Logically speaking, Rust's "enums" are neither enums (in the traditional sense) nor unions. They are tagged unions / disjoint unions / variants / coproducts / algebraic data types. We many names for this concept, but "union" is not one of them, because a union allows for a non-empty intersection.

Under the hood, of course, they are implemented with C-style unions with fields sharing memory. But to conflate Rust's enums with how they are implemented is to disregard the extra safety that they provide.