Detect subscriptions wrapped in startTransition

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

@@ -385,6 +385,9 @@ export function requestUpdateLane(fiber: Fiber): Lane {
 
   const isTransition = requestCurrentTransition() !== NoTransition;
   if (isTransition) {
+    if (__DEV__ && ReactCurrentBatchConfig._updatedFibers) {
+      ReactCurrentBatchConfig._updatedFibers.add(fiber);
+    }
     // The algorithm for assigning an update to a lane should be stable for all
     // updates at the same priority within the same event. To do this, the
     // inputs to the algorithm must be the same.

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

@@ -385,6 +385,9 @@ export function requestUpdateLane(fiber: Fiber): Lane {
 
   const isTransition = requestCurrentTransition() !== NoTransition;
   if (isTransition) {
+    if (__DEV__ && ReactCurrentBatchConfig._updatedFibers) {
+      ReactCurrentBatchConfig._updatedFibers.add(fiber);
+    }
     // The algorithm for assigning an update to a lane should be stable for all
     // updates at the same priority within the same event. To do this, the
     // inputs to the algorithm must be the same.

packages/react/src/ReactCurrentBatchConfig.js

@@ -7,12 +7,19 @@
  * @flow
  */
 
+import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
+
 /**
  * Keeps track of the current batch's configuration such as how long an update
  * should suspend for if it needs to.
  */
 const ReactCurrentBatchConfig = {
   transition: (0: number),
+  _updatedFibers: (null: Set<Fiber> | null),
 };
 
+if (__DEV__) {
+  ReactCurrentBatchConfig._updatedFibers = new Set();
+}
+
 export default ReactCurrentBatchConfig;

packages/react/src/ReactStartTransition.js

@@ -9,12 +9,32 @@
 
 import ReactCurrentBatchConfig from './ReactCurrentBatchConfig';
 
+/**
+ *  If a certain number of fibers are updated inside startTransition then we suspect
+ *  that a user-space defined subscription is being wrapped by startTransition.
+ *  This is no good, they should instead use built-in APIs for concurrent mode to work correctly.
+ */
+
 export function startTransition(scope: () => void) {
   const prevTransition = ReactCurrentBatchConfig.transition;
   ReactCurrentBatchConfig.transition = 1;
   try {
     scope();
   } finally {
     ReactCurrentBatchConfig.transition = prevTransition;
+    if (__DEV__ && ReactCurrentBatchConfig._updatedFibers) {
+      const updatedFibersCount = ReactCurrentBatchConfig._updatedFibers.size;
+      if (updatedFibersCount > 10) {
+        if (__DEV__) {
+          console.warn(
+            'Detected a suspicious number of fibers being updated (10) inside startTransition. ' +
+              'If this is due to a user-space defined subscription please re-write ' +
+              'it to use React provided hooks. Otherwise concurrent mode guarantees ' +
+              'are off the table.',
+          );
+        }
+      }
+      ReactCurrentBatchConfig._updatedFibers.clear();
+    }
   }
 }

packages/react/src/__tests__/ReactStartTransition-test.js

@@ -0,0 +1,66 @@
+/**
+ * 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
+ */
+
+'use strict';
+
+let React;
+let ReactTestRenderer;
+let act;
+let useState;
+
+const SUSPICIOUS_NUMBER_OF_FIBERS_UPDATED = 10;
+
+describe('ReactStartTransition', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    ReactTestRenderer = require('react-test-renderer');
+    act = require('jest-react').act;
+    useState = React.useState;
+  });
+
+  it('Warns if a suspicious number of fibers are updated inside startTransition', () => {
+    const subs = new Set();
+    const useUserSpaceSubscription = () => {
+      const [value, setState] = useState(0);
+      subs.add(setState);
+      return value;
+    };
+    const Component = ({level}) => {
+      useUserSpaceSubscription();
+      if (level < SUSPICIOUS_NUMBER_OF_FIBERS_UPDATED) {
+        return <Component level={level + 1} />;
+      }
+      return null;
+    };
+    act(() => {
+      ReactTestRenderer.create(<Component level={0} />, {
+        unstable_isConcurrent: true,
+      });
+    });
+    expect(() => {
+      act(() => {
+        React.startTransition(() => {
+          subs.forEach(setState => {
+            setState(state => state + 1);
+          });
+        });
+      });
+    }).toWarnDev(
+      [
+        'Detected a suspicious number of fibers being updated ' +
+          `(${SUSPICIOUS_NUMBER_OF_FIBERS_UPDATED}) inside startTransition. ` +
+          'If this is due to a user-space defined subscription please re-write ' +
+          'it to use React provided hooks. Otherwise concurrent mode guarantees ' +
+          'are off the table.',
+      ],
+      {withoutStack: true},
+    );
+  });
+});