[ralfj]

I used "Rust-style enums" because if I just say "enum" then people think I mean something else. ;) And then I added the clarification to make sure nobody thinks I would claim that Rust invented Rust-style enums.

If I just say "algebraic data types" or "sum types" I fear many people won't know what I mean...

[stormbrew]

I think "discriminated union" is a useful bridge term here, even if it does confuse C programmers due to the conflated meanings of both enum and union in that language. It better represents the concept at an implementation level, and maps it to the C structure that enums more closely resemble in practice.