Ceylon had union types, which is the only place i've seen these: https://github.com/eclipse-archived/ceylon-lang.org/blob/mas...

Another thing Rust enums are missing is having each variant be a type. If you have an enum Shape with variants Circle, Rectangle, and Polygon, there is no way to write a function which only takes a Circle. So you end up defining a struct for each case, then making your enum a trivial wrapper round the three structs. You end up with Shape::Circle and Circle, which are different things, and writing code like c.0.radius to get at the fields. It's rather inelegant. So either variants should be types in their own right, or an enum should be defined as a composition of existing types.