useFormState's permalink option changes form target

When the `permalink` option is passed to `useFormState`, and the form is
submitted before it has hydrated, the permalink will be used as the target
of the form action, enabling MPA-style form submissions.

(Note that submitting a form without hydration is a feature of Server Actions;
it doesn't work with regular client actions.)

It does not have any effect after the form has hydrated.

packages/react-server-dom-webpack/src/__tests__/ReactFlightDOMForm-test.js

@@ -30,6 +30,7 @@ let React;
 let ReactDOMServer;
 let ReactServerDOMServer;
 let ReactServerDOMClient;
+let useFormState;
 
 describe('ReactFlightDOMForm', () => {
   beforeEach(() => {
@@ -47,6 +48,7 @@ describe('ReactFlightDOMForm', () => {
     ReactServerDOMServer = require('react-server-dom-webpack/server.edge');
     ReactServerDOMClient = require('react-server-dom-webpack/client.edge');
     ReactDOMServer = require('react-dom/server.edge');
+    useFormState = require('react-dom').experimental_useFormState;
     container = document.createElement('div');
     document.body.appendChild(container);
   });
@@ -308,4 +310,82 @@ describe('ReactFlightDOMForm', () => {
     expect(result).toBe('hello');
     expect(foo).toBe('barobject');
   });
+
+  // @gate enableFormActions
+  // @gate enableAsyncActions
+  it("useFormState's dispatch binds the initial state to the provided action", async () => {
+    let serverActionResult = null;
+
+    const serverAction = serverExports(function action(prevState, formData) {
+      const newState = {
+        count: prevState.count + parseInt(formData.get('incrementAmount'), 10),
+      };
+      serverActionResult = newState;
+      return newState;
+    });
+
+    const initialState = {count: 1};
+    function Client({action}) {
+      const [state, dispatch] = useFormState(action, initialState);
+      return (
+        <form action={dispatch}>
+          <span>Count: {state.count}</span>
+          <input type="text" name="incrementAmount" defaultValue="5" />
+        </form>
+      );
+    }
+    const ClientRef = await clientExports(Client);
+
+    const rscStream = ReactServerDOMServer.renderToReadableStream(
+      <ClientRef action={serverAction} />,
+      webpackMap,
+    );
+    const response = ReactServerDOMClient.createFromReadableStream(rscStream);
+    const ssrStream = await ReactDOMServer.renderToReadableStream(response);
+    await readIntoContainer(ssrStream);
+
+    const form = container.firstChild;
+    const span = container.getElementsByTagName('span')[0];
+    expect(span.textContent).toBe('Count: 1');
+
+    await submit(form);
+    expect(serverActionResult.count).toBe(6);
+  });
+
+  // @gate enableFormActions
+  // @gate enableAsyncActions
+  it("useFormState can change the action's target with the `permalink` argument", async () => {
+    const serverAction = serverExports(function action(prevState) {
+      return {state: prevState.count + 1};
+    });
+
+    const initialState = {count: 1};
+    function Client({action}) {
+      const [state, dispatch] = useFormState(
+        action,
+        initialState,
+        '/permalink',
+      );
+      return (
+        <form action={dispatch}>
+          <span>Count: {state.count}</span>
+        </form>
+      );
+    }
+    const ClientRef = await clientExports(Client);
+
+    const rscStream = ReactServerDOMServer.renderToReadableStream(
+      <ClientRef action={serverAction} />,
+      webpackMap,
+    );
+    const response = ReactServerDOMClient.createFromReadableStream(rscStream);
+    const ssrStream = await ReactDOMServer.renderToReadableStream(response);
+    await readIntoContainer(ssrStream);
+
+    const form = container.firstChild;
+    const span = container.getElementsByTagName('span')[0];
+    expect(span.textContent).toBe('Count: 1');
+
+    expect(form.target).toBe('/permalink');
+  });
 });

packages/react-server/src/ReactFizzHooks.js

@@ -14,6 +14,7 @@ import type {
   StartTransitionOptions,
   Thenable,
   Usable,
+  ReactCustomFormAction,
 } from 'shared/ReactTypes';
 
 import type {ResumableState} from './ReactFizzConfig';
@@ -542,10 +543,6 @@ function unsupportedSetOptimisticState() {
   throw new Error('Cannot update optimistic state while rendering.');
 }
 
-function unsupportedDispatchFormState() {
-  throw new Error('Cannot update form state while rendering.');
-}
-
 function useOptimistic<S, A>(
   passthrough: S,
   reducer: ?(S, A) => S,
@@ -560,7 +557,35 @@ function useFormState<S, P>(
   permalink?: string,
 ): [S, (P) => void] {
   resolveCurrentlyRenderingComponent();
-  return [initialState, unsupportedDispatchFormState];
+
+  // Bind the initial state to the first argument of the action.
+  // TODO: Use the keypath (or permalink) to check if there's matching state
+  // from the previous page.
+  const boundAction = action.bind(null, initialState);
+
+  // Wrap the action so the return value is void.
+  const dispatch = (payload: P): void => {
+    boundAction(payload);
+  };
+
+  // $FlowIgnore[prop-missing]
+  if (typeof boundAction.$$FORM_ACTION === 'function') {
+    // $FlowIgnore[prop-missing]
+    dispatch.$$FORM_ACTION = (prefix: string) => {
+      // $FlowIgnore[prop-missing]
+      const metadata: ReactCustomFormAction = boundAction.$$FORM_ACTION(prefix);
+      // Override the target URL
+      if (typeof permalink === 'string') {
+        metadata.target = permalink;
+      }
+      return metadata;
+    };
+  } else {
+    // This is not a server action, so the permalink argument has
+    // no effect. The form will have to be hydrated before it's submitted.
+  }
+
+  return [initialState, dispatch];
 }
 
 function useId(): string {