Bug 1652744 - Add browser pref for private fields support r=jorendorff

Differential Revision: https://phabricator.services.mozilla.com/D83511

js/src/tests/user.js

@@ -33,3 +33,4 @@ user_pref("browser.webapps.checkForUpdates", 0);
 user_pref("javascript.options.weakrefs", true);
 user_pref("javascript.options.experimental.weakrefs.expose_cleanupSome", true);
 user_pref("javascript.options.experimental.iterator_helpers", true);
+user_pref("javascript.options.experimental.private_fields", true);

js/xpconnect/src/XPCJSContext.cpp

@@ -760,6 +760,7 @@ static mozilla::Atomic<bool> sPropertyErrorMessageFixEnabled(false);
 static mozilla::Atomic<bool> sWeakRefsEnabled(false);
 static mozilla::Atomic<bool> sWeakRefsExposeCleanupSome(false);
 static mozilla::Atomic<bool> sIteratorHelpersEnabled(false);
+static mozilla::Atomic<bool> sPrivateFieldsEnabled(false);
 
 static JS::WeakRefSpecifier GetWeakRefsEnabled() {
   if (!sWeakRefsEnabled) {
@@ -784,7 +785,8 @@ void xpc::SetPrefableRealmOptions(JS::RealmOptions& options) {
           StaticPrefs::javascript_options_writable_streams())
       .setPropertyErrorMessageFixEnabled(sPropertyErrorMessageFixEnabled)
       .setWeakRefsEnabled(GetWeakRefsEnabled())
-      .setIteratorHelpersEnabled(sIteratorHelpersEnabled);
+      .setIteratorHelpersEnabled(sIteratorHelpersEnabled)
+      .setPrivateClassFieldsEnabled(sPrivateFieldsEnabled);
 }
 
 static void LoadStartupJSPrefs(XPCJSContext* xpccx) {
@@ -967,6 +969,8 @@ static void ReloadPrefsCallback(const char* pref, void* aXpccx) {
 #ifdef NIGHTLY_BUILD
   sIteratorHelpersEnabled =
       Preferences::GetBool(JS_OPTIONS_DOT_STR "experimental.iterator_helpers");
+  sPrivateFieldsEnabled =
+      Preferences::GetBool(JS_OPTIONS_DOT_STR "experimental.private_fields");
 #endif
 
 #ifdef JS_GC_ZEAL

modules/libpref/init/StaticPrefList.yaml

@@ -4909,6 +4909,12 @@
   type: RelaxedAtomicBool
   value: false
   mirror: always
+
+  # Experimental support for Private Fields in JavaScript.
+- name: javascript.options.experimental.private_fields
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
 #endif
 
 # The amount of time we wait between a request to GC (due to leaving a page) and doing the actual GC, in ms.