use a feature flag for the missing mock scheduler warning

I also tried writing a test for it, but couldn't get the scheduler to unmock. included the failing test.

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

@@ -15,6 +15,7 @@ let act;
 let container;
 let isConcurrentMode;
 let concurrentRoot = null;
+let ReactFeatureFlags;
 
 function render(el, dom) {
   if (isConcurrentMode) {
@@ -54,6 +55,7 @@ describe('ReactTestUtils.act()', () => {
     ReactDOM = require('react-dom');
     ReactTestUtils = require('react-dom/test-utils');
     SchedulerTracing = require('scheduler/tracing');
+    ReactFeatureFlags = require('shared/ReactFeatureFlags');
     act = ReactTestUtils.act;
     container = document.createElement('div');
     document.body.appendChild(container);
@@ -63,6 +65,27 @@ describe('ReactTestUtils.act()', () => {
     document.body.removeChild(container);
   });
 
+  describe('mocked scheduler', () => {
+    beforeEach(() => {
+      ReactFeatureFlags.warnAboutMissingMockScheduler = true;
+      // todo - this doesn't actually work
+      // jest.unmock('scheduler');
+      // jest.unmock('scheduler/src/SchedulerHostConfig');
+      // neither does this. am investigating.
+      // jest.mock('scheduler', () => {
+      //   return require.requireActual('scheduler');
+      // });
+      // jest.mock('scheduler/src/SchedulerHostConfig', () => {
+      //   return require.requireActual('scheduler/src/forks/SchedulerHostConfig.default');
+      // });
+    });
+    it("should warn when the scheduler isn't mocked", () => {
+      expect(() => act(() => {})).toWarnDev([
+        'Starting from React v17, the "scheduler" module will need to be mocked',
+      ]);
+    });
+  });
+
   [false, true].forEach(concurrent => {
     isConcurrentMode = concurrent;
     describe(isConcurrentMode ? 'concurrent mode' : 'heritage mode', () => {

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

@@ -12,6 +12,7 @@ import type {Thenable} from 'react-reconciler/src/ReactFiberScheduler';
 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';
 
@@ -45,19 +46,20 @@ let hasWarnedAboutMissingMockScheduler = false;
 const flushWork =
   Scheduler.unstable_flushWithoutYielding ||
   function() {
-    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;
+    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;
+      }
     }
-
     while (flushPassiveEffects()) {}
   };
 

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

@@ -31,7 +31,10 @@ import warning from 'shared/warning';
 import enqueueTask from 'shared/enqueueTask';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import warningWithoutStack from 'shared/warningWithoutStack';
-import {enableEventAPI} from 'shared/ReactFeatureFlags';
+import {
+  warnAboutMissingMockScheduler,
+  enableEventAPI,
+} from 'shared/ReactFeatureFlags';
 
 type EventTargetChildElement = {
   type: string,
@@ -654,19 +657,20 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
   const flushWork =
     Scheduler.unstable_flushWithoutYielding ||
     function() {
-      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;
+      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;
+        }
       }
-
       while (flushPassiveEffects()) {}
     };
 

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

@@ -14,6 +14,7 @@ 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';
 
@@ -26,19 +27,20 @@ let hasWarnedAboutMissingMockScheduler = false;
 const flushWork =
   Scheduler.unstable_flushWithoutYielding ||
   function() {
-    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;
+    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;
+      }
     }
-
     while (flushPassiveEffects()) {}
   };
 

packages/shared/ReactFeatureFlags.js

@@ -67,3 +67,7 @@ export const enableEventAPI = false;
 
 // New API for JSX transforms to target - https://github.com/reactjs/rfcs/pull/107
 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;

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

@@ -32,6 +32,7 @@ export const warnAboutDeprecatedLifecycles = true;
 export const warnAboutDeprecatedSetNativeProps = true;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = true;
 
 // Only used in www builds.
 export function addUserTimingListener() {

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

@@ -29,6 +29,7 @@ export const enableSchedulerDebugging = false;
 export const warnAboutDeprecatedSetNativeProps = false;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = false;
 
 // Only used in www builds.
 export function addUserTimingListener() {

packages/shared/forks/ReactFeatureFlags.persistent.js

@@ -29,6 +29,7 @@ export const enableSchedulerDebugging = false;
 export const warnAboutDeprecatedSetNativeProps = false;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = true;
 
 // Only used in www builds.
 export function addUserTimingListener() {

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

@@ -29,6 +29,7 @@ export const enableSchedulerDebugging = false;
 export const warnAboutDeprecatedSetNativeProps = false;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = false;
 
 // Only used in www builds.
 export function addUserTimingListener() {

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

@@ -27,6 +27,7 @@ export const disableJavaScriptURLs = false;
 export const disableYielding = false;
 export const enableEventAPI = true;
 export const enableJSXTransformAPI = true;
+export const warnAboutMissingMockScheduler = true;
 
 // Only used in www builds.
 export function addUserTimingListener() {

packages/shared/forks/ReactFeatureFlags.www.js

@@ -71,6 +71,8 @@ export const enableEventAPI = true;
 
 export const enableJSXTransformAPI = true;
 
+export const warnAboutMissingMockScheduler = true;
+
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;