Fix preservation of order in align_right in cirq-core/cirq/transformers/align.py

cirq-core/cirq/transformers/align.py

@@ -73,4 +73,14 @@ def align_right(
     """
     if context is not None and context.deep is True:
         context = dataclasses.replace(context, deep=False)
-    return align_left(circuit[::-1], context=context)[::-1]
+    # Reverse the circuit, align left, and reverse again. Note each moment also has to have its ops
+    # reversed internally, to avoid edge conditions where non-commuting but can-be-in-same-moment
+    # ops (measurements and classical controls, particularly) could end up getting swapped.
+    backwards = []
+    for moment in circuit[::-1]:
+        backwards.append(circuits.Moment(reversed(moment.operations)))
+    aligned_backwards = align_left(circuits.Circuit(backwards), context=context)
+    forwards = []
+    for moment in aligned_backwards[::-1]:
+        forwards.append(circuits.Moment(reversed(moment.operations)))
+    return circuits.Circuit(forwards)

cirq-core/cirq/transformers/align_test.py

@@ -208,3 +208,16 @@ def test_classical_control():
     )
     cirq.testing.assert_same_circuits(cirq.align_left(circuit), circuit)
     cirq.testing.assert_same_circuits(cirq.align_right(circuit), circuit)
+
+
+def test_measurement_and_classical_control_same_moment_preserve_order():
+    q0, q1 = cirq.LineQubit.range(2)
+    circuit = cirq.Circuit()
+    op_measure = cirq.measure(q0, key='m')
+    op_controlled = cirq.X(q1).with_classical_controls('m')
+    circuit.append(op_measure)
+    circuit.append(op_controlled, cirq.InsertStrategy.INLINE)
+    circuit = cirq.align_right(circuit)
+    ops_in_order = list(circuit.all_operations())
+    assert ops_in_order[0] == op_measure
+    assert ops_in_order[1] == op_controlled