Exponent should be drawn on target qubit in CX and CCX gates (#4462)

`CX` and `CCX` gates currently print the exponent on the bottom most qubit in the circuit diagram. This PR modifies this behavior to always draw the exponent on the target qubit, which is also the expected behavior for other `ControlledOperations`. This came up in discussions on #4167 (comment)

Note that behavior of `CZ` and `CCZ` is left unchanged, i.e. draws exponent on the bottom most qubit, since these gates are symmetrical across all applied qubits. 

BREAKING CHANGE: Tests depending upon diagrams of `CX` and `CCX` can now fail.

cirq-core/cirq/ops/common_gates.py

@@ -1275,7 +1275,7 @@ def _circuit_diagram_info_(
         self, args: 'cirq.CircuitDiagramInfoArgs'
     ) -> 'cirq.CircuitDiagramInfo':
         return protocols.CircuitDiagramInfo(
-            wire_symbols=('@', 'X'), exponent=self._diagram_exponent(args)
+            wire_symbols=('@', 'X'), exponent=self._diagram_exponent(args), exponent_qubit_index=1
         )
 
     def _apply_unitary_(self, args: 'protocols.ApplyUnitaryArgs') -> Optional[np.ndarray]:

cirq-core/cirq/ops/common_gates_test.py

@@ -720,6 +720,8 @@ def test_text_diagrams():
         cirq.CZ(a, b),
         cirq.CNOT(a, b),
         cirq.CNOT(b, a),
+        cirq.CNOT(a, b) ** 0.5,
+        cirq.CNOT(b, a) ** 0.5,
         cirq.H(a) ** 0.5,
         cirq.I(a),
         cirq.IdentityGate(2)(a, b),
@@ -729,18 +731,18 @@ def test_text_diagrams():
     cirq.testing.assert_has_diagram(
         circuit,
         """
-a: ───X───Y───Z───Z^x───Rx(x)───@───@───X───H^0.5───I───I───@─────
-                                │   │   │               │   │
-b: ─────────────────────────────@───X───@───────────────I───@^t───
+a: ───X───Y───Z───Z^x───Rx(x)───@───@───X───@───────X^0.5───H^0.5───I───I───@─────
+                                │   │   │   │       │                   │   │
+b: ─────────────────────────────@───X───@───X^0.5───@───────────────────I───@^t───
 """,
     )
 
     cirq.testing.assert_has_diagram(
         circuit,
         """
-a: ---X---Y---Z---Z^x---Rx(x)---@---@---X---H^0.5---I---I---@-----
-                                |   |   |               |   |
-b: -----------------------------@---X---@---------------I---@^t---
+a: ---X---Y---Z---Z^x---Rx(x)---@---@---X---@-------X^0.5---H^0.5---I---I---@-----
+                                |   |   |   |       |                   |   |
+b: -----------------------------@---X---@---X^0.5---@-------------------I---@^t---
 """,
         use_unicode_characters=False,
     )

cirq-core/cirq/ops/three_qubit_gates.py

@@ -397,7 +397,9 @@ def _decompose_(self, qubits):
     def _circuit_diagram_info_(
         self, args: 'cirq.CircuitDiagramInfoArgs'
     ) -> 'cirq.CircuitDiagramInfo':
-        return protocols.CircuitDiagramInfo(('@', '@', 'X'), exponent=self._diagram_exponent(args))
+        return protocols.CircuitDiagramInfo(
+            ('@', '@', 'X'), exponent=self._diagram_exponent(args), exponent_qubit_index=2
+        )
 
     def _qasm_(self, args: 'cirq.QasmArgs', qubits: Tuple['cirq.Qid', ...]) -> Optional[str]:
         if self._exponent != 1:

cirq-core/cirq/ops/three_qubit_gates_test.py

@@ -234,6 +234,7 @@ def test_diagram():
     circuit = cirq.Circuit(
         cirq.TOFFOLI(a, b, c),
         cirq.TOFFOLI(a, b, c) ** 0.5,
+        cirq.TOFFOLI(c, b, a) ** 0.5,
         cirq.CCX(a, c, b),
         cirq.CCZ(a, d, b),
         cirq.CCZ(a, d, b) ** 0.5,
@@ -243,25 +244,25 @@ def test_diagram():
     cirq.testing.assert_has_diagram(
         circuit,
         """
-0: ───@───@───────@───@───@───────@───@───
-      │   │       │   │   │       │   │
-1: ───@───@───────X───@───@───────┼───×───
-      │   │       │   │   │       │   │
-2: ───X───X^0.5───@───┼───┼───────×───×───
-                      │   │       │
-3: ───────────────────@───@^0.5───×───────
+0: ───@───@───────X^0.5───@───@───@───────@───@───
+      │   │       │       │   │   │       │   │
+1: ───@───@───────@───────X───@───@───────┼───×───
+      │   │       │       │   │   │       │   │
+2: ───X───X^0.5───@───────@───┼───┼───────×───×───
+                              │   │       │
+3: ───────────────────────────@───@^0.5───×───────
 """,
     )
     cirq.testing.assert_has_diagram(
         circuit,
         """
-0: ---@---@-------@---@---@-------@------@------
-      |   |       |   |   |       |      |
-1: ---@---@-------X---@---@-------|------swap---
-      |   |       |   |   |       |      |
-2: ---X---X^0.5---@---|---|-------swap---swap---
-                      |   |       |
-3: -------------------@---@^0.5---swap----------
+0: ---@---@-------X^0.5---@---@---@-------@------@------
+      |   |       |       |   |   |       |      |
+1: ---@---@-------@-------X---@---@-------|------swap---
+      |   |       |       |   |   |       |      |
+2: ---X---X^0.5---@-------@---|---|-------swap---swap---
+                              |   |       |
+3: ---------------------------@---@^0.5---swap----------
 """,
         use_unicode_characters=False,
     )