How do I pass an enum variant to match on as a function parameter?

Question

I would like to pass in the parameters what arm of the enum I need to match, something like this:

enum D {
    A(i64),
    B(u64),
    C(u64, u64),
}

let a = D.A(10);
println!(a.is_of(D.A)); // true
println!(a.is_of(D.B)); // false

I know I can use matching rules for this, but I'd like this is_of method to take as an input of the enum options for my purposes.

Answer 1

You cannot.

• It is not possible to pass types as function parameters.
• Enum variants are not types to start with.

If you are OK using a macro instead of a function, see

• How do I assert an enum is a specific variant if I don't care about its fields?
• The matches crate

See also:

• Compare enums only by variant, not value

• Can traits be used on enum types?

• Can struct-like enums be used as types?
• Is there a way to use existing structs as enum variants?

Answer 2

The discriminant of an enum variant can be passed for compairson

use std::mem::{discriminant,Discriminant};

enum MyEnum {
  A,
  B,
}

fn is_enum_variant(value: &MyEnum, d: Discriminant<MyEnum>) -> bool {
  discriminant(value) == d
}

fn main() {
  println!("Is variant: {}", is_enum_variant(&MyEnum::A, discriminant(&MyEnum::A)));
  println!("Is variant: {}", is_enum_variant(&MyEnum::A, discriminant(&MyEnum::B)));
}

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