Add warnings for common root API mistakes (#23356)

For createRoot, a common mistake is to pass JSX as the second argument,
instead of calling root.render.

For hydrateRoot, a common mistake is to forget to pass children as
the second argument.

The type system will enforce correct usage, but since not everyone uses
types we'll log a helpful warning, too.

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

@@ -420,4 +420,27 @@ describe('ReactDOMRoot', () => {
     // Still works in the legacy API
     ReactDOM.render(<div />, commentNode);
   });
+
+  it('warn if no children passed to hydrateRoot', async () => {
+    expect(() =>
+      ReactDOM.hydrateRoot(container),
+    ).toErrorDev(
+      'Must provide initial children as second argument to hydrateRoot.',
+      {withoutStack: true},
+    );
+  });
+
+  it('warn if JSX passed to createRoot', async () => {
+    function App() {
+      return 'Child';
+    }
+
+    expect(() => ReactDOM.createRoot(container, <App />)).toErrorDev(
+      'You passed a JSX element to createRoot. You probably meant to call ' +
+        'root.render instead',
+      {
+        withoutStack: true,
+      },
+    );
+  });
 });

packages/react-dom/src/client/ReactDOMRoot.js

@@ -15,6 +15,7 @@ import type {
 } from 'react-reconciler/src/ReactInternalTypes';
 
 import {queueExplicitHydrationTarget} from '../events/ReactDOMEventReplaying';
+import {REACT_ELEMENT_TYPE} from 'shared/ReactSymbols';
 
 export type RootType = {
   render(children: ReactNodeList): void,
@@ -174,6 +175,20 @@ export function createRoot(
         console.warn(
           'hydrate through createRoot is deprecated. Use ReactDOM.hydrateRoot(container, <App />) instead.',
         );
+      } else {
+        if (
+          typeof options === 'object' &&
+          options !== null &&
+          (options: any).$$typeof === REACT_ELEMENT_TYPE
+        ) {
+          console.error(
+            'You passed a JSX element to createRoot. You probably meant to ' +
+              'call root.render instead. ' +
+              'Example usage:\n\n' +
+              '  let root = createRoot(domContainer);\n' +
+              '  root.render(<App />);',
+          );
+        }
       }
     }
     if (options.unstable_strictMode === true) {
@@ -237,6 +252,15 @@ export function hydrateRoot(
 
   warnIfReactDOMContainerInDEV(container);
 
+  if (__DEV__) {
+    if (initialChildren === undefined) {
+      console.error(
+        'Must provide initial children as second argument to hydrateRoot. ' +
+          'Example usage: hydrateRoot(domContainer, <App />)',
+      );
+    }
+  }
+
   // For now we reuse the whole bag of options since they contain
   // the hydration callbacks.
   const hydrationCallbacks = options != null ? options : null;