[Fiber/Fizz] Support AsyncIterable as Children and AsyncGenerator Client Components (#28868)

Stacked on #28849, #28854, #28853. Behind a flag.

If you're following along from the side-lines. This is probably not what
you think it is.

It's NOT a way to get updates to a component over time. The
AsyncIterable works like an Iterable already works in React which is how
an Array works. I.e. it's a list of children - not the value of a child
over time.

It also doesn't actually render one component at a time. The way it
works is more like awaiting the entire list to become an array and then
it shows up. Before that it suspends the parent.

To actually get these to display one at a time, you have to opt-in with
`<SuspenseList>` to describe how they should appear. That's really the
interesting part and that not implemented yet.

Additionally, since these are effectively Async Functions and uncached
promises, they're not actually fully "supported" on the client yet for
the same reason rendering plain Promises and Async Functions aren't.
They warn. It's only really useful when paired with RSC that produces
instrumented versions of these. Ideally we'd published instrumented
helpers to help with map/filter style operations that yield new
instrumented AsyncIterables.

The way the implementation works basically just relies on unwrapThenable
and otherwise works like a plain Iterator.

There is one quirk with these that are different than just promises. We
ask for a new iterator each time we rerender. This means that upon retry
we kick off another iteration which itself might kick off new requests
that block iterating further. To solve this and make it actually
efficient enough to use on the client we'd need to stash something like
a buffer of the previous iteration and maybe iterator on the iterable so
that we can continue where we left off or synchronously iterate if we've
seen it before. Similar to our `.value` convention on Promises.

In Fizz, I had to do a special case because when we render an iterator
child we don't actually rerender the parent again like we do in Fiber.
However, it's more efficient to just continue on where we left off by
reusing the entries from the thenable state from before in that case.

DiffTrain build for [9f2eebd](9f2eebd)

Author: sebmarkbage

compiled/facebook-www/REVISION

@@ -1 +1 @@
-3b551c82844bcfde51f0febb8e42c1a0d777df2c
+9f2eebd807bf53b7d9901cf0b768762948224cae

compiled/facebook-www/ReactART-dev.classic.js

@@ -63,7 +63,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = '19.0.0-www-classic-5033c724';
+var ReactVersion = '19.0.0-www-classic-bd72d7f5';
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;
@@ -169,6 +169,7 @@ var enableProfilerNestedUpdatePhase = true;
 var enableAsyncActions = true;
 
 var enableSchedulingProfiler = dynamicFeatureFlags.enableSchedulingProfiler;
+var enableAsyncIterableChildren = false;
 var disableLegacyMode = false;
 
 var FunctionComponent = 0;
@@ -6597,7 +6598,7 @@ function createChildReconciler(shouldTrackSideEffects) {
           }
       }
 
-      if (isArray(newChild) || getIteratorFn(newChild)) {
+      if (isArray(newChild) || getIteratorFn(newChild) || enableAsyncIterableChildren ) {
         var _created3 = createFiberFromFragment(newChild, returnFiber.mode, lanes, null);
 
         _created3.return = returnFiber;
@@ -6682,7 +6683,7 @@ function createChildReconciler(shouldTrackSideEffects) {
           }
       }
 
-      if (isArray(newChild) || getIteratorFn(newChild)) {
+      if (isArray(newChild) || getIteratorFn(newChild) || enableAsyncIterableChildren ) {
         if (key !== null) {
           return null;
         }
@@ -6750,7 +6751,7 @@ function createChildReconciler(shouldTrackSideEffects) {
           return updateFromMap(existingChildren, returnFiber, newIdx, init(payload), lanes, mergeDebugInfo(debugInfo, newChild._debugInfo));
       }
 
-      if (isArray(newChild) || getIteratorFn(newChild)) {
+      if (isArray(newChild) || getIteratorFn(newChild) || enableAsyncIterableChildren ) {
         var _matchedFiber3 = existingChildren.get(newIdx) || null;
 
         return updateFragment(returnFiber, _matchedFiber3, newChild, lanes, null, mergeDebugInfo(debugInfo, newChild._debugInfo));
@@ -6983,7 +6984,7 @@ function createChildReconciler(shouldTrackSideEffects) {
     return resultingFirstChild;
   }
 
-  function reconcileChildrenIterator(returnFiber, currentFirstChild, newChildrenIterable, lanes, debugInfo) {
+  function reconcileChildrenIteratable(returnFiber, currentFirstChild, newChildrenIterable, lanes, debugInfo) {
     // This is the same implementation as reconcileChildrenArray(),
     // but using the iterator instead.
     var iteratorFn = getIteratorFn(newChildrenIterable);
@@ -7022,6 +7023,10 @@ function createChildReconciler(shouldTrackSideEffects) {
       }
     }
 
+    return reconcileChildrenIterator(returnFiber, currentFirstChild, newChildren, lanes, debugInfo);
+  }
+
+  function reconcileChildrenIterator(returnFiber, currentFirstChild, newChildren, lanes, debugInfo) {
     if (newChildren == null) {
       throw new Error('An iterable object provided no iterator.');
     }
@@ -7327,8 +7332,8 @@ function createChildReconciler(shouldTrackSideEffects) {
       }
 
       if (getIteratorFn(newChild)) {
-        return reconcileChildrenIterator(returnFiber, currentFirstChild, newChild, lanes, mergeDebugInfo(debugInfo, newChild._debugInfo));
-      } // Usables are a valid React node type. When React encounters a Usable in
+        return reconcileChildrenIteratable(returnFiber, currentFirstChild, newChild, lanes, mergeDebugInfo(debugInfo, newChild._debugInfo));
+      }
       // a child position, it unwraps it using the same algorithm as `use`. For
       // example, for promises, React will throw an exception to unwind the
       // stack, then replay the component once the promise resolves.
@@ -7836,7 +7841,8 @@ function warnIfAsyncClientComponent(Component) {
     // for transpiled async functions. Neither mechanism is completely
     // bulletproof but together they cover the most common cases.
     var isAsyncFunction = // $FlowIgnore[method-unbinding]
-    Object.prototype.toString.call(Component) === '[object AsyncFunction]';
+    Object.prototype.toString.call(Component) === '[object AsyncFunction]' || // $FlowIgnore[method-unbinding]
+    Object.prototype.toString.call(Component) === '[object AsyncGeneratorFunction]';
 
     if (isAsyncFunction) {
       // Encountered an async Client Component. This is not yet supported.

compiled/facebook-www/ReactART-dev.modern.js

@@ -63,7 +63,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = '19.0.0-www-modern-9c7c2ced';
+var ReactVersion = '19.0.0-www-modern-03c6d7dd';
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;
@@ -169,6 +169,7 @@ var enableProfilerNestedUpdatePhase = true;
 var enableAsyncActions = true;
 
 var enableSchedulingProfiler = dynamicFeatureFlags.enableSchedulingProfiler;
+var enableAsyncIterableChildren = false;
 var disableLegacyMode = true;
 
 var FunctionComponent = 0;
@@ -6386,7 +6387,7 @@ function createChildReconciler(shouldTrackSideEffects) {
           }
       }
 
-      if (isArray(newChild) || getIteratorFn(newChild)) {
+      if (isArray(newChild) || getIteratorFn(newChild) || enableAsyncIterableChildren ) {
         var _created3 = createFiberFromFragment(newChild, returnFiber.mode, lanes, null);
 
         _created3.return = returnFiber;
@@ -6471,7 +6472,7 @@ function createChildReconciler(shouldTrackSideEffects) {
           }
       }
 
-      if (isArray(newChild) || getIteratorFn(newChild)) {
+      if (isArray(newChild) || getIteratorFn(newChild) || enableAsyncIterableChildren ) {
         if (key !== null) {
           return null;
         }
@@ -6539,7 +6540,7 @@ function createChildReconciler(shouldTrackSideEffects) {
           return updateFromMap(existingChildren, returnFiber, newIdx, init(payload), lanes, mergeDebugInfo(debugInfo, newChild._debugInfo));
       }
 
-      if (isArray(newChild) || getIteratorFn(newChild)) {
+      if (isArray(newChild) || getIteratorFn(newChild) || enableAsyncIterableChildren ) {
         var _matchedFiber3 = existingChildren.get(newIdx) || null;
 
         return updateFragment(returnFiber, _matchedFiber3, newChild, lanes, null, mergeDebugInfo(debugInfo, newChild._debugInfo));
@@ -6772,7 +6773,7 @@ function createChildReconciler(shouldTrackSideEffects) {
     return resultingFirstChild;
   }
 
-  function reconcileChildrenIterator(returnFiber, currentFirstChild, newChildrenIterable, lanes, debugInfo) {
+  function reconcileChildrenIteratable(returnFiber, currentFirstChild, newChildrenIterable, lanes, debugInfo) {
     // This is the same implementation as reconcileChildrenArray(),
     // but using the iterator instead.
     var iteratorFn = getIteratorFn(newChildrenIterable);
@@ -6811,6 +6812,10 @@ function createChildReconciler(shouldTrackSideEffects) {
       }
     }
 
+    return reconcileChildrenIterator(returnFiber, currentFirstChild, newChildren, lanes, debugInfo);
+  }
+
+  function reconcileChildrenIterator(returnFiber, currentFirstChild, newChildren, lanes, debugInfo) {
     if (newChildren == null) {
       throw new Error('An iterable object provided no iterator.');
     }
@@ -7116,8 +7121,8 @@ function createChildReconciler(shouldTrackSideEffects) {
       }
 
       if (getIteratorFn(newChild)) {
-        return reconcileChildrenIterator(returnFiber, currentFirstChild, newChild, lanes, mergeDebugInfo(debugInfo, newChild._debugInfo));
-      } // Usables are a valid React node type. When React encounters a Usable in
+        return reconcileChildrenIteratable(returnFiber, currentFirstChild, newChild, lanes, mergeDebugInfo(debugInfo, newChild._debugInfo));
+      }
       // a child position, it unwraps it using the same algorithm as `use`. For
       // example, for promises, React will throw an exception to unwind the
       // stack, then replay the component once the promise resolves.
@@ -7625,7 +7630,8 @@ function warnIfAsyncClientComponent(Component) {
     // for transpiled async functions. Neither mechanism is completely
     // bulletproof but together they cover the most common cases.
     var isAsyncFunction = // $FlowIgnore[method-unbinding]
-    Object.prototype.toString.call(Component) === '[object AsyncFunction]';
+    Object.prototype.toString.call(Component) === '[object AsyncFunction]' || // $FlowIgnore[method-unbinding]
+    Object.prototype.toString.call(Component) === '[object AsyncGeneratorFunction]';
 
     if (isAsyncFunction) {
       // Encountered an async Client Component. This is not yet supported.

compiled/facebook-www/ReactART-prod.classic.js

@@ -2204,22 +2204,19 @@ function createChildReconciler(shouldTrackSideEffects) {
   function reconcileChildrenIterator(
     returnFiber,
     currentFirstChild,
-    newChildrenIterable,
+    newChildren,
     lanes
   ) {
-    var iteratorFn = getIteratorFn(newChildrenIterable);
-    if ("function" !== typeof iteratorFn)
-      throw Error(formatProdErrorMessage(150));
-    newChildrenIterable = iteratorFn.call(newChildrenIterable);
-    if (null == newChildrenIterable) throw Error(formatProdErrorMessage(151));
+    if (null == newChildren) throw Error(formatProdErrorMessage(151));
     for (
-      var previousNewFiber = (iteratorFn = null),
+      var resultingFirstChild = null,
+        previousNewFiber = null,
         oldFiber = currentFirstChild,
         newIdx = (currentFirstChild = 0),
         nextOldFiber = null,
-        step = newChildrenIterable.next();
+        step = newChildren.next();
       null !== oldFiber && !step.done;
-      newIdx++, step = newChildrenIterable.next(), null
+      newIdx++, step = newChildren.next(), null
     ) {
       oldFiber.index > newIdx
         ? ((nextOldFiber = oldFiber), (oldFiber = null))
@@ -2235,28 +2232,30 @@ function createChildReconciler(shouldTrackSideEffects) {
         deleteChild(returnFiber, oldFiber);
       currentFirstChild = placeChild(newFiber, currentFirstChild, newIdx);
       null === previousNewFiber
-        ? (iteratorFn = newFiber)
+        ? (resultingFirstChild = newFiber)
         : (previousNewFiber.sibling = newFiber);
       previousNewFiber = newFiber;
       oldFiber = nextOldFiber;
     }
     if (step.done)
-      return deleteRemainingChildren(returnFiber, oldFiber), iteratorFn;
+      return (
+        deleteRemainingChildren(returnFiber, oldFiber), resultingFirstChild
+      );
     if (null === oldFiber) {
-      for (; !step.done; newIdx++, step = newChildrenIterable.next(), null)
+      for (; !step.done; newIdx++, step = newChildren.next(), null)
         (step = createChild(returnFiber, step.value, lanes)),
           null !== step &&
             ((currentFirstChild = placeChild(step, currentFirstChild, newIdx)),
             null === previousNewFiber
-              ? (iteratorFn = step)
+              ? (resultingFirstChild = step)
               : (previousNewFiber.sibling = step),
             (previousNewFiber = step));
-      return iteratorFn;
+      return resultingFirstChild;
     }
     for (
       oldFiber = mapRemainingChildren(oldFiber);
       !step.done;
-      newIdx++, step = newChildrenIterable.next(), null
+      newIdx++, step = newChildren.next(), null
     )
       (step = updateFromMap(oldFiber, returnFiber, newIdx, step.value, lanes)),
         null !== step &&
@@ -2265,14 +2264,14 @@ function createChildReconciler(shouldTrackSideEffects) {
             oldFiber.delete(null === step.key ? newIdx : step.key),
           (currentFirstChild = placeChild(step, currentFirstChild, newIdx)),
           null === previousNewFiber
-            ? (iteratorFn = step)
+            ? (resultingFirstChild = step)
             : (previousNewFiber.sibling = step),
           (previousNewFiber = step));
     shouldTrackSideEffects &&
       oldFiber.forEach(function (child) {
         return deleteChild(returnFiber, child);
       });
-    return iteratorFn;
+    return resultingFirstChild;
   }
   function reconcileChildFibersImpl(
     returnFiber,
@@ -2404,13 +2403,18 @@ function createChildReconciler(shouldTrackSideEffects) {
           newChild,
           lanes
         );
-      if (getIteratorFn(newChild))
+      if (getIteratorFn(newChild)) {
+        child = getIteratorFn(newChild);
+        if ("function" !== typeof child)
+          throw Error(formatProdErrorMessage(150));
+        newChild = child.call(newChild);
         return reconcileChildrenIterator(
           returnFiber,
           currentFirstChild,
           newChild,
           lanes
         );
+      }
       if ("function" === typeof newChild.then)
         return reconcileChildFibersImpl(
           returnFiber,
@@ -10619,19 +10623,19 @@ var slice = Array.prototype.slice,
     };
     return Text;
   })(React.Component),
-  devToolsConfig$jscomp$inline_1114 = {
+  devToolsConfig$jscomp$inline_1123 = {
     findFiberByHostInstance: function () {
       return null;
     },
     bundleType: 0,
-    version: "19.0.0-www-classic-48f6e4f0",
+    version: "19.0.0-www-classic-6eb34c6f",
     rendererPackageName: "react-art"
   };
-var internals$jscomp$inline_1322 = {
-  bundleType: devToolsConfig$jscomp$inline_1114.bundleType,
-  version: devToolsConfig$jscomp$inline_1114.version,
-  rendererPackageName: devToolsConfig$jscomp$inline_1114.rendererPackageName,
-  rendererConfig: devToolsConfig$jscomp$inline_1114.rendererConfig,
+var internals$jscomp$inline_1331 = {
+  bundleType: devToolsConfig$jscomp$inline_1123.bundleType,
+  version: devToolsConfig$jscomp$inline_1123.version,
+  rendererPackageName: devToolsConfig$jscomp$inline_1123.rendererPackageName,
+  rendererConfig: devToolsConfig$jscomp$inline_1123.rendererConfig,
   overrideHookState: null,
   overrideHookStateDeletePath: null,
   overrideHookStateRenamePath: null,
@@ -10648,26 +10652,26 @@ var internals$jscomp$inline_1322 = {
     return null === fiber ? null : fiber.stateNode;
   },
   findFiberByHostInstance:
-    devToolsConfig$jscomp$inline_1114.findFiberByHostInstance ||
+    devToolsConfig$jscomp$inline_1123.findFiberByHostInstance ||
     emptyFindFiberByHostInstance,
   findHostInstancesForRefresh: null,
   scheduleRefresh: null,
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "19.0.0-www-classic-48f6e4f0"
+  reconcilerVersion: "19.0.0-www-classic-6eb34c6f"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
-  var hook$jscomp$inline_1323 = __REACT_DEVTOOLS_GLOBAL_HOOK__;
+  var hook$jscomp$inline_1332 = __REACT_DEVTOOLS_GLOBAL_HOOK__;
   if (
-    !hook$jscomp$inline_1323.isDisabled &&
-    hook$jscomp$inline_1323.supportsFiber
+    !hook$jscomp$inline_1332.isDisabled &&
+    hook$jscomp$inline_1332.supportsFiber
   )
     try {
-      (rendererID = hook$jscomp$inline_1323.inject(
-        internals$jscomp$inline_1322
+      (rendererID = hook$jscomp$inline_1332.inject(
+        internals$jscomp$inline_1331
       )),
-        (injectedHook = hook$jscomp$inline_1323);
+        (injectedHook = hook$jscomp$inline_1332);
     } catch (err) {}
 }
 var Path = Mode$1.Path;