Definitely if you don't have the C "union" user defined type you should use this keyword for your sum types. Many languages don't have this feature - which is an extremely sharp blade intended only for experts - and that's fine. You don't need an Abrams tank to take the kids to school, beginners should not learn to fly in the F-35A and the language for writing your CRUD app does not need C-style unions.

If Rust didn't have (C-style) unions then its enum should be named union instead. But it does, so they needed a different name. As we work our way through the rough edges of Rust maybe this will stick up more and annoy me, but given Rust 1.95 just finally stabilized core::range::RangeInclusive, the fix for the wonky wheel that is core::ops::RangeInclusive we're not going to get there any time soon.