don't box built-in types

src/mlw/rust_printer.ml

@@ -465,6 +465,10 @@ module MLToRust = struct
       def=def
     }
 
+  let id_of_mlty t =
+    match t with
+    | Tapp (id, _) -> id
+    | _ -> raise (TODO "mlty is not Tapp")
 
   let box_typedefs (itl: its_defn list) box_fields box_enum: Sid.t * Sbe.t =
     (* TODO box only fields that are necessary *)
@@ -486,7 +490,11 @@ module MLToRust = struct
 (*     in *)
 
     (* TODO see if something from itl is reachable through the ty *)
-    let needs_box ty = true in
+    let needs_box ty =
+      let id = id_of_mlty ty in
+      let is_ts ts = id_equal ts.ts_name id in
+      not (is_ts ts_int || is_ts ts_real || is_ts ts_bool)
+      in
 
     let box its =
       (match its.its_def with