.tag());
    }