Yes, you do. There is no idea of a 'default' type here, because each variant of the enum is treated exactly the same. Your C# example assumes an inheritance rel

Yes, you do. There is no idea of a 'default' type here, because each variant of the enum is treated exactly the same. Your C# example assumes an inheritance relationship between T1 and T2 (i.e. T2 inherits from T1, so you can pass it to something expecting a T1 reference). Rust doesn't have type inheritance, so the only way you can define relationships between types is to have something external tell you how to package them together, which is what the enum would do here. So it's not a parent-child relationship between the types, it's a new thing which says "here is a thing which can be a T1 or a T2 but not both at once". Not having OO does require you to think somewhat differently about how to build your data structures in Rust. I've found my intuition from Haskell is a stronger guide when working with Rust, but since Rust's type system is much more like that in an ML-family language this makes quite a bit of sense! Sadly this does increase the learning curve for more mainstream languages. (Rust does have trait inheritance, but all that says is that to implement trait T2 you have to also implement trait T1, and thus anything working with a T2 can assume that thing is also a T1).