[Flight] Encode React Elements in Replies as Temporary References (#28564)

Currently you can accidentally pass React Element to a Server Action. It
warns but in prod it actually works because we can encode the symbol and
otherwise it's mostly a plain object. It only works if you only pass
host components and no function props etc. which makes it potentially
error later. The first thing this does it just early hard error for
elements.

I made Lazy work by unwrapping though since that will be replaced by
Promises later which works.

Our protocol is not fully symmetric in that elements flow from Server ->
Client. Only the Server can resolve Components and only the client
should really be able to receive host components. It's not intended that
a Server can actually do something with them other than passing them to
the client.

In the case of a Reply, we expect the client to be stateful. It's
waiting for a response. So anything we can't serialize we can still pass
by reference to an in memory object. So I introduce the concept of a
TemporaryReferenceSet which is an opaque object that you create before
encoding the reply. This then stashes any unserializable values in this
set and encode the slot by id. When a new response from the Action then
returns we pass the same temporary set into the parser which can then
restore the objects. This lets you pass a value by reference to the
server and back into another slot.

For example it can be used to render children inside a parent tree from
a server action:

```
export async function Component({ children }) {
  "use server";
  return <div>{children}</div>;
}
```

(You wouldn't normally do this due to the waterfalls but for advanced
cases.)

A common scenario where this comes up accidentally today is in
`useActionState`.

```
export function action(state, formData) {
  "use server";
   if (errored) {
     return <div>This action <strong>errored</strong></div>;
   }
   return null;
}
```

```
const [errors, formAction] = useActionState(action);
return <div>{errors}<div>;
```

It feels like I'm just passing the JSX from server to client. However,
because `useActionState` also sends the previous state *back* to the
server this should not actually be valid. Before this PR this actually
worked accidentally. You get a DEV warning but it used to work in prod.
Once you do something like pass a client reference it won't work tho. We
could perhaps make client references work by stashing where we got them
from but it wouldn't work with all possible JSX.

By adding temporary references to the action implementation this will
work again - on the client. It'll also be more efficient since we don't
send back the JSX content that you shouldn't introspect on the server
anyway.

However, a flaw here is that the progressive enhancement of this case
won't work because we can't use temporary references for progressive
enhancement since there's no in memory stash. What is worse is that it
won't error if you hydrate. ~It also will error late in the example
above because the first state is "undefined" so invoking the form once
works - it errors on the second attempt when it tries to send the error
state back again.~ It actually errors on the first invocation because we
need to eagerly serialize "previous state" into the form. So at least
that's better.

I think maybe the solution to this particular pattern would be to allow
JSX to serialize if you have no temporary reference set, and remember
client references so that client references can be returned back to the
server as client references. That way anything you could send from the
server could also be returned to the server. But it would only deopt to
serializing it for progressive enhancement. The consequence of that
would be that there's a lot of JSX that might accidentally seem like it
should work but it's only if you've gotten it from the server before
that it works. This would have to have pair them somehow though since
you can't take a client reference from one implementation of Flight and
use it with another.

DiffTrain build for [83409a1fdd](https://github.com/facebook/react/commit/83409a1fdd14b2e5b33c587935a7ef552607780f)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-0aab065eb3250a9714a62dc05587cbb571da7f71
+83409a1fdd14b2e5b33c587935a7ef552607780f

compiled/facebook-www/ReactDOMTesting-prod.modern.js

@@ -17142,7 +17142,7 @@ Internals.Events = [
 var devToolsConfig$jscomp$inline_1781 = {
   findFiberByHostInstance: getClosestInstanceFromNode,
   bundleType: 0,
-  version: "18.3.0-www-modern-aa2fdf61",
+  version: "18.3.0-www-modern-68b315be",
   rendererPackageName: "react-dom"
 };
 var internals$jscomp$inline_2151 = {
@@ -17173,7 +17173,7 @@ var internals$jscomp$inline_2151 = {
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "18.3.0-www-modern-aa2fdf61"
+  reconcilerVersion: "18.3.0-www-modern-68b315be"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
   var hook$jscomp$inline_2152 = __REACT_DEVTOOLS_GLOBAL_HOOK__;
@@ -17582,4 +17582,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactCurrentDispatcher$2.current.useHostTransitionStatus();
 };
-exports.version = "18.3.0-www-modern-aa2fdf61";
+exports.version = "18.3.0-www-modern-68b315be";

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

@@ -257,6 +257,17 @@ if (__DEV__) {
     var REACT_ELEMENT_TYPE = Symbol.for("react.element");
     var REACT_LAZY_TYPE = Symbol.for("react.lazy");
 
+    function readTemporaryReference(set, id) {
+      if (id < 0 || id >= set.length) {
+        throw new Error(
+          "The RSC response contained a reference that doesn't exist in the temporary reference set. " +
+            "Always pass the matching set that was used to create the reply when parsing its response."
+        );
+      }
+
+      return set[id];
+    }
+
     var knownServerReferences = new WeakMap(); // Serializable values
 
     function registerServerReference(proxy, reference, encodeFormAction) {
@@ -784,19 +795,35 @@ if (__DEV__) {
             return createServerReferenceProxy(response, metadata);
           }
 
-          case "Q": {
-            // Map
+          case "T": {
+            // Temporary Reference
             var _id3 = parseInt(value.slice(2), 16);
 
-            var data = getOutlinedModel(response, _id3);
+            var temporaryReferences = response._tempRefs;
+
+            if (temporaryReferences == null) {
+              throw new Error(
+                "Missing a temporary reference set but the RSC response returned a temporary reference. " +
+                  "Pass a temporaryReference option with the set that was used with the reply."
+              );
+            }
+
+            return readTemporaryReference(temporaryReferences, _id3);
+          }
+
+          case "Q": {
+            // Map
+            var _id4 = parseInt(value.slice(2), 16);
+
+            var data = getOutlinedModel(response, _id4);
             return new Map(data);
           }
 
           case "W": {
             // Set
-            var _id4 = parseInt(value.slice(2), 16);
+            var _id5 = parseInt(value.slice(2), 16);
 
-            var _data = getOutlinedModel(response, _id4);
+            var _data = getOutlinedModel(response, _id5);
 
             return new Set(_data);
           }
@@ -853,9 +880,9 @@ if (__DEV__) {
 
           default: {
             // We assume that anything else is a reference ID.
-            var _id5 = parseInt(value.slice(1), 16);
+            var _id6 = parseInt(value.slice(1), 16);
 
-            var _chunk2 = getChunk(response, _id5);
+            var _chunk2 = getChunk(response, _id6);
 
             switch (_chunk2.status) {
               case RESOLVED_MODEL:
@@ -950,7 +977,8 @@ if (__DEV__) {
       moduleLoading,
       callServer,
       encodeFormAction,
-      nonce
+      nonce,
+      temporaryReferences
     ) {
       var chunks = new Map();
       var response = {
@@ -966,7 +994,8 @@ if (__DEV__) {
         _rowID: 0,
         _rowTag: 0,
         _rowLength: 0,
-        _buffer: []
+        _buffer: [],
+        _tempRefs: temporaryReferences
       }; // Don't inline this call because it causes closure to outline the call above.
 
       response._fromJSON = createFromJSONCallback(response);

compiled/facebook-www/ReactFlightDOMClient-prod.modern.js

@@ -269,6 +269,18 @@ function parseModelString(response, parentObject, key, value) {
           (parentObject = getOutlinedModel(response, parentObject)),
           createServerReferenceProxy(response, parentObject)
         );
+      case "T":
+        parentObject = parseInt(value.slice(2), 16);
+        response = response._tempRefs;
+        if (null == response)
+          throw Error(
+            "Missing a temporary reference set but the RSC response returned a temporary reference. Pass a temporaryReference option with the set that was used with the reply."
+          );
+        if (0 > parentObject || parentObject >= response.length)
+          throw Error(
+            "The RSC response contained a reference that doesn't exist in the temporary reference set. Always pass the matching set that was used to create the reply when parsing its response."
+          );
+        return response[parentObject];
       case "Q":
         return (
           (parentObject = parseInt(value.slice(2), 16)),
@@ -570,7 +582,8 @@ exports.createFromReadableStream = function (stream, options) {
     _rowID: 0,
     _rowTag: 0,
     _rowLength: 0,
-    _buffer: []
+    _buffer: [],
+    _tempRefs: void 0
   };
   options._fromJSON = createFromJSONCallback(options);
   startReadingFromStream(options, stream);

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

@@ -434,6 +434,15 @@ if (__DEV__) {
     var supportsRequestStorage = false;
     var requestStorage = null;
 
+    var TEMPORARY_REFERENCE_TAG = Symbol.for("react.temporary.reference"); // eslint-disable-next-line no-unused-vars
+
+    function isTemporaryReference(reference) {
+      return reference.$$typeof === TEMPORARY_REFERENCE_TAG;
+    }
+    function resolveTemporaryReferenceID(temporaryReference) {
+      return temporaryReference.$$id;
+    }
+
     // ATTENTION
     // When adding new symbols to this file,
     // Please consider also adding to 'react-devtools-shared/src/backend/ReactSymbols'
@@ -1644,7 +1653,7 @@ if (__DEV__) {
       }
 
       if (typeof type === "function") {
-        if (isClientReference(type)) {
+        if (isClientReference(type) || isTemporaryReference(type)) {
           // This is a reference to a Client Component.
           return renderClientElement(task, type, key, props);
         } // This is a Server Component.
@@ -1814,6 +1823,10 @@ if (__DEV__) {
       return "$F" + id.toString(16);
     }
 
+    function serializeTemporaryReferenceID(id) {
+      return "$T" + id;
+    }
+
     function serializeSymbolReference(name) {
       return "$S" + name;
     }
@@ -1942,6 +1955,11 @@ if (__DEV__) {
       return serializeServerReferenceID(metadataId);
     }
 
+    function serializeTemporaryReference(request, temporaryReference) {
+      var id = resolveTemporaryReferenceID(temporaryReference);
+      return serializeTemporaryReferenceID(id);
+    }
+
     function serializeLargeTextString(request, text) {
       request.pendingChunks += 2;
       var textId = request.nextChunkId++;
@@ -2377,6 +2395,10 @@ if (__DEV__) {
           return serializeServerReference(request, value);
         }
 
+        if (isTemporaryReference(value)) {
+          return serializeTemporaryReference(request, value);
+        }
+
         if (/^on[A-Z]/.test(parentPropertyName)) {
           throw new Error(
             "Event handlers cannot be passed to Client Component props." +
@@ -2700,6 +2722,10 @@ if (__DEV__) {
             parentPropertyName,
             value
           );
+        }
+
+        if (isTemporaryReference(value)) {
+          return serializeTemporaryReference(request, value);
         } // Serialize the body of the function as an eval so it can be printed.
         // $FlowFixMe[method-unbinding]
 

compiled/facebook-www/ReactFlightDOMServer-prod.modern.js

@@ -164,7 +164,8 @@ function trimOptions(options) {
       ((hasProperties = !0), (trimmed[key] = options[key]));
   return hasProperties ? trimmed : null;
 }
-var REACT_ELEMENT_TYPE = Symbol.for("react.element"),
+var TEMPORARY_REFERENCE_TAG = Symbol.for("react.temporary.reference"),
+  REACT_ELEMENT_TYPE = Symbol.for("react.element"),
   REACT_FRAGMENT_TYPE = Symbol.for("react.fragment"),
   REACT_CONTEXT_TYPE = Symbol.for("react.context"),
   REACT_FORWARD_REF_TYPE = Symbol.for("react.forward_ref"),
@@ -595,7 +596,7 @@ function renderElement(request, task, type, key, ref, props) {
       "Refs cannot be used in Server Components, nor passed to Client Components."
     );
   if ("function" === typeof type)
-    return isClientReference(type)
+    return isClientReference(type) || type.$$typeof === TEMPORARY_REFERENCE_TAG
       ? renderClientElement(task, type, key, props)
       : renderFunctionComponent(request, task, key, type, props);
   if ("string" === typeof type)

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

@@ -2809,7 +2809,7 @@ if (__DEV__) {
             console["error"](error);
           };
 
-    var ReactVersion = "18.3.0-www-modern-1e9c6c79";
+    var ReactVersion = "18.3.0-www-modern-9f10bc05";
 
     // Patch fetch
     var Children = {

compiled/facebook-www/ReactServer-prod.modern.js

@@ -524,4 +524,4 @@ exports.useId = function () {
 exports.useMemo = function (create, deps) {
   return ReactCurrentDispatcher.current.useMemo(create, deps);
 };
-exports.version = "18.3.0-www-modern-7ba66f9a";
+exports.version = "18.3.0-www-modern-e9e628b6";

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

@@ -26056,7 +26056,7 @@ if (__DEV__) {
       return root;
     }
 
-    var ReactVersion = "18.3.0-www-modern-05b3f7fc";
+    var ReactVersion = "18.3.0-www-modern-34ddd034";
 
     // Might add PROFILE later.
 

compiled/facebook-www/__test_utils__/ReactAllWarnings.js

@@ -221,8 +221,6 @@ export default [
   "Profiler must specify an \"id\" of type `string` as a prop. Received the type `%s` instead.",
   "Prop `%s` did not match. Server: %s Client: %s",
   "React Context Providers cannot be passed to Server Functions from the Client.%s",
-  "React Element cannot be passed to Server Functions from the Client.%s",
-  "React Lazy cannot be passed to Server Functions from the Client.%s",
   "React depends on Map and Set built-in types. Make sure that you load a polyfill in older browsers. https://react.dev/link/react-polyfills",
   "React does not recognize the `%s` prop on a DOM element. If you intentionally want it to appear in the DOM as a custom attribute, spell it as lowercase `%s` instead. If you accidentally passed it from a parent component, remove it from the DOM element.",
   "React encountered a <link rel=\"stylesheet\" href=\"%s\" ... /> with a `precedence` prop that also included %s. The presence of loading and error handlers indicates an intent to manage the stylesheet loading state from your from your Component code and React will not hoist or deduplicate this stylesheet. If your intent was to have React hoist and deduplciate this stylesheet using the `precedence` prop remove the %s, otherwise remove the `precedence` prop.",