rustboot: Only bottom out at opaque IL types after finding two cycles. DON'T ASK ME WHY THIS WORKS.

src/boot/me/semant.ml

@@ -2425,7 +2425,7 @@ and referent_type
                 Some parent_tags
                   when boxed
                     && parent_tags <> []
-                    && List.mem ttag parent_tags
+                    && (list_count ttag parent_tags) > 1
                     && is_back_edge ttag (List.hd parent_tags) ->
                   Il.StructTy [| word; Il.OpaqueTy |]
               | _ -> tag ttag

src/boot/util/common.ml

@@ -380,6 +380,13 @@ let rec list_drop n ls =
   else list_drop (n-1) (List.tl ls)
 ;;
 
+let rec list_count elem lst =
+  match lst with
+      [] -> 0
+    | h::t when h = elem -> 1 + (list_count elem t)
+    | _::t -> list_count elem t
+;;
+
 
 (*
  * Auxiliary pair functions.