There are two major forms of sum type implementations that I'm aware of: tagged and untagged. What you're describing is a tagged union, for exactly the reason you state: there's an extra field for a discriminant which lets you choose the tag. Untagged unions are C's unions, where there's no discriminant, and you just have to know which type is valid at a given time.

Rust supports tagged unions, but can also optimize them to untagged in certain circumstances. We will probably add some kind of untagged union as well, mostly for FFI, but they're very unsafe, for the above reasons.