Switch <Context> to mean <Context.Provider> (facebook#28226)

Previously, `<Context>` was equivalent to `<Context.Consumer>`. However,
since the introduction of Hooks, the `<Context.Consumer>` API is rarely
used. The goal here is to make the common case cleaner:

```js
const ThemeContext = createContext('light')

function App() {
  return (
    <ThemeContext value="dark">
      ...
    </ThemeContext>
  )
}

function Button() {
  const theme = use(ThemeContext)
  // ...
}
```

This is technically a breaking change, but we've been warning about
rendering `<Context>` directly for several years by now, so it's
unlikely much code in the wild depends on the old behavior. [Proof that
it warns today (check
console).](https://codesandbox.io/p/sandbox/peaceful-nobel-pdxtfl)

---

**The relevant commit is 5696782.** It
switches `createContext` implementation so that `Context.Provider ===
Context`.

The main assumption that changed is that a Provider's fiber type is now
the context itself (rather than an intermediate object). Whereas a
Consumer's fiber type is now always an intermediate object (rather than
it being sometimes the context itself and sometimes an intermediate
object).

My methodology was to start with the relevant symbols, work tags, and
types, and work my way backwards to all usages.

This might break tooling that depends on inspecting React's internal
fields. I've added DevTools support in the second commit. This didn't
need explicit versioning—the structure tells us enough.

packages/react-client/src/ReactFlightReplyClient.js

@@ -14,10 +14,12 @@ import type {
   RejectedThenable,
   ReactCustomFormAction,
 } from 'shared/ReactTypes';
+import {enableRenderableContext} from 'shared/ReactFeatureFlags';
 
 import {
   REACT_ELEMENT_TYPE,
   REACT_LAZY_TYPE,
+  REACT_CONTEXT_TYPE,
   REACT_PROVIDER_TYPE,
   getIteratorFn,
 } from 'shared/ReactSymbols';
@@ -302,7 +304,10 @@ export function processReply(
             'React Lazy cannot be passed to Server Functions from the Client.%s',
             describeObjectForErrorMessage(parent, key),
           );
-        } else if ((value: any).$$typeof === REACT_PROVIDER_TYPE) {
+        } else if (
+          (value: any).$$typeof ===
+          (enableRenderableContext ? REACT_CONTEXT_TYPE : REACT_PROVIDER_TYPE)
+        ) {
           console.error(
             'React Context Providers cannot be passed to Server Functions from the Client.%s',
             describeObjectForErrorMessage(parent, key),

packages/react-debug-tools/src/ReactDebugHooks.js

@@ -10,7 +10,6 @@
 import type {
   Awaited,
   ReactContext,
-  ReactProviderType,
   StartTransitionOptions,
   Usable,
   Thenable,
@@ -931,8 +930,11 @@ function setupContexts(contextMap: Map<ReactContext<any>, any>, fiber: Fiber) {
   let current: null | Fiber = fiber;
   while (current) {
     if (current.tag === ContextProvider) {
-      const providerType: ReactProviderType<any> = current.type;
-      const context: ReactContext<any> = providerType._context;
+      let context: ReactContext<any> = current.type;
+      if ((context: any)._context !== undefined) {
+        // Support inspection of pre-19+ providers.
+        context = (context: any)._context;
+      }
       if (!contextMap.has(context)) {
         // Store the current value that we're going to restore later.
         contextMap.set(context, context._currentValue);

packages/react-devtools-shared/src/backend/ReactSymbols.js

@@ -51,6 +51,8 @@ export const PROFILER_SYMBOL_STRING = 'Symbol(react.profiler)';
 export const PROVIDER_NUMBER = 0xeacd;
 export const PROVIDER_SYMBOL_STRING = 'Symbol(react.provider)';
 
+export const CONSUMER_SYMBOL_STRING = 'Symbol(react.consumer)';
+
 export const SCOPE_NUMBER = 0xead7;
 export const SCOPE_SYMBOL_STRING = 'Symbol(react.scope)';
 

packages/react-devtools-shared/src/backend/renderer.js

@@ -79,6 +79,7 @@ import {
   PROVIDER_SYMBOL_STRING,
   CONTEXT_NUMBER,
   CONTEXT_SYMBOL_STRING,
+  CONSUMER_SYMBOL_STRING,
   STRICT_MODE_NUMBER,
   STRICT_MODE_SYMBOL_STRING,
   PROFILER_NUMBER,
@@ -525,6 +526,15 @@ export function getInternalReactConstants(version: string): {
           case CONTEXT_NUMBER:
           case CONTEXT_SYMBOL_STRING:
           case SERVER_CONTEXT_SYMBOL_STRING:
+            if (
+              fiber.type._context === undefined &&
+              fiber.type.Provider === fiber.type
+            ) {
+              // In 19+, Context.Provider === Context, so this is a provider.
+              resolvedContext = fiber.type;
+              return `${resolvedContext.displayName || 'Context'}.Provider`;
+            }
+
             // 16.3-16.5 read from "type" because the Consumer is the actual context object.
             // 16.6+ should read from "type._context" because Consumer can be different (in DEV).
             // NOTE Keep in sync with inspectElementRaw()
@@ -533,6 +543,10 @@ export function getInternalReactConstants(version: string): {
             // NOTE: TraceUpdatesBackendManager depends on the name ending in '.Consumer'
             // If you change the name, figure out a more resilient way to detect it.
             return `${resolvedContext.displayName || 'Context'}.Consumer`;
+          case CONSUMER_SYMBOL_STRING:
+            // 19+
+            resolvedContext = fiber.type._context;
+            return `${resolvedContext.displayName || 'Context'}.Consumer`;
           case STRICT_MODE_NUMBER:
           case STRICT_MODE_SYMBOL_STRING:
             return null;
@@ -3178,8 +3192,14 @@ export function attach(
         }
       }
     } else if (
-      typeSymbol === CONTEXT_NUMBER ||
-      typeSymbol === CONTEXT_SYMBOL_STRING
+      // Detect pre-19 Context Consumers
+      (typeSymbol === CONTEXT_NUMBER || typeSymbol === CONTEXT_SYMBOL_STRING) &&
+      !(
+        // In 19+, CONTEXT_SYMBOL_STRING means a Provider instead.
+        // It will be handled in a different branch below.
+        // Eventually, this entire branch can be removed.
+        (type._context === undefined && type.Provider === type)
+      )
     ) {
       // 16.3-16.5 read from "type" because the Consumer is the actual context object.
       // 16.6+ should read from "type._context" because Consumer can be different (in DEV).
@@ -3209,6 +3229,35 @@ export function attach(
           }
         }
 
+        current = current.return;
+      }
+    } else if (
+      // Detect 19+ Context Consumers
+      typeSymbol === CONSUMER_SYMBOL_STRING
+    ) {
+      // This branch is 19+ only, where Context.Provider === Context.
+      // NOTE Keep in sync with getDisplayNameForFiber()
+      const consumerResolvedContext = type._context;
+
+      // Global context value.
+      context = consumerResolvedContext._currentValue || null;
+
+      // Look for overridden value.
+      let current = ((fiber: any): Fiber).return;
+      while (current !== null) {
+        const currentType = current.type;
+        const currentTypeSymbol = getTypeSymbol(currentType);
+        if (
+          // In 19+, these are Context Providers
+          currentTypeSymbol === CONTEXT_SYMBOL_STRING
+        ) {
+          const providerResolvedContext = currentType;
+          if (providerResolvedContext === consumerResolvedContext) {
+            context = current.memoizedProps.value;
+            break;
+          }
+        }
+
         current = current.return;
       }
     }

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

@@ -31,8 +31,7 @@ function initModules() {
   };
 }
 
-const {resetModules, itRenders, clientRenderOnBadMarkup} =
-  ReactDOMServerIntegrationUtils(initModules);
+const {resetModules, itRenders} = ReactDOMServerIntegrationUtils(initModules);
 
 describe('ReactDOMServerIntegration', () => {
   beforeEach(() => {
@@ -296,115 +295,35 @@ describe('ReactDOMServerIntegration', () => {
       expect(e.querySelector('#language3').textContent).toBe('french');
     });
 
-    itRenders(
-      'should warn with an error message when using Context as consumer in DEV',
-      async render => {
-        const Theme = React.createContext('dark');
-        const Language = React.createContext('french');
+    itRenders('should treat Context as Context.Provider', async render => {
+      // The `itRenders` helpers don't work with the gate pragma, so we have to do
+      // this instead.
+      if (gate(flags => !flags.enableRenderableContext)) {
+        return;
+      }
 
-        const App = () => (
-          <div>
-            <Theme.Provider value="light">
-              <Language.Provider value="english">
-                <Theme.Provider value="dark">
-                  <Theme>{theme => <div id="theme1">{theme}</div>}</Theme>
-                </Theme.Provider>
-              </Language.Provider>
-            </Theme.Provider>
-          </div>
-        );
-        // We expect 1 error.
-        await render(<App />, 1);
-      },
-    );
-
-    // False positive regression test.
-    itRenders(
-      'should not warn when using Consumer from React < 16.6 with newer renderer',
-      async render => {
-        const Theme = React.createContext('dark');
-        const Language = React.createContext('french');
-        // React 16.5 and earlier didn't have a separate object.
-        Theme.Consumer = Theme;
-
-        const App = () => (
-          <div>
-            <Theme.Provider value="light">
-              <Language.Provider value="english">
-                <Theme.Provider value="dark">
-                  <Theme>{theme => <div id="theme1">{theme}</div>}</Theme>
-                </Theme.Provider>
-              </Language.Provider>
-            </Theme.Provider>
-          </div>
-        );
-        // We expect 0 errors.
-        await render(<App />, 0);
-      },
-    );
-
-    itRenders(
-      'should warn with an error message when using nested context consumers in DEV',
-      async render => {
-        const App = () => {
-          const Theme = React.createContext('dark');
-          const Language = React.createContext('french');
+      const Theme = React.createContext('dark');
+      const Language = React.createContext('french');
 
-          return (
-            <div>
-              <Theme.Provider value="light">
-                <Language.Provider value="english">
-                  <Theme.Provider value="dark">
-                    <Theme.Consumer.Consumer>
-                      {theme => <div id="theme1">{theme}</div>}
-                    </Theme.Consumer.Consumer>
-                  </Theme.Provider>
-                </Language.Provider>
-              </Theme.Provider>
-            </div>
-          );
-        };
-        await render(
-          <App />,
-          render === clientRenderOnBadMarkup
-            ? // On hydration mismatch we retry and therefore log the warning again.
-              2
-            : 1,
-        );
-      },
-    );
+      expect(Theme.Provider).toBe(Theme);
 
-    itRenders(
-      'should warn with an error message when using Context.Consumer.Provider DEV',
-      async render => {
-        const App = () => {
-          const Theme = React.createContext('dark');
-          const Language = React.createContext('french');
+      const App = () => (
+        <div>
+          <Theme value="light">
+            <Language value="english">
+              <Theme value="dark">
+                <Theme.Consumer>
+                  {theme => <div id="theme1">{theme}</div>}
+                </Theme.Consumer>
+              </Theme>
+            </Language>
+          </Theme>
+        </div>
+      );
 
-          return (
-            <div>
-              <Theme.Provider value="light">
-                <Language.Provider value="english">
-                  <Theme.Consumer.Provider value="dark">
-                    <Theme.Consumer>
-                      {theme => <div id="theme1">{theme}</div>}
-                    </Theme.Consumer>
-                  </Theme.Consumer.Provider>
-                </Language.Provider>
-              </Theme.Provider>
-            </div>
-          );
-        };
-
-        await render(
-          <App />,
-          render === clientRenderOnBadMarkup
-            ? // On hydration mismatch we retry and therefore log the warning again.
-              2
-            : 1,
-        );
-      },
-    );
+      const e = await render(<App />, 0);
+      expect(e.textContent).toBe('dark');
+    });
 
     it('does not pollute parallel node streams', () => {
       const LoggedInUser = React.createContext();

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

@@ -1000,22 +1000,15 @@ describe('ReactDOMServer', () => {
     ]);
   });
 
+  // @gate enableRenderableContext || !__DEV__
   it('should warn if an invalid contextType is defined', () => {
     const Context = React.createContext();
-
     class ComponentA extends React.Component {
-      // It should warn for both Context.Consumer and Context.Provider
       static contextType = Context.Consumer;
       render() {
         return <div />;
       }
     }
-    class ComponentB extends React.Component {
-      static contextType = Context.Provider;
-      render() {
-        return <div />;
-      }
-    }
 
     expect(() => {
       ReactDOMServer.renderToString(<ComponentA />);
@@ -1028,13 +1021,14 @@ describe('ReactDOMServer', () => {
     // Warnings should be deduped by component type
     ReactDOMServer.renderToString(<ComponentA />);
 
-    expect(() => {
-      ReactDOMServer.renderToString(<ComponentB />);
-    }).toErrorDev(
-      'Warning: ComponentB defines an invalid contextType. ' +
-        'contextType should point to the Context object returned by React.createContext(). ' +
-        'Did you accidentally pass the Context.Provider instead?',
-    );
+    class ComponentB extends React.Component {
+      static contextType = Context.Provider;
+      render() {
+        return <div />;
+      }
+    }
+    // Does not warn because Context === Context.Provider.
+    ReactDOMServer.renderToString(<ComponentB />);
   });
 
   it('should not warn when class contextType is null', () => {