[Fizz] Fragments and Iterable support

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

@@ -351,10 +351,12 @@ describe('ReactDOMFizzServer', () => {
     await act(async () => {
       const {startWriting} = ReactDOMFizzServer.pipeToNodeWritable(
         <Suspense fallback={<Text text="Loading A..." />}>
-          <Text text="This will show A: " />
-          <div>
-            <AsyncText text="A" />
-          </div>
+          <>
+            <Text text="This will show A: " />
+            <div>
+              <AsyncText text="A" />
+            </div>
+          </>
         </Suspense>,
         writableA,
         {
@@ -432,11 +434,11 @@ describe('ReactDOMFizzServer', () => {
     }
 
     function AsyncPath({id}) {
-      return <path id={readText(id)}>{[]}</path>;
+      return <path id={readText(id)} />;
     }
 
     function AsyncMi({id}) {
-      return <mi id={readText(id)}>{[]}</mi>;
+      return <mi id={readText(id)} />;
     }
 
     function App() {
@@ -601,7 +603,7 @@ describe('ReactDOMFizzServer', () => {
   // @gate experimental
   it('can stream into an SVG container', async () => {
     function AsyncPath({id}) {
-      return <path id={readText(id)}>{[]}</path>;
+      return <path id={readText(id)} />;
     }
 
     function App() {

packages/react-server/src/ReactFizzServer.js

@@ -62,6 +62,12 @@ import {
   REACT_PORTAL_TYPE,
   REACT_LAZY_TYPE,
   REACT_SUSPENSE_TYPE,
+  REACT_LEGACY_HIDDEN_TYPE,
+  REACT_DEBUG_TRACING_MODE_TYPE,
+  REACT_STRICT_MODE_TYPE,
+  REACT_PROFILER_TYPE,
+  REACT_SUSPENSE_LIST_TYPE,
+  REACT_FRAGMENT_TYPE,
 } from 'shared/ReactSymbols';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import {
@@ -521,6 +527,8 @@ const didWarnAboutContextTypeOnFunctionComponent = {};
 const didWarnAboutGetDerivedStateOnFunctionComponent = {};
 let didWarnAboutReassigningProps = false;
 const didWarnAboutDefaultPropsOnFunctionComponent = {};
+let didWarnAboutGenerators = false;
+let didWarnAboutMaps = false;
 
 // This would typically be a function component but we still support module pattern
 // components for some reason.
@@ -701,10 +709,67 @@ function renderElement(
     }
   } else if (typeof type === 'string') {
     renderHostElement(request, task, type, props);
-  } else if (type === REACT_SUSPENSE_TYPE) {
-    renderSuspenseBoundary(request, task, props);
   } else {
-    throw new Error('Not yet implemented element type.');
+    switch (type) {
+      // TODO: LegacyHidden acts the same as a fragment. This only works
+      // because we currently assume that every instance of LegacyHidden is
+      // accompanied by a host component wrapper. In the hidden mode, the host
+      // component is given a `hidden` attribute, which ensures that the
+      // initial HTML is not visible. To support the use of LegacyHidden as a
+      // true fragment, without an extra DOM node, we would have to hide the
+      // initial HTML in some other way.
+      // TODO: Add REACT_OFFSCREEN_TYPE here too with the same capability.
+      case REACT_LEGACY_HIDDEN_TYPE:
+      case REACT_DEBUG_TRACING_MODE_TYPE:
+      case REACT_STRICT_MODE_TYPE:
+      case REACT_PROFILER_TYPE:
+      case REACT_SUSPENSE_LIST_TYPE: // TODO: SuspenseList should control the boundaries.
+      case REACT_FRAGMENT_TYPE: {
+        renderNodeDestructive(request, task, props.children);
+        break;
+      }
+      case REACT_SUSPENSE_TYPE: {
+        renderSuspenseBoundary(request, task, props);
+        break;
+      }
+      default: {
+        throw new Error('Not yet implemented element type.');
+      }
+    }
+  }
+}
+
+function validateIterable(iterable, iteratorFn: Function): void {
+  if (__DEV__) {
+    // We don't support rendering Generators because it's a mutation.
+    // See https://github.com/facebook/react/issues/12995
+    if (
+      typeof Symbol === 'function' &&
+      // $FlowFixMe Flow doesn't know about toStringTag
+      iterable[Symbol.toStringTag] === 'Generator'
+    ) {
+      if (!didWarnAboutGenerators) {
+        console.error(
+          'Using Generators as children is unsupported and will likely yield ' +
+            'unexpected results because enumerating a generator mutates it. ' +
+            'You may convert it to an array with `Array.from()` or the ' +
+            '`[...spread]` operator before rendering. Keep in mind ' +
+            'you might need to polyfill these features for older browsers.',
+        );
+      }
+      didWarnAboutGenerators = true;
+    }
+
+    // Warn about using Maps as children
+    if ((iterable: any).entries === iteratorFn) {
+      if (!didWarnAboutMaps) {
+        console.error(
+          'Using Maps as children is not supported. ' +
+            'Use an array of keyed ReactElements instead.',
+        );
+      }
+      didWarnAboutMaps = true;
+    }
   }
 }
 
@@ -756,7 +821,30 @@ function renderNodeDestructive(
 
     const iteratorFn = getIteratorFn(node);
     if (iteratorFn) {
-      throw new Error('Not yet implemented node type.');
+      if (__DEV__) {
+        validateIterable(node, iteratorFn());
+      }
+      const iterator = iteratorFn.call(node);
+      if (iterator) {
+        let step = iterator.next();
+        // If there are not entries, we need to push an empty so we start by checking that.
+        if (!step.done) {
+          do {
+            // Recursively render the rest. We need to use the non-destructive form
+            // so that we can safely pop back up and render the sibling if something
+            // suspends.
+            renderNode(request, task, step.value);
+            step = iterator.next();
+          } while (!step.done);
+          return;
+        }
+      }
+      pushEmpty(
+        task.blockedSegment.chunks,
+        request.responseState,
+        task.assignID,
+      );
+      task.assignID = null;
     }
 
     const childString = Object.prototype.toString.call(node);
@@ -805,6 +893,7 @@ function renderNodeDestructive(
 
   // Any other type is assumed to be empty.
   pushEmpty(task.blockedSegment.chunks, request.responseState, task.assignID);
+  task.assignID = null;
 }
 
 function spawnNewSuspendedTask(