[lmm]

I would say the notion of something being first-class is that you can manipulate it in the same way as a regular value in the language; in particular, you can use an expression that evaluates to such a thing in all the same ways that you can use a built-in version of that thing. Certainly Java does not have first-class types: you can pass a value that is sort of a representation a flattened form of a type, but you can't use that kind of value in a "new" expression or as a function return type.

AIUI Rust's async/await still has quite a lot of special case support. IMO concurrency is only really first-class in languages like Haskell where you can manipulate async actions the same way as a user-defined type and implement concurrency-related operations in plain old code.