How do I convert an enum reference to a number?

I have an enum:

enum Foo {
    Bar = 1,
}

How do I convert a reference to this enum into an integer to be used in math?

fn f(foo: &Foo) {
    let f = foo as u8;  // error[E0606]: casting `&Foo` as `u8` is invalid
    let f = foo as &u8; // error[E0605]: non-primitive cast: `&Foo` as `&u8`
    let f = *foo as u8; // error[E0507]: cannot move out of borrowed content
}

Solution 1:

*foo as u8 is correct, but you have to implement Copy because otherwise you would leave behind an invalid reference.

#[derive(Copy, Clone)]
enum Foo {
    Bar = 1,
}

fn f(foo: &Foo) -> u8 {
    *foo as u8
}

Since your enum will be a very lightweight object you should pass it around by value anyway, for which you would need Copy as well.