Yeah, that's when you would use trait objects in Rust (and virtual dispatch protocols in Swift?), but you still probably won't use them exactly the same way.
enum Car {
case Red
case Blue
}
extension Car {
func hello() {
print("hello")
}
}
let x = Car.Blue
x.hello()
edit: although you can't add a new case to the enum.
That's the point. We all know you can add impls or extensions.