Cache the hash of frozen circuits to avoid recomputing (quantumlib#5738)

This follows what we previously did for GridQubit, but reworked to use `_compat.cached_method` and to compute the hash lazily.

cirq/circuits/frozen_circuit.py

@@ -55,9 +55,20 @@ def __init__(
     def moments(self) -> Sequence['cirq.Moment']:
         return self._moments
 
-    def __hash__(self):
+    @_compat.cached_method
+    def __hash__(self) -> int:
+        # Explicitly cached for performance
         return hash((self.moments,))
 
+    def __getstate__(self):
+        # Don't save hash when pickling; see #3777.
+        state = self.__dict__
+        hash_cache = _compat._method_cache_name(self.__hash__)
+        if hash_cache in state:
+            state = state.copy()
+            del state[hash_cache]
+        return state
+
     @_compat.cached_method
     def _num_qubits_(self) -> int:
         return len(self.all_qubits())

cirq/devices/grid_qubit.py

@@ -19,7 +19,7 @@
 
 import numpy as np
 
-from cirq import ops, protocols
+from cirq import _compat, ops, protocols
 
 if TYPE_CHECKING:
     import cirq
@@ -294,23 +294,19 @@ class GridQubit(_BaseGridQid):
         cirq.GridQubit(5, 4)
     """
 
-    def __init__(self, row: int, col: int):
-        super().__init__(row, col)
-        self._hash = super().__hash__()
-
     def __getstate__(self):
         # Don't save hash when pickling; see #3777.
-        state = self.__dict__.copy()
-        del state['_hash']
+        state = self.__dict__
+        hash_key = _compat._method_cache_name(self.__hash__)
+        if hash_key in state:
+            state = state.copy()
+            del state[hash_key]
         return state
 
-    def __setstate__(self, state):
-        self.__dict__.update(state)
-        self._hash = super().__hash__()
-
-    def __hash__(self):
+    @_compat.cached_method
+    def __hash__(self) -> int:
         # Explicitly cached for performance (vs delegating to Qid).
-        return self._hash
+        return super().__hash__()
 
     def __eq__(self, other):
         # Explicitly implemented for performance (vs delegating to Qid).

cirq/devices/grid_qubit_test.py

@@ -19,6 +19,7 @@
 import pytest
 
 import cirq
+from cirq import _compat
 
 
 def test_init():
@@ -43,7 +44,9 @@ def test_eq():
 def test_pickled_hash():
     q = cirq.GridQubit(3, 4)
     q_bad = cirq.GridQubit(3, 4)
-    q_bad._hash += 1
+    _ = hash(q_bad)  # compute hash to ensure it is cached.
+    hash_key = _compat._method_cache_name(cirq.GridQubit.__hash__)
+    setattr(q_bad, hash_key, getattr(q_bad, hash_key) + 1)
     assert q_bad == q
     assert hash(q_bad) != hash(q)
     data = pickle.dumps(q_bad)