> If you prefer, Rust tooling is still quite far behind from languages like Kotlin and Scala
I'm not sure that's true, at least when it comes to specifically build tooling. I'd say Cargo is far ahead of Gradle, Ant, or worst of all SBT, and probably even slightly ahead of Maven (which never really reached critical mass in the Kotlin or Scala ecosystems sadly).
> This sounds more like "this is what I like in rust" than "features any modern language should have" though
Good build tooling has been around since 2004, and all of the rest of those features have been around since the late 1970s. There's really no excuse for a language not having all of them.
I'm not sure what you mean by "usable abstractions" and tagged unions are a little verbose because they are defined in terms of closed sets of subtypes, but otherwise Dart has all of those.