[ctypes] [dune] Constant pass to know struct size

I am using the new dune ctypes stanza.
I was wondering if it was possible to have a constant generation pass, before the type generation pass, in order to use the constants to build types.

for example, consider this C code:

#define SIZE 42
struct foo {
  int array [SIZE];
};

I would like to do something like this:

module Types(T : Ctypes.TYPE) = struct
  type foo
  let size = T.constant "SIZE" T.int64_t
  let foo : foo Ctypes.structure T.typ = T.structure "foo"
  ...
  let () = T.seal foo
end

but to use the array field:

let arr = T.field foo "array" (T.array (Int64.of_int size) T.int) (* error *)

seems impossible, as size is still a int64 T.const…

Do I have to go back from the fantastic dune ctypes stanza to custom rules, or is there a trick I am missing?

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