AbstractCircuit.freeze should not reallocate moments (quantumlib#5878)

cirq-core/cirq/circuits/circuit_test.py

@@ -4486,6 +4486,13 @@ def test_concat_ragged_alignment():
     )
 
 
+def test_freeze_not_relocate_moments():
+    q = cirq.q(0)
+    c = cirq.Circuit(cirq.X(q), cirq.measure(q))
+    f = c.freeze()
+    assert [mc is fc for mc, fc in zip(c, f)] == [True, True]
+
+
 def test_factorize_one_factor():
     circuit = cirq.Circuit()
     q0, q1, q2 = cirq.LineQubit.range(3)

cirq-core/cirq/circuits/moment.py

@@ -192,6 +192,9 @@ def with_operations(self, *contents: 'cirq.OP_TREE') -> 'cirq.Moment':
         """
         flattened_contents = tuple(op_tree.flatten_to_ops(contents))
 
+        if not flattened_contents:
+            return self
+
         m = Moment()
         # Use private variables to facilitate a quick copy.
         m._qubit_to_op = self._qubit_to_op.copy()

cirq-core/cirq/circuits/moment_test.py

@@ -467,6 +467,7 @@ def test_add():
 
     assert m1 + [[[[cirq.Y(b)]]]] == cirq.Moment(cirq.X(a), cirq.Y(b))
     assert m1 + [] == m1
+    assert m1 + [] is m1
 
 
 def test_sub():
@@ -703,7 +704,6 @@ def test_kraus():
     m = cirq.Moment(cirq.CNOT(a, b))
     assert cirq.has_kraus(m)
     k = cirq.kraus(m)
-    print(k[0])
     assert len(k) == 1
     assert np.allclose(k[0], np.array([[1, 0, 0, 0], [0, 1, 0, 0], [0, 0, 0, 1], [0, 0, 1, 0]]))