Remove client caching from cache API

We haven't yet decided how we want `cache` to work on the client. The
lifetime of the cache is more complex than on the server, where it only
has to live as long as a single request.

Since it's more important to ship this on the server, we're removing the
existing behavior from the client for now. On the client (i.e. not a
Server Components environment) `cache` will have not have any caching
behavior.

The rest of the behavior is the same as the server implementation — it
returns a new reference, extra properties like `displayName` are not
preserved, the length of the new function is 0, etc. That way apps can't
accidentally depend on those details.

We intend to implement client caching in a future major release. In the
meantime, it's only exposed as an API so that Shared Components can use
per-request caching on the server without breaking on the client.

packages/react-reconciler/src/__tests__/ReactCache-test.js

@@ -199,4 +199,25 @@ describe('ReactCache', () => {
     expect(x).toBe(y);
     expect(z).not.toBe(x);
   });
+
+  // @gate enableCache
+  it('introspection of returned wrapper function is same on client and server', async () => {
+    // When the variant flag is true, test the client version of `cache`.
+    if (gate(flags => flags.variant)) {
+      jest.resetModules();
+      jest.mock('react', () => jest.requireActual('react'));
+      const ClientReact = require('react');
+      cache = ClientReact.cache;
+    }
+
+    function foo(a, b, c) {
+      return a + b + c;
+    }
+    foo.displayName = 'Custom display name';
+
+    const cachedFoo = cache(foo);
+    expect(cachedFoo).not.toBe(foo);
+    expect(cachedFoo.length).toBe(0);
+    expect(cachedFoo.displayName).toBe(undefined);
+  });
 });

packages/react/src/ReactCacheClient.js

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+export function cache<A: Iterable<mixed>, T>(fn: (...A) => T): (...A) => T {
+  // On the client (i.e. not a Server Components environment) `cache` has
+  // no caching behavior. We just return the function as-is.
+  //
+  // We intend to implement client caching in a future major release. In the
+  // meantime, it's only exposed as an API so that Shared Components can use
+  // per-request caching on the server without breaking on the client. But it
+  // does mean they need to be aware of the behavioral difference.
+  //
+  // The rest of the behavior is the same as the server implementation — it
+  // returns a new reference, extra properties like `displayName` are not
+  // preserved, the length of the new function is 0, etc. That way apps can't
+  // accidentally depend on those details.
+  return function () {
+    // $FlowFixMe[incompatible-call]: We don't want to use rest arguments since we transpile the code.
+    return fn.apply(null, arguments);
+  };
+}

packages/react/src/ReactClient.js

@@ -35,7 +35,7 @@ import {createContext} from './ReactContext';
 import {lazy} from './ReactLazy';
 import {forwardRef} from './ReactForwardRef';
 import {memo} from './ReactMemo';
-import {cache} from './ReactCache';
+import {cache} from './ReactCacheClient';
 import {postpone} from './ReactPostpone';
 import {
   getCacheSignal,

packages/react/src/ReactServer.experimental.js

@@ -38,7 +38,7 @@ import {
 import {forwardRef} from './ReactForwardRef';
 import {lazy} from './ReactLazy';
 import {memo} from './ReactMemo';
-import {cache} from './ReactCache';
+import {cache} from './ReactCacheServer';
 import {startTransition} from './ReactStartTransition';
 import {postpone} from './ReactPostpone';
 import version from 'shared/ReactVersion';

packages/react/src/ReactServer.js

@@ -35,7 +35,7 @@ import {
 import {forwardRef} from './ReactForwardRef';
 import {lazy} from './ReactLazy';
 import {memo} from './ReactMemo';
-import {cache} from './ReactCache';
+import {cache} from './ReactCacheServer';
 import {startTransition} from './ReactStartTransition';
 import version from 'shared/ReactVersion';