The problem with unions/enums is that is hard to pick a subset or superset of them, despite it will be not hard to add to any language. Is similar to projection on the relational model (aka:select), sometimes I need:

    enum Value {
        Int32(i32),
        Int64(i32),
        Str(String),    
    }
    fn only_int(of:Value[Int32, Int64]) -> Value.Str

And other times:

    enum ExtraValue:Value {
        Bool(bool), 
    }

This is analog to structs, where eventually is "discovered" that is nice to:

    let user2 = User {
        email: String::from("another@example.com"),
        username: String::from("anotherusername567"),
        active: user1.active,
        sign_in_count: user1.sign_in_count,
    };


    let user2 = User {
        email: String::from("another@example.com"),
        username: String::from("anotherusername567"),
        ..user1
    };
     

But somehow enums are still considered second-class in the algebraic type space.