diff --git a/src/hb-iter.hh b/src/hb-iter.hh
index df7372e4e..c74ea951e 100644
--- a/src/hb-iter.hh
+++ b/src/hb-iter.hh
@@ -339,7 +339,7 @@ struct hb_zip_iter_t :
 	    hb_pair_t<typename A::item_t, typename B::item_t> >
 {
   hb_zip_iter_t () {}
-  hb_zip_iter_t (A a, B b) : a (a), b (b) {}
+  hb_zip_iter_t (const A& a, const B& b) : a (a), b (b) {}
 
   typedef hb_pair_t<typename A::item_t, typename B::item_t> __item_t__;
   static constexpr bool is_random_access_iterator =
@@ -366,10 +366,45 @@ static const struct
   template <typename A, typename B,
 	    hb_enable_if (hb_is_iterable (A) && hb_is_iterable (B))>
   hb_zip_iter_t<hb_iter_t (A), hb_iter_t (B)>
-  operator () (const A& a, const B &b) const
+  operator () (A& a, B &b) const
   { return hb_zip_iter_t<hb_iter_t (A), hb_iter_t (B)> (hb_iter (a), hb_iter (b)); }
 } hb_zip HB_UNUSED;
 
+/* hb_enumerate */
+
+template <typename Iter,
+	 hb_enable_if (hb_is_iterator (Iter))>
+struct hb_enumerate_iter_t :
+  hb_iter_t<hb_enumerate_iter_t<Iter>,
+	    hb_pair_t<unsigned, typename Iter::item_t> >
+{
+  hb_enumerate_iter_t (const Iter& it) : i (0), it (it) {}
+
+  typedef hb_pair_t<unsigned, typename Iter::item_t> __item_t__;
+  static constexpr bool is_random_access_iterator = Iter::is_random_access_iterator;
+  static constexpr bool is_sorted_iterator = true;
+  __item_t__ __item__ () const { return __item_t__ (+i, *it); }
+  __item_t__ __item_at__ (unsigned j) const { return __item_t__ (i + j, it[j]); }
+  bool __more__ () const { return bool (it); }
+  unsigned __len__ () const { return it.len (); }
+  void __next__ () { ++i; ++it; }
+  void __forward__ (unsigned n) { i += n; it += n; }
+  void __prev__ () { --i; --it; }
+  void __rewind__ (unsigned n) { i -= n; it -= n; }
+
+  private:
+  unsigned i;
+  Iter it;
+};
+static const struct
+{
+  template <typename Iterable,
+	    hb_enable_if (hb_is_iterable (Iterable))>
+  hb_enumerate_iter_t<hb_iter_t (Iterable)>
+  operator () (Iterable& it) const
+  { return hb_enumerate_iter_t<hb_iter_t (Iterable)> (hb_iter (it)); }
+} hb_enumerate HB_UNUSED;
+
 /* hb_apply() */
 
 template <typename Appl>
diff --git a/src/hb-ot-layout-gsubgpos.hh b/src/hb-ot-layout-gsubgpos.hh
index 70ddc28e6..dabb98c39 100644
--- a/src/hb-ot-layout-gsubgpos.hh
+++ b/src/hb-ot-layout-gsubgpos.hh
@@ -1533,10 +1533,9 @@ struct ContextFormat2
       &class_def
     };
 
-    unsigned int count = ruleSet.len;
-    for (unsigned int i = 0; i < count; i++)
-      if (class_def.intersects_class (glyphs, i) &&
-	  (this+ruleSet[i]).intersects (glyphs, lookup_context))
+    for (auto it = hb_enumerate (ruleSet); it; ++it)
+      if (class_def.intersects_class (glyphs, (*it).first) &&
+	  (this+(*it).second).intersects (glyphs, lookup_context))
 	return true;
 
     return false;
@@ -1554,12 +1553,9 @@ struct ContextFormat2
       &class_def
     };
 
-    unsigned int count = ruleSet.len;
-    for (unsigned int i = 0; i < count; i++)
-      if (class_def.intersects_class (c->glyphs, i)) {
-	const RuleSet &rule_set = this+ruleSet[i];
-	rule_set.closure (c, lookup_context);
-      }
+    for (auto it = hb_enumerate (ruleSet); it; ++it)
+      if (class_def.intersects_class (c->glyphs, (*it).first))
+	(this+(*it).second).closure (c, lookup_context);
   }
 
   void collect_glyphs (hb_collect_glyphs_context_t *c) const
@@ -2171,10 +2167,9 @@ struct ChainContextFormat2
        &lookahead_class_def}
     };
 
-    unsigned int count = ruleSet.len;
-    for (unsigned int i = 0; i < count; i++)
-      if (input_class_def.intersects_class (glyphs, i) &&
-	  (this+ruleSet[i]).intersects (glyphs, lookup_context))
+    for (auto it = hb_enumerate (ruleSet); it; ++it)
+      if (input_class_def.intersects_class (glyphs, (*it).first) &&
+	  (this+(*it).second).intersects (glyphs, lookup_context))
 	return true;
 
     return false;
@@ -2195,12 +2190,9 @@ struct ChainContextFormat2
        &lookahead_class_def}
     };
 
-    unsigned int count = ruleSet.len;
-    for (unsigned int i = 0; i < count; i++)
-      if (input_class_def.intersects_class (c->glyphs, i)) {
-	const ChainRuleSet &rule_set = this+ruleSet[i];
-	rule_set.closure (c, lookup_context);
-      }
+    for (auto it = hb_enumerate (ruleSet); it; ++it)
+      if (input_class_def.intersects_class (c->glyphs, (*it).first))
+	(this+(*it).second).closure (c, lookup_context);
   }
 
   void collect_glyphs (hb_collect_glyphs_context_t *c) const