Add support for React.pure in ReactDOMServer (facebook#13855)

* Add support for React.pure in ReactDOMServer

* Unwrap pure wrappers by creating an additional element as a single child

This is very slow but meh. We're rewriting this whole thing anyway.

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

@@ -14,13 +14,30 @@ const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegratio
 let React;
 let ReactDOM;
 let ReactDOMServer;
+let forwardRef;
+let pure;
+let yieldedValues;
+let yieldValue;
+let clearYields;
 
 function initModules() {
   // Reset warning cache.
   jest.resetModuleRegistry();
   React = require('react');
   ReactDOM = require('react-dom');
   ReactDOMServer = require('react-dom/server');
+  forwardRef = React.forwardRef;
+  pure = React.pure;
+
+  yieldedValues = [];
+  yieldValue = value => {
+    yieldedValues.push(value);
+  };
+  clearYields = () => {
+    const ret = yieldedValues;
+    yieldedValues = [];
+    return ret;
+  };
 
   // Make them available to the helpers.
   return {
@@ -40,7 +57,7 @@ describe('ReactDOMServerIntegration', () => {
     const FunctionComponent = ({label, forwardedRef}) => (
       <div ref={forwardedRef}>{label}</div>
     );
-    const WrappedFunctionComponent = React.forwardRef((props, ref) => (
+    const WrappedFunctionComponent = forwardRef((props, ref) => (
       <FunctionComponent {...props} forwardedRef={ref} />
     ));
 
@@ -65,4 +82,57 @@ describe('ReactDOMServerIntegration', () => {
     expect(div.tagName).toBe('DIV');
     expect(div.textContent).toBe('Test');
   });
+
+  describe('pure functional components', () => {
+    beforeEach(() => {
+      resetModules();
+    });
+
+    function Text({text}) {
+      yieldValue(text);
+      return <span>{text}</span>;
+    }
+
+    function Counter({count}) {
+      return <Text text={'Count: ' + count} />;
+    }
+
+    itRenders('basic render', async render => {
+      const PureCounter = pure(Counter);
+      const domNode = await render(<PureCounter count={0} />);
+      expect(domNode.textContent).toEqual('Count: 0');
+    });
+
+    itRenders('composition with forwardRef', async render => {
+      const RefCounter = (props, ref) => <Counter count={ref.current} />;
+      const PureRefCounter = pure(forwardRef(RefCounter));
+
+      const ref = React.createRef();
+      ref.current = 0;
+      await render(<PureRefCounter ref={ref} />);
+
+      expect(clearYields()).toEqual(['Count: 0']);
+    });
+
+    itRenders('with comparator', async render => {
+      const PureCounter = pure(Counter, (oldProps, newProps) => false);
+      await render(<PureCounter count={0} />);
+      expect(clearYields()).toEqual(['Count: 0']);
+    });
+
+    itRenders(
+      'comparator functions are not invoked on the server',
+      async render => {
+        const PureCounter = React.pure(Counter, (oldProps, newProps) => {
+          yieldValue(
+            `Old count: ${oldProps.count}, New count: ${newProps.count}`,
+          );
+          return oldProps.count === newProps.count;
+        });
+
+        await render(<PureCounter count={0} />);
+        expect(clearYields()).toEqual(['Count: 0']);
+      },
+    );
+  });
 });

packages/react-dom/src/server/ReactPartialRenderer.js

@@ -39,6 +39,7 @@ import {
   REACT_PROVIDER_TYPE,
   REACT_CONTEXT_TYPE,
   REACT_LAZY_TYPE,
+  REACT_PURE_TYPE,
 } from 'shared/ReactSymbols';
 
 import {
@@ -1001,6 +1002,28 @@ class ReactDOMServerRenderer {
             this.stack.push(frame);
             return '';
           }
+          case REACT_PURE_TYPE: {
+            const element: ReactElement = ((nextChild: any): ReactElement);
+            let nextChildren = [
+              React.createElement(
+                elementType.type,
+                Object.assign({ref: element.ref}, element.props),
+              ),
+            ];
+            const frame: Frame = {
+              type: null,
+              domNamespace: parentNamespace,
+              children: nextChildren,
+              childIndex: 0,
+              context: context,
+              footer: '',
+            };
+            if (__DEV__) {
+              ((frame: any): FrameDev).debugElementStack = [];
+            }
+            this.stack.push(frame);
+            return '';
+          }
           case REACT_PROVIDER_TYPE: {
             const provider: ReactProvider<any> = (nextChild: any);
             const nextProps = provider.props;