Remove _owner field from JSX elements in prod if string refs are disabled (facebook#28739)

In prod, the `_owner` field is only used for string refs so if we have
string refs disabled, we don't need this field. In fact, that's one of
the big benefits of deprecating them.

Author: sebmarkbage

packages/jest-react/src/JestReact.js

@@ -6,7 +6,7 @@
  */
 
 import {REACT_ELEMENT_TYPE, REACT_FRAGMENT_TYPE} from 'shared/ReactSymbols';
-import {enableRefAsProp} from 'shared/ReactFeatureFlags';
+import {disableStringRefs, enableRefAsProp} from 'shared/ReactFeatureFlags';
 
 import isArray from 'shared/isArray';
 
@@ -54,6 +54,14 @@ function createJSXElementForTestComparison(type, props) {
       value: null,
     });
     return element;
+  } else if (!__DEV__ && disableStringRefs) {
+    return {
+      $$typeof: REACT_ELEMENT_TYPE,
+      type: type,
+      key: null,
+      ref: null,
+      props: props,
+    };
   } else {
     return {
       $$typeof: REACT_ELEMENT_TYPE,

packages/react-client/src/ReactFlightClient.js

@@ -38,6 +38,7 @@ import type {Postpone} from 'react/src/ReactPostpone';
 import type {TemporaryReferenceSet} from './ReactFlightTemporaryReferences';
 
 import {
+  disableStringRefs,
   enableBinaryFlight,
   enablePostpone,
   enableRefAsProp,
@@ -498,6 +499,16 @@ function createElement(
       enumerable: false,
       get: nullRefGetter,
     });
+  } else if (!__DEV__ && disableStringRefs) {
+    element = ({
+      // This tag allows us to uniquely identify this as a React Element
+      $$typeof: REACT_ELEMENT_TYPE,
+
+      type,
+      key,
+      ref: null,
+      props,
+    }: any);
   } else {
     element = ({
       // This tag allows us to uniquely identify this as a React Element

packages/react-noop-renderer/src/createReactNoop.js

@@ -32,7 +32,11 @@ import {
   ConcurrentRoot,
   LegacyRoot,
 } from 'react-reconciler/constants';
-import {enableRefAsProp, disableLegacyMode} from 'shared/ReactFeatureFlags';
+import {
+  enableRefAsProp,
+  disableLegacyMode,
+  disableStringRefs,
+} from 'shared/ReactFeatureFlags';
 
 type Container = {
   rootID: string,
@@ -799,6 +803,14 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
         value: null,
       });
       return element;
+    } else if (!__DEV__ && disableStringRefs) {
+      return {
+        $$typeof: REACT_ELEMENT_TYPE,
+        type: type,
+        key: null,
+        ref: null,
+        props: props,
+      };
     } else {
       return {
         $$typeof: REACT_ELEMENT_TYPE,

packages/react/src/__tests__/ReactCreateElement-test.js

@@ -275,7 +275,11 @@ describe('ReactCreateElement', () => {
     }
     const root = ReactDOMClient.createRoot(document.createElement('div'));
     await act(() => root.render(React.createElement(Wrapper)));
-    expect(element._owner.stateNode).toBe(instance);
+    if (__DEV__ || !gate(flags => flags.disableStringRefs)) {
+      expect(element._owner.stateNode).toBe(instance);
+    } else {
+      expect('_owner' in element).toBe(false);
+    }
   });
 
   it('merges an additional argument onto the children prop', () => {

packages/react/src/jsx/ReactJSXElement.js

@@ -239,6 +239,19 @@ function ReactElement(type, key, _ref, self, source, owner, props) {
         value: null,
       });
     }
+  } else if (!__DEV__ && disableStringRefs) {
+    // In prod, `ref` is a regular property and _owner doesn't exist.
+    element = {
+      // This tag allows us to uniquely identify this as a React Element
+      $$typeof: REACT_ELEMENT_TYPE,
+
+      // Built-in properties that belong on the element
+      type,
+      key,
+      ref,
+
+      props,
+    };
   } else {
     // In prod, `ref` is a regular property. It will be removed in a
     // future release.
@@ -774,7 +787,7 @@ export function cloneAndReplaceKey(oldElement, newKey) {
     enableRefAsProp ? null : oldElement.ref,
     undefined,
     undefined,
-    oldElement._owner,
+    !__DEV__ && disableStringRefs ? undefined : oldElement._owner,
     oldElement.props,
   );
 }
@@ -800,7 +813,7 @@ export function cloneElement(element, config, children) {
   let ref = enableRefAsProp ? null : element.ref;
 
   // Owner will be preserved, unless ref is overridden
-  let owner = element._owner;
+  let owner = !__DEV__ && disableStringRefs ? undefined : element._owner;
 
   if (config != null) {
     if (hasValidRef(config)) {

packages/shared/ReactElementType.js

@@ -13,7 +13,7 @@ export type ReactElement = {
   key: any,
   ref: any,
   props: any,
-  // ReactFiber
+  // __DEV__ or for string refs
   _owner: any,
 
   // __DEV__