derive-discriminant

```rust

[derive(Discriminant)]

[derive(Debug, PartialEq, Eq)]

enum Abc { A, B { x: usize, y: usize }, }

fn main() { // tryfrom implementations let a = A::tryfrom(Abc::A).unwrap(); asserteq!(a, A); let nota = A::tryfrom(Abc::B { x: 1, y: 2 }); assert!(nota.is_err());

let b = B::try_from(Abc::B { x: 1, y: 2 }).unwrap();
assert_eq!(b, B { x: 1, y: 2 });
let not_b = B::try_from(Abc::A);
assert!(not_b.is_err());

// from implementations
let a = Abc::from(A);
assert_eq!(a, Abc::A);
let b = Abc::from(B { x: 1, y: 2 });
assert_eq!(b, Abc::B { x: 1, y: 2 });

// casting
let a = Abc::A;
let a: Box<dyn Debug> = a.cast();
assert_eq!(format!("{a:?}"), "A");

let b = Abc::B { x: 1, y: 2 };
let b: Box<dyn Debug> = b.cast();
assert_eq!(format!("{b:?}"), "B { x: 1, y: 2 }");

// is_{variant} methods
assert!(Abc::A.is_a());
assert!(!Abc::A.is_b());

assert!(!Abc::B { x: 1, y: 2 }.is_a());
assert!(Abc::B { x: 1, y: 2 }.is_b());

} ```