Add argument check to move_pauli_strings_into_circuit (#7985)

The `circuit_left` argument of `move_pauli_strings_into_circuit`
should contain only a PauliStringPhasor-type operations.
Raise exception if that is not the case.

Fixes #7934

---------

Co-authored-by: Pavol Juhas <juhas@google.com>

Author: Acciaccatura

cirq-core/cirq/contrib/paulistring/recombine.py

@@ -69,6 +69,14 @@ def _sorted_best_string_placements(
 def move_pauli_strings_into_circuit(
     circuit_left: circuits.Circuit | circuitdag.CircuitDag, circuit_right: circuits.Circuit
 ) -> circuits.Circuit:
+    non_pauli_string_phasor = next(
+        (o for o in circuit_left.all_operations() if not isinstance(o, ops.PauliStringPhasor)), None
+    )
+    if non_pauli_string_phasor is not None:
+        raise ValueError(
+            f'Expected only PauliStringPhasor operations in circuit_left. '
+            f'Found {non_pauli_string_phasor}.'
+        )
     if isinstance(circuit_left, circuitdag.CircuitDag):
         string_dag = circuitdag.CircuitDag(
             incoming_graph_data=circuit_left, can_reorder=pauli_string_reorder_pred

cirq-core/cirq/contrib/paulistring/recombine_test.py

@@ -14,6 +14,8 @@
 
 from __future__ import annotations
 
+import pytest
+
 import cirq
 from cirq.contrib.paulistring import (
     convert_and_separate_circuit,
@@ -49,3 +51,12 @@ def test_move_non_clifford_into_clifford() -> None:
     opt_len2 = len(cirq.optimize_for_target_gateset(c_recombined2, gateset=gateset))
     assert opt_len1 <= baseline_len
     assert opt_len2 <= baseline_len
+
+
+def test_move_pauli_strings_into_circuit_raises() -> None:
+    # check that example in #7934 raises exception
+    q0 = cirq.LineQubit(0)
+    with pytest.raises(
+        ValueError, match='Expected only PauliStringPhasor operations in circuit_left.'
+    ):
+        move_pauli_strings_into_circuit(cirq.Circuit(cirq.X(q0)), cirq.Circuit())