Support use in act testing API

`use` can avoid suspending on already resolved data by yielding to
microtasks. In a real, browser environment, we do this by scheduling
a platform task (i.e. postTask).

In a test environment, tasks are scheduled on a special internal queue
so that they can be flushed by the `act` testing API. So we need to add
support for this in `act`.

This behavior only works if you `await` the thenable returning by
`act` call. We currently do not require that users do this. So I added
a warning, but it only fires if `use` was called. The old Suspense
pattern will not trigger a warning. This is to avoid breaking existing
tests that use Suspense.

The implementation of `act` has gotten extremely complicated because of
the subtle changes in behavior over the years, and our commitment to
maintaining backwards compatibility. We really should consider being
more restrictive in a future major release.

The changes are a bit confusing so I did my best to add inline comments
explaining how it works.

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

@@ -15,6 +15,9 @@ import type {
   RejectedThenable,
 } from 'shared/ReactTypes';
 
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+const {ReactCurrentActQueue} = ReactSharedInternals;
+
 let suspendedThenable: Thenable<mixed> | null = null;
 let adHocSuspendCount: number = 0;
 
@@ -124,6 +127,10 @@ export function trackUsedThenable<T>(thenable: Thenable<T>, index: number) {
   }
   usedThenables[index] = thenable;
   lastUsedThenable = thenable;
+
+  if (__DEV__ && ReactCurrentActQueue.current !== null) {
+    ReactCurrentActQueue.didUsePromise = true;
+  }
 }
 
 export function getPreviouslyUsedThenableAtIndex<T>(

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

@@ -15,6 +15,9 @@ import type {
   RejectedThenable,
 } from 'shared/ReactTypes';
 
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+const {ReactCurrentActQueue} = ReactSharedInternals;
+
 let suspendedThenable: Thenable<mixed> | null = null;
 let adHocSuspendCount: number = 0;
 
@@ -124,6 +127,10 @@ export function trackUsedThenable<T>(thenable: Thenable<T>, index: number) {
   }
   usedThenables[index] = thenable;
   lastUsedThenable = thenable;
+
+  if (__DEV__ && ReactCurrentActQueue.current !== null) {
+    ReactCurrentActQueue.didUsePromise = true;
+  }
 }
 
 export function getPreviouslyUsedThenableAtIndex<T>(

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

@@ -12,15 +12,22 @@
 let React;
 let ReactNoop;
 let act;
+let use;
+let Suspense;
 let DiscreteEventPriority;
+let startTransition;
 
 describe('isomorphic act()', () => {
   beforeEach(() => {
     React = require('react');
+
     ReactNoop = require('react-noop-renderer');
     DiscreteEventPriority = require('react-reconciler/constants')
       .DiscreteEventPriority;
     act = React.unstable_act;
+    use = React.experimental_use;
+    Suspense = React.Suspense;
+    startTransition = React.startTransition;
   });
 
   beforeEach(() => {
@@ -133,4 +140,154 @@ describe('isomorphic act()', () => {
       expect(root).toMatchRenderedOutput('C');
     });
   });
+
+  // @gate __DEV__
+  // @gate enableUseHook
+  test('unwraps promises by yielding to microtasks (async act scope)', async () => {
+    const promise = Promise.resolve('Async');
+
+    function Fallback() {
+      throw new Error('Fallback should never be rendered');
+    }
+
+    function App() {
+      return use(promise);
+    }
+
+    const root = ReactNoop.createRoot();
+    await act(async () => {
+      startTransition(() => {
+        root.render(
+          <Suspense fallback={<Fallback />}>
+            <App />
+          </Suspense>,
+        );
+      });
+    });
+    expect(root).toMatchRenderedOutput('Async');
+  });
+
+  // @gate __DEV__
+  // @gate enableUseHook
+  test('unwraps promises by yielding to microtasks (non-async act scope)', async () => {
+    const promise = Promise.resolve('Async');
+
+    function Fallback() {
+      throw new Error('Fallback should never be rendered');
+    }
+
+    function App() {
+      return use(promise);
+    }
+
+    const root = ReactNoop.createRoot();
+
+    // Note that the scope function is not an async function
+    await act(() => {
+      startTransition(() => {
+        root.render(
+          <Suspense fallback={<Fallback />}>
+            <App />
+          </Suspense>,
+        );
+      });
+    });
+    expect(root).toMatchRenderedOutput('Async');
+  });
+
+  // @gate __DEV__
+  // @gate enableUseHook
+  test('warns if a promise is used in a non-awaited `act` scope', async () => {
+    const promise = new Promise(() => {});
+
+    function Fallback() {
+      throw new Error('Fallback should never be rendered');
+    }
+
+    function App() {
+      return use(promise);
+    }
+
+    spyOnDev(console, 'error');
+    const root = ReactNoop.createRoot();
+    act(() => {
+      startTransition(() => {
+        root.render(
+          <Suspense fallback={<Fallback />}>
+            <App />
+          </Suspense>,
+        );
+      });
+    });
+
+    // `act` warns after a few microtasks, instead of a macrotask, so that it's
+    // more likely to be attributed to the correct test case.
+    //
+    // The exact number of microtasks is an implementation detail; just needs
+    // to happen when the microtask queue is flushed.
+    await null;
+    await null;
+    await null;
+
+    expect(console.error.calls.count()).toBe(1);
+    expect(console.error.calls.argsFor(0)[0]).toContain(
+      'Warning: A component suspended inside an `act` scope, but the `act` ' +
+        'call was not awaited. When testing React components that ' +
+        'depend on asynchronous data, you must await the result:\n\n' +
+        'await act(() => ...)',
+    );
+  });
+
+  // @gate __DEV__
+  test('does not warn when suspending via legacy `throw` API  in non-awaited `act` scope', async () => {
+    let didResolve = false;
+    let resolvePromise;
+    const promise = new Promise(r => {
+      resolvePromise = () => {
+        didResolve = true;
+        r();
+      };
+    });
+
+    function Fallback() {
+      return 'Loading...';
+    }
+
+    function App() {
+      if (!didResolve) {
+        throw promise;
+      }
+      return 'Async';
+    }
+
+    spyOnDev(console, 'error');
+    const root = ReactNoop.createRoot();
+    act(() => {
+      startTransition(() => {
+        root.render(
+          <Suspense fallback={<Fallback />}>
+            <App />
+          </Suspense>,
+        );
+      });
+    });
+    expect(root).toMatchRenderedOutput('Loading...');
+
+    // `act` warns after a few microtasks, instead of a macrotask, so that it's
+    // more likely to be attributed to the correct test case.
+    //
+    // The exact number of microtasks is an implementation detail; just needs
+    // to happen when the microtask queue is flushed.
+    await null;
+    await null;
+    await null;
+
+    expect(console.error.calls.count()).toBe(0);
+
+    // Finish loading the data
+    await act(async () => {
+      resolvePromise();
+    });
+    expect(root).toMatchRenderedOutput('Async');
+  });
 });