Warn if act is called in wrong environment

Adds a warning if `act` is called but `IS_REACT_ACT_ENVIRONMENT` is not
enabled. The goal is to prompt users to correctly configure their
testing environment, so that if they forget to use `act` in a different
test, we can detect and warn about.

It's expected that the environment flag will be configured by the
testing framework. For example, a Jest plugin. We will link to the
relevant documentation page, once it exists.

The warning only fires in concurrent mode. Legacy roots will keep the
existing behavior.

packages/react-devtools-scheduling-profiler/src/import-worker/__tests__/preprocessData-test.internal.js

@@ -17,6 +17,8 @@ import {
 } from '../../constants';
 import REACT_VERSION from 'shared/ReactVersion';
 
+global.IS_REACT_ACT_ENVIRONMENT = true;
+
 describe('getLanesFromTransportDecimalBitmask', () => {
   it('should return array of lane numbers from bitmask string', () => {
     expect(getLanesFromTransportDecimalBitmask('1')).toEqual([0]);

packages/react-devtools-shared/src/__tests__/inspectedElement-test.js

@@ -38,6 +38,8 @@ describe('InspectedElement', () => {
   let ErrorBoundary;
   let errorBoundaryInstance;
 
+  global.IS_REACT_ACT_ENVIRONMENT = true;
+
   beforeEach(() => {
     utils = require('./utils');
     utils.beforeEachProfiling();

packages/react-reconciler/src/ReactFiberAct.new.js

@@ -8,9 +8,14 @@
  */
 
 import type {Fiber} from './ReactFiber.new';
+
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+
 import {warnsIfNotActing} from './ReactFiberHostConfig';
 import {ConcurrentMode} from './ReactTypeOfMode';
 
+const {ReactCurrentActQueue} = ReactSharedInternals;
+
 export function isActEnvironment(fiber: Fiber) {
   if (__DEV__) {
     const isReactActEnvironmentGlobal =
@@ -20,6 +25,16 @@ export function isActEnvironment(fiber: Fiber) {
         : undefined;
 
     if (fiber.mode & ConcurrentMode) {
+      if (
+        !isReactActEnvironmentGlobal &&
+        ReactCurrentActQueue.current !== null
+      ) {
+        // TODO: Include link to relevant documentation page.
+        console.error(
+          'The current testing environment is not configured to support ' +
+            'act(...)',
+        );
+      }
       return isReactActEnvironmentGlobal;
     } else {
       // Legacy mode. We preserve the behavior of React 17's act. It assumes an

packages/react-reconciler/src/ReactFiberAct.old.js

@@ -8,9 +8,14 @@
  */
 
 import type {Fiber} from './ReactFiber.old';
+
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+
 import {warnsIfNotActing} from './ReactFiberHostConfig';
 import {ConcurrentMode} from './ReactTypeOfMode';
 
+const {ReactCurrentActQueue} = ReactSharedInternals;
+
 export function isActEnvironment(fiber: Fiber) {
   if (__DEV__) {
     const isReactActEnvironmentGlobal =
@@ -20,6 +25,16 @@ export function isActEnvironment(fiber: Fiber) {
         : undefined;
 
     if (fiber.mode & ConcurrentMode) {
+      if (
+        !isReactActEnvironmentGlobal &&
+        ReactCurrentActQueue.current !== null
+      ) {
+        // TODO: Include link to relevant documentation page.
+        console.error(
+          'The current testing environment is not configured to support ' +
+            'act(...)',
+        );
+      }
       return isReactActEnvironmentGlobal;
     } else {
       // Legacy mode. We preserve the behavior of React 17's act. It assumes an

packages/react-reconciler/src/__tests__/DebugTracing-test.internal.js

@@ -19,6 +19,8 @@ describe('DebugTracing', () => {
   const DEFAULT_LANE_STRING = '0b0000000000000000000000000010000';
   const RETRY_LANE_STRING = '0b0000000010000000000000000000000';
 
+  global.IS_REACT_ACT_ENVIRONMENT = true;
+
   beforeEach(() => {
     jest.resetModules();
 

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

@@ -11,15 +11,18 @@ let React;
 let Scheduler;
 let ReactNoop;
 let useState;
+let act;
 
 // These tests are mostly concerned with concurrent roots. The legacy root
 // behavior is covered by other older test suites and is unchanged from
 // React 17.
 describe('act warnings', () => {
   beforeEach(() => {
+    jest.resetModules();
     React = require('react');
     Scheduler = require('scheduler');
     ReactNoop = require('react-noop-renderer');
+    act = React.unstable_act;
     useState = React.useState;
   });
 
@@ -73,4 +76,55 @@ describe('act warnings', () => {
       expect(root).toMatchRenderedOutput('3');
     });
   });
+
+  // @gate __DEV__
+  test('act warns if the environment flag is not enabled', () => {
+    let setState;
+    function App() {
+      const [state, _setState] = useState(0);
+      setState = _setState;
+      return <Text text={state} />;
+    }
+
+    const root = ReactNoop.createRoot();
+    root.render(<App />);
+    expect(Scheduler).toFlushAndYield([0]);
+    expect(root).toMatchRenderedOutput('0');
+
+    // Default behavior. Flag is undefined. Warn.
+    expect(global.IS_REACT_ACT_ENVIRONMENT).toBe(undefined);
+    expect(() => {
+      act(() => {
+        setState(1);
+      });
+    }).toErrorDev(
+      'The current testing environment is not configured to support act(...)',
+      {withoutStack: true},
+    );
+    expect(Scheduler).toHaveYielded([1]);
+    expect(root).toMatchRenderedOutput('1');
+
+    // Flag is true. Don't warn.
+    withActEnvironment(true, () => {
+      act(() => {
+        setState(2);
+      });
+      expect(Scheduler).toHaveYielded([2]);
+      expect(root).toMatchRenderedOutput('2');
+    });
+
+    // Flag is false. Warn.
+    withActEnvironment(false, () => {
+      expect(() => {
+        act(() => {
+          setState(1);
+        });
+      }).toErrorDev(
+        'The current testing environment is not configured to support act(...)',
+        {withoutStack: true},
+      );
+      expect(Scheduler).toHaveYielded([1]);
+      expect(root).toMatchRenderedOutput('1');
+    });
+  });
 });

packages/react-reconciler/src/__tests__/SchedulingProfilerLabels-test.internal.js

@@ -21,6 +21,8 @@ describe('SchedulingProfiler labels', () => {
   let featureDetectionMarkName = null;
   let marks;
 
+  global.IS_REACT_ACT_ENVIRONMENT = true;
+
   function polyfillJSDomUserTiming() {
     featureDetectionMarkName = null;