Improve warning message for setState-on-unmounted (#12347)

This is one of the most common warnings people see, and I don't think the old text is especially clear. Improve it.

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

@@ -244,10 +244,11 @@ describe('ReactCompositeComponent', () => {
     ReactDOM.unmountComponentAtNode(container);
 
     expect(() => instance.forceUpdate()).toWarnDev(
-      'Can only update a mounted or mounting component. This usually means ' +
-        'you called setState, replaceState, or forceUpdate on an unmounted ' +
-        'component. This is a no-op.\n\nPlease check the code for the ' +
-        'Component component.',
+      "Warning: Can't call setState (or forceUpdate) on an unmounted " +
+        'component. This is a no-op, but it indicates a memory leak in your ' +
+        'application. To fix, cancel all subscriptions and asynchronous ' +
+        'tasks in the componentWillUnmount method.\n' +
+        '    in Component (at **)',
     );
 
     // No additional warning should be recorded
@@ -269,26 +270,33 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    let instance = <Component />;
-    expect(instance.setState).not.toBeDefined();
-
-    instance = ReactDOM.render(instance, container);
+    let instance;
+    ReactDOM.render(
+      <div>
+        <span>
+          <Component ref={c => (instance = c || instance)} />
+        </span>
+      </div>,
+      container,
+    );
 
     expect(renders).toBe(1);
 
     instance.setState({value: 1});
 
     expect(renders).toBe(2);
 
-    ReactDOM.unmountComponentAtNode(container);
+    ReactDOM.render(<div />, container);
 
     expect(() => {
       instance.setState({value: 2});
     }).toWarnDev(
-      'Can only update a mounted or mounting component. This usually means ' +
-        'you called setState, replaceState, or forceUpdate on an unmounted ' +
-        'component. This is a no-op.\n\nPlease check the code for the ' +
-        'Component component.',
+      "Warning: Can't call setState (or forceUpdate) on an unmounted " +
+        'component. This is a no-op, but it indicates a memory leak in your ' +
+        'application. To fix, cancel all subscriptions and asynchronous ' +
+        'tasks in the componentWillUnmount method.\n' +
+        '    in Component (at **)\n' +
+        '    in span',
     );
 
     expect(renders).toBe(2);

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -14,6 +14,7 @@ import type {HydrationContext} from './ReactFiberHydrationContext';
 import type {ExpirationTime} from './ReactFiberExpirationTime';
 
 import ReactErrorUtils from 'shared/ReactErrorUtils';
+import {getStackAddendumByWorkInProgressFiber} from 'shared/ReactFiberComponentTreeHook';
 import {ReactCurrentOwner} from 'shared/ReactGlobalSharedState';
 import ReactStrictModeWarnings from './ReactStrictModeWarnings';
 import {
@@ -112,17 +113,19 @@ if (__DEV__) {
   const didWarnStateUpdateForUnmountedComponent = {};
 
   warnAboutUpdateOnUnmounted = function(fiber: Fiber) {
+    // We show the whole stack but dedupe on the top component's name because
+    // the problematic code almost always lies inside that component.
     const componentName = getComponentName(fiber) || 'ReactClass';
     if (didWarnStateUpdateForUnmountedComponent[componentName]) {
       return;
     }
     warning(
       false,
-      'Can only update a mounted or mounting ' +
-        'component. This usually means you called setState, replaceState, ' +
-        'or forceUpdate on an unmounted component. This is a no-op.\n\nPlease ' +
-        'check the code for the %s component.',
-      componentName,
+      "Can't call setState (or forceUpdate) on an unmounted component. This " +
+        'is a no-op, but it indicates a memory leak in your application. To ' +
+        'fix, cancel all subscriptions and asynchronous tasks in the ' +
+        'componentWillUnmount method.%s',
+      getStackAddendumByWorkInProgressFiber(fiber),
     );
     didWarnStateUpdateForUnmountedComponent[componentName] = true;
   };