Feature flag to disable legacy context for function components (#30319)

While the goal is to remove legacy context completely, I think we can
already land the removal of legacy context for function components. I
didn't even know this feature existed until reading the code recently.

The win is just a couple of property lookups on function renders, but it
trims down the API already as the full removal will likely still take a
bit more time.

www: Starting with enabled test renderer and a feature flag for
production rollout.

RN: Not enabled, will follow up on this.

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

@@ -86,6 +86,9 @@ describe('ReactDOMServerIntegration', () => {
     });
 
     itRenders('stateless child with context', async render => {
+      if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+        return;
+      }
       function FunctionChildWithContext(props, context) {
         return <div>{context.text}</div>;
       }
@@ -118,6 +121,9 @@ describe('ReactDOMServerIntegration', () => {
     });
 
     itRenders('stateless child without context', async render => {
+      if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+        return;
+      }
       function FunctionChildWithoutContext(props, context) {
         // this should render blank; context isn't passed to this component.
         return <div>{context.text}</div>;
@@ -151,6 +157,9 @@ describe('ReactDOMServerIntegration', () => {
     });
 
     itRenders('stateless child with wrong context', async render => {
+      if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+        return;
+      }
       function FunctionChildWithWrongContext(props, context) {
         // this should render blank; context.text isn't passed to this component.
         return <div id="statelessWrongChild">{context.text}</div>;
@@ -169,6 +178,9 @@ describe('ReactDOMServerIntegration', () => {
     });
 
     itRenders('with context passed through to a grandchild', async render => {
+      if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+        return;
+      }
       function Grandchild(props, context) {
         return <div>{context.text}</div>;
       }
@@ -186,6 +198,9 @@ describe('ReactDOMServerIntegration', () => {
     });
 
     itRenders('a child context overriding a parent context', async render => {
+      if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+        return;
+      }
       const Grandchild = (props, context) => {
         return <div>{context.text}</div>;
       };
@@ -203,6 +218,9 @@ describe('ReactDOMServerIntegration', () => {
     });
 
     itRenders('a child context merged with a parent context', async render => {
+      if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+        return;
+      }
       class Parent extends React.Component {
         getChildContext() {
           return {text1: 'purple'};
@@ -244,6 +262,9 @@ describe('ReactDOMServerIntegration', () => {
     itRenders(
       'with a call to componentWillMount before getChildContext',
       async render => {
+        if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+          return;
+        }
         class WillMountContext extends React.Component {
           getChildContext() {
             return {text: this.state.text};
@@ -270,6 +291,9 @@ describe('ReactDOMServerIntegration', () => {
     itRenders(
       'if getChildContext exists but childContextTypes is missing with a warning',
       async render => {
+        if (gate(flags => flags.disableLegacyContextForFunctionComponents)) {
+          return;
+        }
         function HopefulChild(props, context) {
           return context.foo || 'nope';
         }

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

@@ -451,7 +451,7 @@ describe('ReactFunctionComponent', () => {
     ]);
   });
 
-  // @gate !disableLegacyContext
+  // @gate !disableLegacyContext && !disableLegacyContextForFunctionComponents
   it('should receive context', async () => {
     class Parent extends React.Component {
       static childContextTypes = {

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -95,6 +95,7 @@ import {
 import {
   debugRenderPhaseSideEffectsForStrictMode,
   disableLegacyContext,
+  disableLegacyContextForFunctionComponents,
   enableProfilerCommitHooks,
   enableProfilerTimer,
   enableScopeAPI,
@@ -1158,7 +1159,7 @@ function updateFunctionComponent(
   }
 
   let context;
-  if (!disableLegacyContext) {
+  if (!disableLegacyContext && !disableLegacyContextForFunctionComponents) {
     const unmaskedContext = getUnmaskedContext(workInProgress, Component, true);
     context = getMaskedContext(workInProgress, unmaskedContext);
   }

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

@@ -1701,7 +1701,7 @@ describe('ReactIncremental', () => {
     expect(instance.state.n).toEqual(3);
   });
 
-  // @gate !disableLegacyContext
+  // @gate !disableLegacyContext && !disableLegacyContextForFunctionComponents
   it('merges and masks context', async () => {
     class Intl extends React.Component {
       static childContextTypes = {
@@ -1954,7 +1954,7 @@ describe('ReactIncremental', () => {
     ]);
   });
 
-  // @gate !disableLegacyContext
+  // @gate !disableLegacyContext && !disableLegacyContextForFunctionComponents
   it('reads context when setState is below the provider', async () => {
     let statefulInst;
 
@@ -2046,7 +2046,7 @@ describe('ReactIncremental', () => {
     assertLog([]);
   });
 
-  // @gate !disableLegacyContext
+  // @gate !disableLegacyContext && !disableLegacyContextForFunctionComponents
   it('reads context when setState is above the provider', async () => {
     let statefulInst;
 

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

@@ -1158,7 +1158,7 @@ describe('ReactIncrementalErrorHandling', () => {
   // because it's used for new context, suspense, and many other features.
   // It has to be tested independently for each feature anyway. So although it
   // doesn't look like it, this test is specific to legacy context.
-  // @gate !disableLegacyContext
+  // @gate !disableLegacyContext && !disableLegacyContextForFunctionComponents
   it('unwinds the context stack correctly on error', async () => {
     class Provider extends React.Component {
       static childContextTypes = {message: PropTypes.string};

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

@@ -1562,7 +1562,7 @@ describe('ReactUse', () => {
     expect(root).toMatchRenderedOutput('Async!');
   });
 
-  // @gate !disableLegacyContext
+  // @gate !disableLegacyContext && !disableLegacyContextForFunctionComponents
   it('unwrap uncached promises in component that accesses legacy context', async () => {
     class ContextProvider extends React.Component {
       static childContextTypes = {

packages/react-server/src/ReactFizzServer.js

@@ -152,6 +152,7 @@ import {
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import {
   disableLegacyContext,
+  disableLegacyContextForFunctionComponents,
   enableScopeAPI,
   enableSuspenseAvoidThisFallbackFizz,
   enableCache,
@@ -1654,7 +1655,7 @@ function renderFunctionComponent(
   props: any,
 ): void {
   let legacyContext;
-  if (!disableLegacyContext) {
+  if (!disableLegacyContext && !disableLegacyContextForFunctionComponents) {
     legacyContext = getMaskedContext(Component, task.legacyContext);
   }
   if (__DEV__) {

packages/shared/ReactFeatureFlags.js

@@ -155,8 +155,14 @@ export const transitionLaneExpirationMs = 5000;
 // Renames the internal symbol for elements since they have changed signature/constructor
 export const renameElementSymbol = true;
 
-// Removes legacy style context
+/**
+ * Removes legacy style context defined using static `contextTypes` and consumed with static `childContextTypes`.
+ */
 export const disableLegacyContext = true;
+/**
+ * Removes legacy style context just from function components.
+ */
+export const disableLegacyContextForFunctionComponents = true;
 
 // Not ready to break experimental yet.
 // Modern <StrictMode /> behaviour aligns more with what components

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -37,6 +37,7 @@ export const disableCommentsAsDOMContainers = true;
 export const disableIEWorkarounds = true;
 export const disableInputAttributeSyncing = false;
 export const disableLegacyContext = false;
+export const disableLegacyContextForFunctionComponents = false;
 export const disableLegacyMode = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const disableStringRefs = true;

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -28,6 +28,7 @@ export const disableDefaultPropsExceptForClasses = true;
 export const disableIEWorkarounds = true;
 export const disableInputAttributeSyncing = false;
 export const disableLegacyContext = true;
+export const disableLegacyContextForFunctionComponents = true;
 export const disableLegacyMode = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const disableStringRefs = true;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -92,6 +92,7 @@ export const disableStringRefs = true;
 export const enableFastJSX = true;
 export const disableLegacyMode = true;
 export const disableLegacyContext = true;
+export const disableLegacyContextForFunctionComponents = true;
 export const enableRenderableContext = true;
 export const enableReactTestRendererWarning = true;
 export const disableDefaultPropsExceptForClasses = true;

packages/shared/forks/ReactFeatureFlags.test-renderer.native-fb.js

@@ -20,6 +20,7 @@ export const disableDefaultPropsExceptForClasses = false;
 export const disableIEWorkarounds = true;
 export const disableInputAttributeSyncing = false;
 export const disableLegacyContext = false;
+export const disableLegacyContextForFunctionComponents = false;
 export const disableLegacyMode = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const disableStringRefs = true;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -32,6 +32,7 @@ export const enableScopeAPI = true;
 export const enableCreateEventHandleAPI = false;
 export const enableSuspenseCallback = true;
 export const disableLegacyContext = false;
+export const disableLegacyContextForFunctionComponents = false;
 export const enableTrustedTypesIntegration = false;
 export const disableTextareaChildren = false;
 export const enableSuspenseAvoidThisFallback = true;

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -15,6 +15,7 @@
 
 export const alwaysThrottleRetries = true;
 export const disableDefaultPropsExceptForClasses = __VARIANT__;
+export const disableLegacyContextForFunctionComponents = __VARIANT__;
 export const disableLegacyMode = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
 export const enableAddPropertiesFastPath = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -17,6 +17,7 @@ const dynamicFeatureFlags: DynamicFeatureFlags = require('ReactFeatureFlags');
 export const {
   alwaysThrottleRetries,
   disableDefaultPropsExceptForClasses,
+  disableLegacyContextForFunctionComponents,
   disableSchedulerTimeoutInWorkLoop,
   enableAddPropertiesFastPath,
   enableDebugTracing,