Derive Macro HasVariant 

#[derive(HasVariant)]

Dervies HasVariant for an enum. Note: it ignores variants with multiple or no contents. Currently does not support generics nor lifetimes.

For example:

#[derive(HasVariant)]
enum MyEnum {
    Integer(i32),
    String(String),
}

MyEnum now implements HasVariant<i32> and HasVariant<String>.