Warn when rendering tests in concurrent/batched mode without a mocked scheduler (#16207)

Concurrent/Batched mode tests should always be run with a mocked scheduler (v17 or not). This PR adds a warning for the same. I'll put up a separate PR to the docs with a page detailing how to mock the scheduler.

fixtures/dom/src/__tests__/nested-act-test.js

@@ -0,0 +1,97 @@
+/**
+ * Copyright (c) Facebook, 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.
+ *
+ * @emails react-core
+ */
+
+let React;
+let TestUtils;
+let TestRenderer;
+
+global.__DEV__ = process.env.NODE_ENV !== 'production';
+
+expect.extend(require('../toWarnDev'));
+
+describe('unmocked scheduler', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    TestUtils = require('react-dom/test-utils');
+    TestRenderer = require('react-test-renderer');
+  });
+
+  it('flushes work only outside the outermost act() corresponding to its own renderer', () => {
+    let log = [];
+    function Effecty() {
+      React.useEffect(() => {
+        log.push('called');
+      }, []);
+      return null;
+    }
+    // in legacy mode, this tests whether an act only flushes its own effects
+    TestRenderer.act(() => {
+      TestUtils.act(() => {
+        TestRenderer.create(<Effecty />);
+      });
+      expect(log).toEqual([]);
+    });
+    expect(log).toEqual(['called']);
+
+    log = [];
+    // for doublechecking, we flip it inside out, and assert on the outermost
+    TestUtils.act(() => {
+      TestRenderer.act(() => {
+        TestRenderer.create(<Effecty />);
+      });
+      expect(log).toEqual(['called']);
+    });
+    expect(log).toEqual(['called']);
+  });
+});
+
+describe('mocked scheduler', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    jest.mock('scheduler', () =>
+      require.requireActual('scheduler/unstable_mock')
+    );
+    React = require('react');
+    TestUtils = require('react-dom/test-utils');
+    TestRenderer = require('react-test-renderer');
+  });
+
+  afterEach(() => {
+    jest.unmock('scheduler');
+  });
+
+  it('flushes work only outside the outermost act()', () => {
+    let log = [];
+    function Effecty() {
+      React.useEffect(() => {
+        log.push('called');
+      }, []);
+      return null;
+    }
+    // with a mocked scheduler, this tests whether it flushes all work only on the outermost act
+    TestRenderer.act(() => {
+      TestUtils.act(() => {
+        TestRenderer.create(<Effecty />);
+      });
+      expect(log).toEqual([]);
+    });
+    expect(log).toEqual(['called']);
+
+    log = [];
+    // for doublechecking, we flip it inside out, and assert on the outermost
+    TestUtils.act(() => {
+      TestRenderer.act(() => {
+        TestRenderer.create(<Effecty />);
+      });
+      expect(log).toEqual([]);
+    });
+    expect(log).toEqual(['called']);
+  });
+});

fixtures/dom/src/__tests__/wrong-act-test.js

@@ -0,0 +1,194 @@
+/**
+ * Copyright (c) Facebook, 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.
+ *
+ * @emails react-core
+ */
+
+let React;
+let ReactDOM;
+let ReactART;
+let ARTSVGMode;
+let ARTCurrentMode;
+let TestUtils;
+let TestRenderer;
+let ARTTest;
+
+global.__DEV__ = process.env.NODE_ENV !== 'production';
+
+expect.extend(require('../toWarnDev'));
+
+function App(props) {
+  return 'hello world';
+}
+
+beforeEach(() => {
+  jest.resetModules();
+  React = require('react');
+  ReactDOM = require('react-dom');
+  ReactART = require('react-art');
+  ARTSVGMode = require('art/modes/svg');
+  ARTCurrentMode = require('art/modes/current');
+  TestUtils = require('react-dom/test-utils');
+  TestRenderer = require('react-test-renderer');
+
+  ARTCurrentMode.setCurrent(ARTSVGMode);
+
+  ARTTest = function ARTTestComponent(props) {
+    return (
+      <ReactART.Surface width={150} height={200}>
+        <ReactART.Group>
+          <ReactART.Shape
+            d="M0,0l50,0l0,50l-50,0z"
+            fill={new ReactART.LinearGradient(['black', 'white'])}
+            key="a"
+            width={50}
+            height={50}
+            x={50}
+            y={50}
+            opacity={0.1}
+          />
+          <ReactART.Shape
+            fill="#3C5A99"
+            key="b"
+            scale={0.5}
+            x={50}
+            y={50}
+            title="This is an F"
+            cursor="pointer">
+            M64.564,38.583H54l0.008-5.834c0-3.035,0.293-4.666,4.657-4.666
+            h5.833V16.429h-9.33c-11.213,0-15.159,5.654-15.159,15.16v6.994
+            h-6.99v11.652h6.99v33.815H54V50.235h9.331L64.564,38.583z
+          </ReactART.Shape>
+        </ReactART.Group>
+      </ReactART.Surface>
+    );
+  };
+});
+
+it("doesn't warn when you use the right act + renderer: dom", () => {
+  TestUtils.act(() => {
+    TestUtils.renderIntoDocument(<App />);
+  });
+});
+
+it("doesn't warn when you use the right act + renderer: test", () => {
+  TestRenderer.act(() => {
+    TestRenderer.create(<App />);
+  });
+});
+
+it('resets correctly across renderers', () => {
+  function Effecty() {
+    React.useEffect(() => {}, []);
+    return null;
+  }
+  TestUtils.act(() => {
+    TestRenderer.act(() => {});
+    expect(() => {
+      TestRenderer.create(<Effecty />);
+    }).toWarnDev(["It looks like you're using the wrong act()"], {
+      withoutStack: true,
+    });
+  });
+});
+
+it('warns when using the wrong act version - test + dom: render', () => {
+  expect(() => {
+    TestRenderer.act(() => {
+      TestUtils.renderIntoDocument(<App />);
+    });
+  }).toWarnDev(["It looks like you're using the wrong act()"], {
+    withoutStack: true,
+  });
+});
+
+it('warns when using the wrong act version - test + dom: updates', () => {
+  let setCtr;
+  function Counter(props) {
+    const [ctr, _setCtr] = React.useState(0);
+    setCtr = _setCtr;
+    return ctr;
+  }
+  TestUtils.renderIntoDocument(<Counter />);
+  expect(() => {
+    TestRenderer.act(() => {
+      setCtr(1);
+    });
+  }).toWarnDev(["It looks like you're using the wrong act()"]);
+});
+
+it('warns when using the wrong act version - dom + test: .create()', () => {
+  expect(() => {
+    TestUtils.act(() => {
+      TestRenderer.create(<App />);
+    });
+  }).toWarnDev(["It looks like you're using the wrong act()"], {
+    withoutStack: true,
+  });
+});
+
+it('warns when using the wrong act version - dom + test: .update()', () => {
+  const root = TestRenderer.create(<App key="one" />);
+  expect(() => {
+    TestUtils.act(() => {
+      root.update(<App key="two" />);
+    });
+  }).toWarnDev(["It looks like you're using the wrong act()"], {
+    withoutStack: true,
+  });
+});
+
+it('warns when using the wrong act version - dom + test: updates', () => {
+  let setCtr;
+  function Counter(props) {
+    const [ctr, _setCtr] = React.useState(0);
+    setCtr = _setCtr;
+    return ctr;
+  }
+  TestRenderer.create(<Counter />);
+  expect(() => {
+    TestUtils.act(() => {
+      setCtr(1);
+    });
+  }).toWarnDev(["It looks like you're using the wrong act()"]);
+});
+
+it('does not warn when nesting react-act inside react-dom', () => {
+  TestUtils.act(() => {
+    TestUtils.renderIntoDocument(<ARTTest />);
+  });
+});
+
+it('does not warn when nesting react-act inside react-test-renderer', () => {
+  TestRenderer.act(() => {
+    TestRenderer.create(<ARTTest />);
+  });
+});
+
+it("doesn't warn if you use nested acts from different renderers", () => {
+  TestRenderer.act(() => {
+    TestUtils.act(() => {
+      TestRenderer.create(<App />);
+    });
+  });
+});
+
+it('warns when using createRoot() + .render', () => {
+  const root = ReactDOM.unstable_createRoot(document.createElement('div'));
+  expect(() => {
+    TestRenderer.act(() => {
+      root.render(<App />);
+    });
+  }).toWarnDev(
+    [
+      'In Concurrent or Sync modes, the "scheduler" module needs to be mocked',
+      "It looks like you're using the wrong act()",
+    ],
+    {
+      withoutStack: true,
+    }
+  );
+});

fixtures/dom/src/index.test.js

@@ -1,233 +0,0 @@
-/**
- * Copyright (c) Facebook, 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.
- *
- * @emails react-core
- */
-
-let React;
-let ReactDOM;
-let ReactART;
-let ARTSVGMode;
-let ARTCurrentMode;
-let TestUtils;
-let TestRenderer;
-let ARTTest;
-
-global.__DEV__ = process.env.NODE_ENV !== 'production';
-
-expect.extend(require('./toWarnDev'));
-
-function App(props) {
-  return 'hello world';
-}
-
-describe('legacy mode', () => {
-  runTests();
-});
-
-describe('mocked scheduler', () => {
-  beforeEach(() => {
-    jest.mock('scheduler', () =>
-      require.requireActual('scheduler/unstable_mock')
-    );
-  });
-  afterEach(() => {
-    jest.unmock('scheduler');
-  });
-  runTests();
-});
-
-function runTests() {
-  beforeEach(() => {
-    jest.resetModules();
-    React = require('react');
-    ReactDOM = require('react-dom');
-    ReactART = require('react-art');
-    ARTSVGMode = require('art/modes/svg');
-    ARTCurrentMode = require('art/modes/current');
-    TestUtils = require('react-dom/test-utils');
-    TestRenderer = require('react-test-renderer');
-
-    ARTCurrentMode.setCurrent(ARTSVGMode);
-
-    ARTTest = function ARTTest(props) {
-      return (
-        <ReactART.Surface width={150} height={200}>
-          <ReactART.Group>
-            <ReactART.Shape
-              d="M0,0l50,0l0,50l-50,0z"
-              fill={new ReactART.LinearGradient(['black', 'white'])}
-              key="a"
-              width={50}
-              height={50}
-              x={50}
-              y={50}
-              opacity={0.1}
-            />
-            <ReactART.Shape
-              fill="#3C5A99"
-              key="b"
-              scale={0.5}
-              x={50}
-              y={50}
-              title="This is an F"
-              cursor="pointer">
-              M64.564,38.583H54l0.008-5.834c0-3.035,0.293-4.666,4.657-4.666
-              h5.833V16.429h-9.33c-11.213,0-15.159,5.654-15.159,15.16v6.994
-              h-6.99v11.652h6.99v33.815H54V50.235h9.331L64.564,38.583z
-            </ReactART.Shape>
-          </ReactART.Group>
-        </ReactART.Surface>
-      );
-    };
-  });
-  it("doesn't warn when you use the right act + renderer: dom", () => {
-    TestUtils.act(() => {
-      TestUtils.renderIntoDocument(<App />);
-    });
-  });
-
-  it("doesn't warn when you use the right act + renderer: test", () => {
-    TestRenderer.act(() => {
-      TestRenderer.create(<App />);
-    });
-  });
-
-  it('resets correctly across renderers', () => {
-    function Effecty() {
-      React.useEffect(() => {}, []);
-      return null;
-    }
-    TestUtils.act(() => {
-      TestRenderer.act(() => {});
-      expect(() => {
-        TestRenderer.create(<Effecty />);
-      }).toWarnDev(["It looks like you're using the wrong act()"], {
-        withoutStack: true,
-      });
-    });
-  });
-
-  it('warns when using createRoot() + .render', () => {
-    const root = ReactDOM.unstable_createRoot(document.createElement('div'));
-    expect(() => {
-      TestRenderer.act(() => {
-        root.render(<App />);
-      });
-    }).toWarnDev(["It looks like you're using the wrong act()"], {
-      withoutStack: true,
-    });
-  });
-
-  it('warns when using the wrong act version - test + dom: render', () => {
-    expect(() => {
-      TestRenderer.act(() => {
-        TestUtils.renderIntoDocument(<App />);
-      });
-    }).toWarnDev(["It looks like you're using the wrong act()"], {
-      withoutStack: true,
-    });
-  });
-
-  it('warns when using the wrong act version - test + dom: updates', () => {
-    let setCtr;
-    function Counter(props) {
-      const [ctr, _setCtr] = React.useState(0);
-      setCtr = _setCtr;
-      return ctr;
-    }
-    TestUtils.renderIntoDocument(<Counter />);
-    expect(() => {
-      TestRenderer.act(() => {
-        setCtr(1);
-      });
-    }).toWarnDev(["It looks like you're using the wrong act()"]);
-  });
-
-  it('warns when using the wrong act version - dom + test: .create()', () => {
-    expect(() => {
-      TestUtils.act(() => {
-        TestRenderer.create(<App />);
-      });
-    }).toWarnDev(["It looks like you're using the wrong act()"], {
-      withoutStack: true,
-    });
-  });
-
-  it('warns when using the wrong act version - dom + test: .update()', () => {
-    const root = TestRenderer.create(<App key="one" />);
-    expect(() => {
-      TestUtils.act(() => {
-        root.update(<App key="two" />);
-      });
-    }).toWarnDev(["It looks like you're using the wrong act()"], {
-      withoutStack: true,
-    });
-  });
-
-  it('warns when using the wrong act version - dom + test: updates', () => {
-    let setCtr;
-    function Counter(props) {
-      const [ctr, _setCtr] = React.useState(0);
-      setCtr = _setCtr;
-      return ctr;
-    }
-    const root = TestRenderer.create(<Counter />);
-    expect(() => {
-      TestUtils.act(() => {
-        setCtr(1);
-      });
-    }).toWarnDev(["It looks like you're using the wrong act()"]);
-  });
-
-  it('does not warn when nesting react-act inside react-dom', () => {
-    TestUtils.act(() => {
-      TestUtils.renderIntoDocument(<ARTTest />);
-    });
-  });
-
-  it('does not warn when nesting react-act inside react-test-renderer', () => {
-    TestRenderer.act(() => {
-      TestRenderer.create(<ARTTest />);
-    });
-  });
-
-  it("doesn't warn if you use nested acts from different renderers", () => {
-    TestRenderer.act(() => {
-      TestUtils.act(() => {
-        TestRenderer.create(<App />);
-      });
-    });
-  });
-
-  it('flushes work only outside the outermost act(), even when nested from different renderers', () => {
-    const log = [];
-    function Effecty() {
-      React.useEffect(() => {
-        log.push('called');
-      }, []);
-      return null;
-    }
-    // in legacy mode, this tests whether an act only flushes its own effects
-    // with a mocked scheduler, this tests whether it flushes all work only on the outermost act
-    TestRenderer.act(() => {
-      TestUtils.act(() => {
-        TestRenderer.create(<Effecty />);
-      });
-      expect(log).toEqual([]);
-    });
-    expect(log).toEqual(['called']);
-
-    log.splice(0);
-    // for doublechecking, we flip it inside out, and assert on the outermost
-    TestUtils.act(() => {
-      TestRenderer.act(() => {
-        TestRenderer.create(<Effecty />);
-      });
-    });
-    expect(log).toEqual(['called']);
-  });
-}

packages/react-dom/src/__tests__/ReactTestUtilsAct-test.internal.js

@@ -1,28 +0,0 @@
-/**
- * Copyright (c) Facebook, 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.
- *
- * @emails react-core
- */
-
-let ReactFeatureFlags;
-let act;
-describe('mocked scheduler', () => {
-  beforeEach(() => {
-    jest.resetModules();
-    ReactFeatureFlags = require('shared/ReactFeatureFlags');
-    ReactFeatureFlags.warnAboutMissingMockScheduler = true;
-    jest.unmock('scheduler');
-    act = require('react-dom/test-utils').act;
-  });
-  it("should warn when the scheduler isn't mocked", () => {
-    expect(() => act(() => {})).toWarnDev(
-      [
-        'Starting from React v17, the "scheduler" module will need to be mocked',
-      ],
-      {withoutStack: true},
-    );
-  });
-});

packages/react-dom/src/__tests__/ReactUnmockedSchedulerWarning-test.internal.js

@@ -0,0 +1,42 @@
+/**
+ * Copyright (c) Facebook, 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.
+ *
+ * @emails react-core
+ */
+
+let React;
+let ReactDOM;
+let ReactFeatureFlags;
+
+function App() {
+  return null;
+}
+
+beforeEach(() => {
+  jest.resetModules();
+  jest.unmock('scheduler');
+  React = require('react');
+  ReactDOM = require('react-dom');
+  ReactFeatureFlags = require('shared/ReactFeatureFlags');
+  ReactFeatureFlags.warnAboutUnmockedScheduler = true;
+});
+
+afterEach(() => {
+  ReactFeatureFlags.warnAboutUnmockedScheduler = false;
+});
+
+it('should warn in sync mode', () => {
+  expect(() => {
+    ReactDOM.render(<App />, document.createElement('div'));
+  }).toWarnDev(
+    ['Starting from React v17, the "scheduler" module will need to be mocked'],
+    {withoutStack: true},
+  );
+  // does not warn twice
+  expect(() => {
+    ReactDOM.render(<App />, document.createElement('div'));
+  }).toWarnDev([]);
+});

packages/react-dom/src/__tests__/ReactUnmockedSchedulerWarning-test.js

@@ -0,0 +1,60 @@
+/**
+ * Copyright (c) Facebook, 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.
+ *
+ * @emails react-core
+ */
+
+let React;
+let ReactDOM;
+
+function App() {
+  return null;
+}
+
+beforeEach(() => {
+  jest.resetModules();
+  jest.unmock('scheduler');
+  React = require('react');
+  ReactDOM = require('react-dom');
+});
+
+it('does not warn when rendering in sync mode', () => {
+  expect(() => {
+    ReactDOM.render(<App />, document.createElement('div'));
+  }).toWarnDev([]);
+});
+
+it('should warn when rendering in concurrent mode', () => {
+  expect(() => {
+    ReactDOM.unstable_createRoot(document.createElement('div')).render(<App />);
+  }).toWarnDev(
+    'In Concurrent or Sync modes, the "scheduler" module needs to be mocked ' +
+      'to guarantee consistent behaviour across tests and browsers.',
+    {withoutStack: true},
+  );
+  // does not warn twice
+  expect(() => {
+    ReactDOM.unstable_createRoot(document.createElement('div')).render(<App />);
+  }).toWarnDev([]);
+});
+
+it('should warn when rendering in batched mode', () => {
+  expect(() => {
+    ReactDOM.unstable_createSyncRoot(document.createElement('div')).render(
+      <App />,
+    );
+  }).toWarnDev(
+    'In Concurrent or Sync modes, the "scheduler" module needs to be mocked ' +
+      'to guarantee consistent behaviour across tests and browsers.',
+    {withoutStack: true},
+  );
+  // does not warn twice
+  expect(() => {
+    ReactDOM.unstable_createSyncRoot(document.createElement('div')).render(
+      <App />,
+    );
+  }).toWarnDev([]);
+});

packages/react-dom/src/test-utils/ReactTestUtilsAct.js

@@ -12,7 +12,6 @@ import type {Thenable} from 'react-reconciler/src/ReactFiberWorkLoop';
 import warningWithoutStack from 'shared/warningWithoutStack';
 import ReactDOM from 'react-dom';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
-import {warnAboutMissingMockScheduler} from 'shared/ReactFeatureFlags';
 import enqueueTask from 'shared/enqueueTask';
 import * as Scheduler from 'scheduler';
 
@@ -43,27 +42,11 @@ const {IsSomeRendererActing} = ReactSharedInternals;
 // this implementation should be exactly the same in
 // ReactTestUtilsAct.js, ReactTestRendererAct.js, createReactNoop.js
 
-let hasWarnedAboutMissingMockScheduler = false;
 const isSchedulerMocked =
   typeof Scheduler.unstable_flushAllWithoutAsserting === 'function';
 const flushWork =
   Scheduler.unstable_flushAllWithoutAsserting ||
   function() {
-    if (warnAboutMissingMockScheduler === true) {
-      if (hasWarnedAboutMissingMockScheduler === false) {
-        warningWithoutStack(
-          null,
-          'Starting from React v17, the "scheduler" module will need to be mocked ' +
-            'to guarantee consistent behaviour across tests and browsers. To fix this, add the following ' +
-            "to the top of your tests, or in your framework's global config file -\n\n" +
-            'As an example, for jest - \n' +
-            "jest.mock('scheduler', () => require.requireActual('scheduler/unstable_mock'));\n\n" +
-            'For more info, visit https://fb.me/react-mock-scheduler',
-        );
-        hasWarnedAboutMissingMockScheduler = true;
-      }
-    }
-
     let didFlushWork = false;
     while (flushPassiveEffects()) {
       didFlushWork = true;

packages/react-noop-renderer/src/createReactNoop.js

@@ -27,7 +27,6 @@ import {REACT_FRAGMENT_TYPE, REACT_ELEMENT_TYPE} from 'shared/ReactSymbols';
 import enqueueTask from 'shared/enqueueTask';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import warningWithoutStack from 'shared/warningWithoutStack';
-import {warnAboutMissingMockScheduler} from 'shared/ReactFeatureFlags';
 import {ConcurrentRoot, BatchedRoot, LegacyRoot} from 'shared/ReactRootTags';
 
 type Container = {
@@ -599,27 +598,11 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
   // this act() implementation should be exactly the same in
   // ReactTestUtilsAct.js, ReactTestRendererAct.js, createReactNoop.js
 
-  let hasWarnedAboutMissingMockScheduler = false;
   const isSchedulerMocked =
     typeof Scheduler.unstable_flushAllWithoutAsserting === 'function';
   const flushWork =
     Scheduler.unstable_flushAllWithoutAsserting ||
     function() {
-      if (warnAboutMissingMockScheduler === true) {
-        if (hasWarnedAboutMissingMockScheduler === false) {
-          warningWithoutStack(
-            null,
-            'Starting from React v17, the "scheduler" module will need to be mocked ' +
-              'to guarantee consistent behaviour across tests and browsers. To fix this, add the following ' +
-              "to the top of your tests, or in your framework's global config file -\n\n" +
-              'As an example, for jest - \n' +
-              "jest.mock('scheduler', () => require.requireActual('scheduler/unstable_mock'));\n\n" +
-              'For more info, visit https://fb.me/react-mock-scheduler',
-          );
-          hasWarnedAboutMissingMockScheduler = true;
-        }
-      }
-
       let didFlushWork = false;
       while (flushPassiveEffects()) {
         didFlushWork = true;

packages/react-reconciler/src/ReactFiberReconciler.js

@@ -58,6 +58,7 @@ import {
   flushDiscreteUpdates,
   flushPassiveEffects,
   warnIfNotScopedWithMatchingAct,
+  warnIfUnmockedScheduler,
   IsThisRendererActing,
 } from './ReactFiberWorkLoop';
 import {createUpdate, enqueueUpdate} from './ReactUpdateQueue';
@@ -314,6 +315,7 @@ export function updateContainer(
   if (__DEV__) {
     // $FlowExpectedError - jest isn't a global, and isn't recognized outside of tests
     if ('undefined' !== typeof jest) {
+      warnIfUnmockedScheduler(current);
       warnIfNotScopedWithMatchingAct(current);
     }
   }

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -25,6 +25,7 @@ import {
   enableProfilerTimer,
   enableSchedulerTracing,
   revertPassiveEffectsChange,
+  warnAboutUnmockedScheduler,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -47,6 +48,9 @@ import {
   scheduleSyncCallback,
 } from './SchedulerWithReactIntegration';
 
+// The scheduler is imported here *only* to detect whether it's been mocked
+import * as Scheduler from 'scheduler';
+
 import {__interactionsRef, __subscriberRef} from 'scheduler/tracing';
 
 import {
@@ -2522,6 +2526,41 @@ function warnIfNotCurrentlyActingUpdatesInDEV(fiber: Fiber): void {
 
 export const warnIfNotCurrentlyActingUpdatesInDev = warnIfNotCurrentlyActingUpdatesInDEV;
 
+// In tests, we want to enforce a mocked scheduler.
+let didWarnAboutUnmockedScheduler = false;
+// TODO Before we release concurrent mode, revisit this and decide whether a mocked
+// scheduler is the actual recommendation. The alternative could be a testing build,
+// a new lib, or whatever; we dunno just yet. This message is for early adopters
+// to get their tests right.
+
+export function warnIfUnmockedScheduler(fiber: Fiber) {
+  if (__DEV__) {
+    if (didWarnAboutUnmockedScheduler === false) {
+      if (fiber.mode & BatchedMode || fiber.mode & ConcurrentMode) {
+        didWarnAboutUnmockedScheduler = true;
+        warningWithoutStack(
+          Scheduler.unstable_flushAllWithoutAsserting !== undefined,
+          'In Concurrent or Sync modes, the "scheduler" module needs to be mocked ' +
+            'to guarantee consistent behaviour across tests and browsers. ' +
+            'For example, with jest: \n' +
+            "jest.mock('scheduler', () => require('scheduler/unstable_mock'));\n\n" +
+            'For more info, visit https://fb.me/react-mock-scheduler',
+        );
+      } else if (warnAboutUnmockedScheduler === true) {
+        didWarnAboutUnmockedScheduler = true;
+        warningWithoutStack(
+          null,
+          'Starting from React v17, the "scheduler" module will need to be mocked ' +
+            'to guarantee consistent behaviour across tests and browsers. ' +
+            'For example, with jest: \n' +
+            "jest.mock('scheduler', () => require('scheduler/unstable_mock'));\n\n" +
+            'For more info, visit https://fb.me/react-mock-scheduler',
+        );
+      }
+    }
+  }
+}
+
 let componentsThatSuspendedAtHighPri = null;
 let componentsThatTriggeredHighPriSuspend = null;
 export function checkForWrongSuspensePriorityInDEV(sourceFiber: Fiber) {

packages/react-test-renderer/src/ReactTestRendererAct.js

@@ -15,7 +15,6 @@ import {
 } from 'react-reconciler/inline.test';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import warningWithoutStack from 'shared/warningWithoutStack';
-import {warnAboutMissingMockScheduler} from 'shared/ReactFeatureFlags';
 import enqueueTask from 'shared/enqueueTask';
 import * as Scheduler from 'scheduler';
 
@@ -24,27 +23,11 @@ const {IsSomeRendererActing} = ReactSharedInternals;
 // this implementation should be exactly the same in
 // ReactTestUtilsAct.js, ReactTestRendererAct.js, createReactNoop.js
 
-let hasWarnedAboutMissingMockScheduler = false;
 const isSchedulerMocked =
   typeof Scheduler.unstable_flushAllWithoutAsserting === 'function';
 const flushWork =
   Scheduler.unstable_flushAllWithoutAsserting ||
   function() {
-    if (warnAboutMissingMockScheduler === true) {
-      if (hasWarnedAboutMissingMockScheduler === false) {
-        warningWithoutStack(
-          null,
-          'Starting from React v17, the "scheduler" module will need to be mocked ' +
-            'to guarantee consistent behaviour across tests and browsers. To fix this, add the following ' +
-            "to the top of your tests, or in your framework's global config file -\n\n" +
-            'As an example, for jest - \n' +
-            "jest.mock('scheduler', () => require.requireActual('scheduler/unstable_mock'));\n\n" +
-            'For more info, visit https://fb.me/react-mock-scheduler',
-        );
-        hasWarnedAboutMissingMockScheduler = true;
-      }
-    }
-
     let didFlushWork = false;
     while (flushPassiveEffects()) {
       didFlushWork = true;

packages/shared/ReactFeatureFlags.js

@@ -70,7 +70,7 @@ export const enableJSXTransformAPI = false;
 
 // We will enforce mocking scheduler with scheduler/unstable_mock at some point. (v17?)
 // Till then, we warn about the missing mock, but still fallback to a sync mode compatible version
-export const warnAboutMissingMockScheduler = false;
+export const warnAboutUnmockedScheduler = false;
 // Temporary flag to revert the fix in #15650
 export const revertPassiveEffectsChange = false;
 

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -34,7 +34,7 @@ export const warnAboutDeprecatedSetNativeProps = true;
 export const enableFlareAPI = false;
 export const enableFundamentalAPI = false;
 export const enableJSXTransformAPI = false;
-export const warnAboutMissingMockScheduler = true;
+export const warnAboutUnmockedScheduler = true;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -29,7 +29,7 @@ export const warnAboutDeprecatedSetNativeProps = false;
 export const enableFlareAPI = false;
 export const enableFundamentalAPI = false;
 export const enableJSXTransformAPI = false;
-export const warnAboutMissingMockScheduler = false;
+export const warnAboutUnmockedScheduler = false;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;

packages/shared/forks/ReactFeatureFlags.persistent.js

@@ -29,7 +29,7 @@ export const warnAboutDeprecatedSetNativeProps = false;
 export const enableFlareAPI = false;
 export const enableFundamentalAPI = false;
 export const enableJSXTransformAPI = false;
-export const warnAboutMissingMockScheduler = true;
+export const warnAboutUnmockedScheduler = true;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -29,7 +29,7 @@ export const warnAboutDeprecatedSetNativeProps = false;
 export const enableFlareAPI = false;
 export const enableFundamentalAPI = false;
 export const enableJSXTransformAPI = false;
-export const warnAboutMissingMockScheduler = false;
+export const warnAboutUnmockedScheduler = false;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -30,7 +30,7 @@ export const disableJavaScriptURLs = false;
 export const enableFlareAPI = true;
 export const enableFundamentalAPI = false;
 export const enableJSXTransformAPI = true;
-export const warnAboutMissingMockScheduler = true;
+export const warnAboutUnmockedScheduler = true;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = true;
 export const warnAboutDefaultPropsOnFunctionComponents = false;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -74,7 +74,7 @@ export const enableFundamentalAPI = false;
 
 export const enableJSXTransformAPI = true;
 
-export const warnAboutMissingMockScheduler = true;
+export const warnAboutUnmockedScheduler = true;
 
 export const enableSuspenseCallback = true;