Multiple enum variants with same value?

Question

What is the best way to have multiple enum variants that are the same value? This is an example of what I would like, except that Rust doesn't like it.

pub enum Nums {
    Num1            = 0,
    Num2            = 1,
    Num3            = 2,
    Num4            = 3,
    Num5            = 4,

    FirstNum        = 0,
    MiddleNum       = 2,
    LastNum         = 4,
}

Answer 1

enum variants cannot have the same tag value as other variants. As of Rust 1.20, you can use associated constants. This lets you use Nums::FIRST_NUM, etc.

pub enum Nums {
    Num1 = 0,
    Num2 = 1,
    Num3 = 2,
    Num4 = 3,
    Num5 = 4,
}

impl Nums {
    pub const FIRST_NUM: Nums = Nums::Num1;
    pub const MIDDLE_NUM: Nums = Nums::Num3;
    pub const LAST_NUM: Nums = Nums::Num5;
}

Before that, you will need to use constants:

pub const FIRST_NUM: Nums = Nums::Num1;
pub const MIDDLE_NUM: Nums = Nums::Num3;
pub const LAST_NUM: Nums = Nums::Num5;

Answer 2

You can also add methods to the enum:

pub enum Nums {
    Num1 = 0,
    Num2 = 1,
    Num3 = 2,
    Num4 = 3,
    Num5 = 4,
}

impl Nums {
    pub fn first()  -> Self { Nums::Num1 }
    pub fn middle() -> Self { Nums::Num3 }
    pub fn last()   -> Self { Nums::Num5 }
}

fn main() {
    println!("{}", Nums::first() as u8);
}