Remove Numeric Fallback of Symbols

This was already defeating the XSS issue that Symbols was meant to protect
against. So you were already supposed to use a polyfill for security.

We rely on real Symbol.for in Flight for Server Components so those require
real symbols anyway.

We also don't really support IE without additional polyfills anyway.

packages/shared/ReactSymbols.js

@@ -11,50 +11,27 @@
 // When adding new symbols to this file,
 // Please consider also adding to 'react-devtools-shared/src/backend/ReactSymbols'
 
-// The Symbol used to tag the ReactElement-like types. If there is no native Symbol
-// nor polyfill, then a plain number is used for performance.
-export let REACT_ELEMENT_TYPE = 0xeac7;
-export let REACT_PORTAL_TYPE = 0xeaca;
-export let REACT_FRAGMENT_TYPE = 0xeacb;
-export let REACT_STRICT_MODE_TYPE = 0xeacc;
-export let REACT_PROFILER_TYPE = 0xead2;
-export let REACT_PROVIDER_TYPE = 0xeacd;
-export let REACT_CONTEXT_TYPE = 0xeace;
-export let REACT_FORWARD_REF_TYPE = 0xead0;
-export let REACT_SUSPENSE_TYPE = 0xead1;
-export let REACT_SUSPENSE_LIST_TYPE = 0xead8;
-export let REACT_MEMO_TYPE = 0xead3;
-export let REACT_LAZY_TYPE = 0xead4;
-export let REACT_SCOPE_TYPE = 0xead7;
-export let REACT_DEBUG_TRACING_MODE_TYPE = 0xeae1;
-export let REACT_OFFSCREEN_TYPE = 0xeae2;
-export let REACT_LEGACY_HIDDEN_TYPE = 0xeae3;
-export let REACT_CACHE_TYPE = 0xeae4;
-export let REACT_TRACING_MARKER_TYPE = 0xeae5;
+// The Symbol used to tag the ReactElement-like types.
+export let REACT_ELEMENT_TYPE = Symbol.for('react.element');
+export let REACT_PORTAL_TYPE = Symbol.for('react.portal');
+export let REACT_FRAGMENT_TYPE = Symbol.for('react.fragment');
+export let REACT_STRICT_MODE_TYPE = Symbol.for('react.strict_mode');
+export let REACT_PROFILER_TYPE = Symbol.for('react.profiler');
+export let REACT_PROVIDER_TYPE = Symbol.for('react.provider');
+export let REACT_CONTEXT_TYPE = Symbol.for('react.context');
+export let REACT_FORWARD_REF_TYPE = Symbol.for('react.forward_ref');
+export let REACT_SUSPENSE_TYPE = Symbol.for('react.suspense');
+export let REACT_SUSPENSE_LIST_TYPE = Symbol.for('react.suspense_list');
+export let REACT_MEMO_TYPE = Symbol.for('react.memo');
+export let REACT_LAZY_TYPE = Symbol.for('react.lazy');
+export let REACT_SCOPE_TYPE = Symbol.for('react.scope');
+export let REACT_DEBUG_TRACING_MODE_TYPE = Symbol.for('react.debug_trace_mode');
+export let REACT_OFFSCREEN_TYPE = Symbol.for('react.offscreen');
+export let REACT_LEGACY_HIDDEN_TYPE = Symbol.for('react.legacy_hidden');
+export let REACT_CACHE_TYPE = Symbol.for('react.cache');
+export let REACT_TRACING_MARKER_TYPE = Symbol.for('react.tracing_marker');
 
-if (typeof Symbol === 'function' && Symbol.for) {
-  const symbolFor = Symbol.for;
-  REACT_ELEMENT_TYPE = symbolFor('react.element');
-  REACT_PORTAL_TYPE = symbolFor('react.portal');
-  REACT_FRAGMENT_TYPE = symbolFor('react.fragment');
-  REACT_STRICT_MODE_TYPE = symbolFor('react.strict_mode');
-  REACT_PROFILER_TYPE = symbolFor('react.profiler');
-  REACT_PROVIDER_TYPE = symbolFor('react.provider');
-  REACT_CONTEXT_TYPE = symbolFor('react.context');
-  REACT_FORWARD_REF_TYPE = symbolFor('react.forward_ref');
-  REACT_SUSPENSE_TYPE = symbolFor('react.suspense');
-  REACT_SUSPENSE_LIST_TYPE = symbolFor('react.suspense_list');
-  REACT_MEMO_TYPE = symbolFor('react.memo');
-  REACT_LAZY_TYPE = symbolFor('react.lazy');
-  REACT_SCOPE_TYPE = symbolFor('react.scope');
-  REACT_DEBUG_TRACING_MODE_TYPE = symbolFor('react.debug_trace_mode');
-  REACT_OFFSCREEN_TYPE = symbolFor('react.offscreen');
-  REACT_LEGACY_HIDDEN_TYPE = symbolFor('react.legacy_hidden');
-  REACT_CACHE_TYPE = symbolFor('react.cache');
-  REACT_TRACING_MARKER_TYPE = symbolFor('react.tracing_marker');
-}
-
-const MAYBE_ITERATOR_SYMBOL = typeof Symbol === 'function' && Symbol.iterator;
+const MAYBE_ITERATOR_SYMBOL = Symbol.iterator;
 const FAUX_ITERATOR_SYMBOL = '@@iterator';
 
 export function getIteratorFn(maybeIterable: ?any): ?() => ?Iterator<*> {

packages/shared/__tests__/ReactSymbols-test.internal.js

@@ -26,19 +26,4 @@ describe('ReactSymbols', () => {
   it('Symbol values should be unique', () => {
     expectToBeUnique(Object.entries(require('shared/ReactSymbols')));
   });
-
-  it('numeric values should be unique', () => {
-    const originalSymbolFor = global.Symbol.for;
-    global.Symbol.for = null;
-    try {
-      const entries = Object.entries(require('shared/ReactSymbols')).filter(
-        // REACT_ASYNC_MODE_TYPE and REACT_CONCURRENT_MODE_TYPE have the same numeric value
-        // for legacy backwards compatibility
-        ([key]) => key !== 'REACT_ASYNC_MODE_TYPE',
-      );
-      expectToBeUnique(entries);
-    } finally {
-      global.Symbol.for = originalSymbolFor;
-    }
-  });
 });

packages/shared/isValidElementType.js

@@ -31,10 +31,7 @@ import {
   enableTransitionTracing,
 } from './ReactFeatureFlags';
 
-let REACT_MODULE_REFERENCE: number | Symbol = 0;
-if (typeof Symbol === 'function') {
-  REACT_MODULE_REFERENCE = Symbol.for('react.module.reference');
-}
+let REACT_MODULE_REFERENCE: Symbol = Symbol.for('react.module.reference');
 
 export default function isValidElementType(type: mixed) {
   if (typeof type === 'string' || typeof type === 'function') {