mercurial/revset.py

--- a/mercurial/revset.py	Mon Sep 22 16:14:08 2014 -0500
+++ b/mercurial/revset.py	Sat Sep 27 13:18:10 2014 -0500
@@ -2228,6 +2228,10 @@
             self._set = set(self)
         return self._set
 
+    @util.propertycache
+    def __contains__(self):
+        return self.set().__contains__
+
     def __sub__(self, other):
         """Returns a new object with the substraction of the two collections.