diff -r 000000000000 -r 6474c204b198 xpcom/analysis/must-override.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/analysis/must-override.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,52 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Detect classes that should have overridden members of their parent
+ * classes but didn't.
+ *
+ * Example:
+ *
+ * struct S {
+ *   virtual NS_MUST_OVERRIDE void f();
+ *   virtual void g();
+ * };
+ *
+ * struct A : S { virtual void f(); }; // ok
+ * struct B : S { virtual NS_MUST_OVERRIDE void f(); }; // also ok
+ *
+ * struct C : S { virtual void g(); }; // ERROR: must override f()
+ * struct D : S { virtual void f(int); }; // ERROR: different overload
+ * struct E : A { }; // ok: A's definition of f() is good for subclasses
+ * struct F : B { }; // ERROR: B's definition of f() is still must-override
+ *
+ * We don't care if you define the method or not.
+ */
+
+function get_must_overrides(cls)
+{
+  let mos = {};
+  for each (let base in cls.bases)
+    for each (let m in base.type.members)
+      if (hasAttribute(m, 'NS_must_override')) 
+       mos[m.shortName] = m;
+
+  return mos;
+}
+
+function process_type(t)
+{
+  if (t.isIncomplete || (t.kind != 'class' && t.kind != 'struct'))
+    return;
+
+  let mos = get_must_overrides(t);
+  for each (let m in t.members) {
+    let mos_m = mos[m.shortName]
+    if (mos_m && signaturesMatch(mos_m, m))
+      delete mos[m.shortName];
+  }
+
+  for each (let u in mos)
+    error(t.kind + " " + t.name + " must override " + u.name, t.loc);
+}