Make XModuleDescriptor hashable again (#22501)

common/lib/xmodule/xmodule/x_module.py

@@ -1263,6 +1263,17 @@ class XModuleDescriptor(XModuleDescriptorToXBlockMixin, HTMLSnippet, ResourceTem
                 all(getattr(self, field.name) == getattr(other, field.name)
                     for field in self.fields.values()))
 
+    def __hash__(self):  # pylint: disable=useless-super-delegation
+        """
+        This isn't technically appropriate since descriptors are actually mutable,
+        but in practice we rarely modify them after creation or instantiate two
+        equivalent descriptors in the same process.  And we perform graph
+        operations on large collections of XBlocks that have simply unacceptable
+        performance if we have to rely on lists and equality rather than sets,
+        dictionaries, and identity-based hash functions.
+        """
+        return super(XModuleDescriptor, self).__hash__()
+
     def __repr__(self):
         return (
             "{0.__class__.__name__}("