@unboxed in *.ml files?

module X = {
  type t = Foo(Int)

Now, if we write this as a *.ml files:

module X = struct
  type t = Foo of int 

Is there a way to specify the @unboxed ?

In ml syntax, you can write type t = Foo of int [@@unboxed].

Edit: and also with single field records: type greeting = { message : string } [@@unboxed].