Attribute Macro fields 

#[fields]

Insert fields for each variant, and generate getter for each fields

Example

#[enum_struct::fields {
    id: u64,
}]
#[derive(Debug, PartialEq)]
enum Foo {
    Named(String),
    Complex { name: String, age: u32, level: u16 },
    Empty,
}

let named = Foo::Named(2, "jack".into());
let complex = Foo::Complex { id: 3, name: "john".into(), age: 22, level: 4 };
let empty = Foo::Empty { id: 4 };

assert_eq!(named.id(), &2);
assert_eq!(complex.id(), &3);
assert_eq!(empty.id(), &4);

let mut named = named;

*named.id_mut() = 8;
assert_eq!(named.id(), &8);
assert_eq!(named, Foo::Named(8, "jack".into()));