Is there any reason why you've singled out Rust as particularly notable here and not any of the many other languages with them? OCaml, Elm, F#, Scala, I think more recent versions of Java, Kotlin, Nim, TypeScript, and Swift all support ADTs. Python already supports them, albeit with very little runtime support. Rust doesn't particularly stand out in such a broad field of languages. They're so useful a language needs a good reason these days to not support them.