Ceylon had a really neat type system - sadly it didn't take off. However before that these types in particular were a feature of OCaml: https://v2.ocaml.org/man

Ceylon had a really neat type system - sadly it didn't take off. However before that these types in particular were a feature of OCaml: https://v2.ocaml.org/manual/polyvariant.html https://v2.ocaml.org/manual/polyvariant.html There they're called Polymorphic Variants. Note also the implementation efficiency concerns on that page - given that Rust is a systems language, defaulting to the current sum types instead of set-theoretic unions was a reasonable choice. Having the option to use them natively and without macros (there's some crates) would be nice still when aware of the additional performance overhead.