It's static typing with inference. Essentially what you have in Haskell/OCaml/F# where you declare a variable `x` through a let-binding witout specifying its type (`let x = something`), and the compiler analyses `something` and infers the type of `x`.
See also https://github.com/mflatt/shplait, which is a language with (1) the HM type system and (2) the same syntax that Rhombus uses. The language itself is implemented in Rhombus.
Absolutely: you can make that with some macros and using type annotations. I do not know how difficult can it be, but surely there are thing already done.
