Fix DepolarizingChannel._circuit_diagram_info_ assuming it is a singl…

…e qubit gate (#3780)

cirq/ops/common_channels.py

@@ -310,10 +310,15 @@ def _act_on_(self, args: Any) -> bool:
             return True
         return NotImplemented
 
-    def _circuit_diagram_info_(self, args: 'protocols.CircuitDiagramInfoArgs') -> str:
+    def _circuit_diagram_info_(self, args: 'protocols.CircuitDiagramInfoArgs') -> Tuple[str, ...]:
+        result: Tuple[str, ...]
         if args.precision is not None:
-            return f"D({self._p:.{args.precision}g})"
-        return f"D({self._p})"
+            result = (f"D({self._p:.{args.precision}g})",)
+        else:
+            result = (f"D({self._p})",)
+        while len(result) < self.num_qubits():
+            result += (f"#{len(result) + 1}",)
+        return result
 
     @property
     def p(self) -> float:

cirq/ops/common_channels_test.py

@@ -164,6 +164,16 @@ def test_depolarizing_channel_two_qubits():
     )
     assert cirq.has_channel(d)
 
+    assert d.num_qubits() == 2
+    cirq.testing.assert_has_diagram(
+        cirq.Circuit(d(*cirq.LineQubit.range(2))),
+        """
+0: ───D(0.15)───
+      │
+1: ───#2────────
+        """,
+    )
+
 
 def test_depolarizing_mixture():
     d = cirq.depolarize(0.3)
@@ -275,13 +285,13 @@ def test_depolarizing_channel_text_diagram():
 def test_depolarizing_channel_text_diagram_two_qubits():
     d = cirq.depolarize(0.1234567, n_qubits=2)
     assert cirq.circuit_diagram_info(d, args=round_to_6_prec) == cirq.CircuitDiagramInfo(
-        wire_symbols=('D(0.123457)',)
+        wire_symbols=('D(0.123457)', '#2')
     )
     assert cirq.circuit_diagram_info(d, args=round_to_2_prec) == cirq.CircuitDiagramInfo(
-        wire_symbols=('D(0.12)',)
+        wire_symbols=('D(0.12)', '#2')
     )
     assert cirq.circuit_diagram_info(d, args=no_precision) == cirq.CircuitDiagramInfo(
-        wire_symbols=('D(0.1234567)',)
+        wire_symbols=('D(0.1234567)', '#2')
     )