Convert ReactLazy-test to waitFor pattern

I'm in the process of codemodding our test suite to the waitFor pattern.
See #26285 for full context.

This module required a lot of manual changes so I'm doing it as its own
PR. The reason is that most of the tests involved simulating an async
import by wrapping them in `Promise.resolve()`, which means they would
immediately resolve the next time the microtask queue was flushed. I
rewrote the tests to resolve the simulated import explicitly.

While converting these tests, I also realized that the `waitFor` helpers
weren't properly waiting for the entire microtask queue to recursively
finish — if a microtask schedules another microtask, the subsequent one
wouldn't fire until after `waitFor` had resolved. To fix this, I used
the same strategy as `act` — wait for a real task to finish before
proceeding, such as a message event.

packages/internal-test-utils/ReactInternalTestUtils.js

@@ -10,6 +10,7 @@
 import * as SchedulerMock from 'scheduler/unstable_mock';
 import {diff} from 'jest-diff';
 import {equals} from '@jest/expect-utils';
+import enqueueTask from './enqueueTask';
 
 function assertYieldsWereCleared(Scheduler) {
   const actualYields = Scheduler.unstable_clearYields();
@@ -22,6 +23,12 @@ function assertYieldsWereCleared(Scheduler) {
   }
 }
 
+async function waitForMicrotasks() {
+  return new Promise(resolve => {
+    enqueueTask(() => resolve());
+  });
+}
+
 export async function waitFor(expectedLog) {
   assertYieldsWereCleared(SchedulerMock);
 
@@ -33,7 +40,7 @@ export async function waitFor(expectedLog) {
   const actualLog = [];
   do {
     // Wait until end of current task/microtask.
-    await null;
+    await waitForMicrotasks();
     if (SchedulerMock.unstable_hasPendingWork()) {
       SchedulerMock.unstable_flushNumberOfYields(
         expectedLog.length - actualLog.length,
@@ -44,7 +51,7 @@ export async function waitFor(expectedLog) {
       } else {
         // Once we've reached the expected sequence, wait one more microtask to
         // flush any remaining synchronous work.
-        await null;
+        await waitForMicrotasks();
         actualLog.push(...SchedulerMock.unstable_clearYields());
         break;
       }
@@ -72,11 +79,11 @@ export async function waitForAll(expectedLog) {
   // Create the error object before doing any async work, to get a better
   // stack trace.
   const error = new Error();
-  Error.captureStackTrace(error, waitFor);
+  Error.captureStackTrace(error, waitForAll);
 
   do {
     // Wait until end of current task/microtask.
-    await null;
+    await waitForMicrotasks();
     if (!SchedulerMock.unstable_hasPendingWork()) {
       // There's no pending work, even after a microtask. Stop flushing.
       break;
@@ -103,11 +110,11 @@ export async function waitForThrow(expectedError: mixed) {
   // Create the error object before doing any async work, to get a better
   // stack trace.
   const error = new Error();
-  Error.captureStackTrace(error, waitFor);
+  Error.captureStackTrace(error, waitForThrow);
 
   do {
     // Wait until end of current task/microtask.
-    await null;
+    await waitForMicrotasks();
     if (!SchedulerMock.unstable_hasPendingWork()) {
       // There's no pending work, even after a microtask. Stop flushing.
       error.message = 'Expected something to throw, but nothing did.';
@@ -119,7 +126,13 @@ export async function waitForThrow(expectedError: mixed) {
       if (equals(x, expectedError)) {
         return;
       }
-      if (typeof x === 'object' && x !== null && x.message === expectedError) {
+      if (
+        typeof expectedError === 'string' &&
+        typeof x === 'object' &&
+        x !== null &&
+        typeof x.message === 'string' &&
+        x.message.includes(expectedError)
+      ) {
         return;
       }
       error.message = `
@@ -142,15 +155,15 @@ export async function waitForPaint(expectedLog) {
   // Create the error object before doing any async work, to get a better
   // stack trace.
   const error = new Error();
-  Error.captureStackTrace(error, waitFor);
+  Error.captureStackTrace(error, waitForPaint);
 
   // Wait until end of current task/microtask.
-  await null;
+  await waitForMicrotasks();
   if (SchedulerMock.unstable_hasPendingWork()) {
     // Flush until React yields.
     SchedulerMock.unstable_flushUntilNextPaint();
     // Wait one more microtask to flush any remaining synchronous work.
-    await null;
+    await waitForMicrotasks();
   }
 
   const actualLog = SchedulerMock.unstable_clearYields();

packages/internal-test-utils/enqueueTask.js

@@ -0,0 +1,48 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+let didWarnAboutMessageChannel = false;
+let enqueueTaskImpl = null;
+
+export default function enqueueTask(task: () => void): void {
+  if (enqueueTaskImpl === null) {
+    try {
+      // read require off the module object to get around the bundlers.
+      // we don't want them to detect a require and bundle a Node polyfill.
+      const requireString = ('require' + Math.random()).slice(0, 7);
+      const nodeRequire = module && module[requireString];
+      // assuming we're in node, let's try to get node's
+      // version of setImmediate, bypassing fake timers if any.
+      enqueueTaskImpl = nodeRequire.call(module, 'timers').setImmediate;
+    } catch (_err) {
+      // we're in a browser
+      // we can't use regular timers because they may still be faked
+      // so we try MessageChannel+postMessage instead
+      enqueueTaskImpl = function (callback: () => void) {
+        if (__DEV__) {
+          if (didWarnAboutMessageChannel === false) {
+            didWarnAboutMessageChannel = true;
+            if (typeof MessageChannel === 'undefined') {
+              console.error(
+                'This browser does not have a MessageChannel implementation, ' +
+                  'so enqueuing tasks via await act(async () => ...) will fail. ' +
+                  'Please file an issue at https://github.com/facebook/react/issues ' +
+                  'if you encounter this warning.',
+              );
+            }
+          }
+        }
+        const channel = new MessageChannel();
+        channel.port1.onmessage = callback;
+        channel.port2.postMessage(undefined);
+      };
+    }
+  }
+  return enqueueTaskImpl(task);
+}

packages/react-cache/src/__tests__/ReactCacheOld-test.internal.js

@@ -183,7 +183,7 @@ describe('ReactCache', () => {
     );
 
     if (__DEV__) {
-      expect(async () => {
+      await expect(async () => {
         await waitForAll(['App', 'Loading...']);
       }).toErrorDev([
         'Invalid key type. Expected a string, number, symbol, or ' +

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

@@ -93,7 +93,7 @@ describe('ReactIncrementalScheduling', () => {
     expect(ReactNoop).toMatchRenderedOutput(<span prop={5} />);
   });
 
-  it('works on deferred roots in the order they were scheduled', () => {
+  it('works on deferred roots in the order they were scheduled', async () => {
     const {useEffect} = React;
     function Text({text}) {
       useEffect(() => {
@@ -114,7 +114,7 @@ describe('ReactIncrementalScheduling', () => {
     expect(ReactNoop.getChildrenAsJSX('c')).toEqual('c:1');
 
     // Schedule deferred work in the reverse order
-    act(async () => {
+    await act(async () => {
       React.startTransition(() => {
         ReactNoop.renderToRootWithID(<Text text="c:2" />, 'c');
         ReactNoop.renderToRootWithID(<Text text="b:2" />, 'b');

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

@@ -518,7 +518,7 @@ describe('ReactIncrementalUpdates', () => {
     expect(ReactNoop).toMatchRenderedOutput(<span prop="derived state" />);
   });
 
-  it('regression: does not expire soon due to layout effects in the last batch', () => {
+  it('regression: does not expire soon due to layout effects in the last batch', async () => {
     const {useState, useLayoutEffect} = React;
 
     let setCount;
@@ -533,7 +533,7 @@ describe('ReactIncrementalUpdates', () => {
       return null;
     }
 
-    act(async () => {
+    await act(async () => {
       React.startTransition(() => {
         ReactNoop.render(<App />);
       });