rtop: work around #use .. syntax errors

rtop/reason_toploop.cppo.ml

@@ -5,27 +5,37 @@
  * LICENSE file in the root directory of this source tree.
  *)
 
-(* this file's triggered by utop/rtop *)
-let main () =
-  if List.exists ((=) "camlp4o") !Topfind.predicates ||
-     List.exists ((=) "camlp4r") !Topfind.predicates then
-    print_endline "Reason is incompatible with camlp4!"
-  else begin
-    Toploop.parse_toplevel_phrase := Reason_util.correctly_catch_parse_errors
-        (fun x ->
-          let r =  Reason_toolchain.To_current.copy_toplevel_phrase
-            (Reason_toolchain.RE.toplevel_phrase x)
-          in
+let default_parse_toplevel_phrase = !Toploop.parse_toplevel_phrase
+let reason_parse_toplevel_phrase =
+  Reason_util.correctly_catch_parse_errors
+  (fun x ->
+    let r =
+      Reason_toolchain.To_current.copy_toplevel_phrase
+        (Reason_toolchain.RE.toplevel_phrase x)
+  in
 #if OCAML_VERSION >= (5,2,0)
-(* NOTE(anmonteiro): after https://github.com/ocaml/ocaml/pull/12029, we get a
+  (* NOTE(anmonteiro): after https://github.com/ocaml/ocaml/pull/12029, we get a
 
   Fatal error: exception Invalid_argument("index out of bounds")
   Raised by primitive operation at Toploop.ends_with_lf in file "toplevel/toploop.ml"
 
   Setting `lex_eof_reached` seems to avoid whatever check upstream is doing. *)
-          x.lex_eof_reached <- true;
+  x.lex_eof_reached <- true;
 #endif
-          r);
+  r)
+
+
+(* this file's triggered by utop/rtop *)
+let main () =
+  if List.exists ((=) "camlp4o") !Topfind.predicates ||
+     List.exists ((=) "camlp4r") !Topfind.predicates then
+    print_endline "Reason is incompatible with camlp4!"
+  else begin
+    Toploop.parse_toplevel_phrase := (fun t ->
+      if !Reason_utop.current_top = UTop then
+        default_parse_toplevel_phrase t
+      else
+        reason_parse_toplevel_phrase t);
     Toploop.parse_use_file := Reason_util.correctly_catch_parse_errors
         (fun x -> List.map Reason_toolchain.To_current.copy_toplevel_phrase
             (Reason_toolchain.RE.use_file x));