Defer extraction of moment keys until necessary

Skip potentially expensive lookup of measurement and control
keys from a Moment when inserted operation has no such keys.

No change in code function.

Author: pavoljuhas

cirq-core/cirq/circuits/circuit.py

@@ -2159,14 +2159,21 @@ def _can_add_op_at(self, moment_index: int, operation: cirq.Operation) -> bool:
         op_measurement_keys = protocols.measurement_key_objs(operation)
         op_control_keys = protocols.control_keys(operation)
 
-        moment_measurement_keys = protocols.measurement_key_objs(self._moments[moment_index])
-        moment_control_keys = protocols.control_keys(self._moments[moment_index])
-
-        return (
-            op_measurement_keys.isdisjoint(moment_measurement_keys)
-            and op_control_keys.isdisjoint(moment_measurement_keys)
-            and moment_control_keys.isdisjoint(op_measurement_keys)
-        )
+        # defer extraction of moment keys until truly needed
+        result = True
+        if op_control_keys or op_measurement_keys:
+            moment_measurement_keys = protocols.measurement_key_objs(self._moments[moment_index])
+            result = op_control_keys.isdisjoint(moment_measurement_keys) and (
+                (
+                    op_measurement_keys.isdisjoint(moment_measurement_keys)
+                    and op_measurement_keys.isdisjoint(
+                        protocols.control_keys(self._moments[moment_index])
+                    )
+                )
+                if op_measurement_keys
+                else True
+            )
+        return result
 
     def _latest_available_moment(self, op: cirq.Operation, *, start_moment_index: int = 0) -> int:
         """Finds the index of the latest (i.e. right most) moment which can accommodate `op`.