diff --git a/src/hb-iter.hh b/src/hb-iter.hh
index 775fba46f..48cc68738 100644
--- a/src/hb-iter.hh
+++ b/src/hb-iter.hh
@@ -75,6 +75,7 @@ struct hb_iter_t
   void prev () { thiz()->__prev__ (); }
   void rewind (unsigned n) { thiz()->__rewind__ (n); }
   unsigned len () const { return thiz()->__len__ (); }
+  bool random_access () const { return thiz()->__random_access__ (); }
 
   /*
    * Subclasses overrides:
@@ -99,6 +100,9 @@ struct hb_iter_t
   /* Population: Implement __len__() if known. */
   unsigned __len__ () const
   { iter_t c (*thiz()); unsigned l = 0; while (c) { c++; l++; }; return l; }
+
+  /* Random access: Return true if len(), forward(), item_at() are fast. */
+  bool __random_access__ () const { return false; }
 };
 
 
diff --git a/src/test-iter.cc b/src/test-iter.cc
index 32fb9f831..f54bf0ae3 100644
--- a/src/test-iter.cc
+++ b/src/test-iter.cc
@@ -39,6 +39,7 @@ struct array_iter_t : hb_iter_t<array_iter_t<T>, T>
   void __forward__ (unsigned n) { arr += n; }
   void __rewind__ (unsigned n) { arr -= n; }
   unsigned __len__ () const { return arr.len; }
+  bool __random_access__ () const { return true; }
 
   private:
   hb_array_t<T> arr;