Bug 1529758 - Add a pref for fields. r=tcampbell

This creates a shell command-line option, `--enable-experimental-fields`, and a
Gecko pref, `javascript.options.experimental.fields`.

Both are off by default everywhere, for now.

Differential Revision: https://phabricator.services.mozilla.com/D22045

--HG--
extra : moz-landing-system : lando

js/public/CompileOptions.h

@@ -117,6 +117,7 @@ class JS_PUBLIC_API TransitiveCompileOptions {
   bool isProbablySystemCode = false;
   bool hideScriptFromDebugger = false;
   bool bigIntEnabledOption = false;
+  bool fieldsEnabledOption = false;
 
   /**
    * |introductionType| is a statically allocated C string: one of "eval",

js/src/frontend/Parser.cpp

@@ -6779,11 +6779,10 @@ bool GeneralParser<ParseHandler, Unit>::classMember(
   }
 
   if (propType == PropertyType::Field) {
-    // TODO(khyperia): Delete the two lines below once fields are fully
-    // supported in the backend. We can't fail in BytecodeCompiler because of
-    // lazy parsing.
-    errorAt(propNameOffset, JSMSG_FIELDS_NOT_SUPPORTED);
-    return false;
+    if (!options().fieldsEnabledOption) {
+      errorAt(propNameOffset, JSMSG_FIELDS_NOT_SUPPORTED);
+      return null();
+    }
 
     if (isStatic) {
       errorAt(propNameOffset, JSMSG_BAD_METHOD_DEF);

js/src/frontend/TokenStream.cpp

@@ -1994,9 +1994,10 @@ MOZ_MUST_USE bool TokenStreamSpecific<Unit, AnyCharsAccess>::identifierName(
                "Private identifier starts with #");
     newPrivateNameToken(atom->asPropertyName(), start, modifier, out);
 
-    // TODO(khypera): Delete the below once private names are supported.
-    errorAt(start.offset(), JSMSG_FIELDS_NOT_SUPPORTED);
-    return false;
+    if (!anyCharsAccess().options().fieldsEnabledOption) {
+      errorAt(start.offset(), JSMSG_FIELDS_NOT_SUPPORTED);
+      return false;
+    }
   } else {
     newNameToken(atom->asPropertyName(), start, modifier, out);
   }

js/src/fuzz-tests/tests.cpp

@@ -42,8 +42,10 @@ static JSObject* jsfuzz_createGlobal(JSContext* cx, JSPrincipals* principals) {
   /* Create the global object. */
   JS::RootedObject newGlobal(cx);
   JS::RealmOptions options;
-  options.creationOptions().setStreamsEnabled(true);
-  options.creationOptions().setBigIntEnabled(true);
+  options.creationOptions()
+      .setStreamsEnabled(true)
+      .setBigIntEnabled(true)
+      .setFieldsEnabled(false);
   newGlobal = JS_NewGlobalObject(cx, getGlobalClass(), principals,
                                  JS::FireOnNewGlobalHook, options);
   if (!newGlobal) {

js/src/jsapi-tests/tests.cpp

@@ -79,8 +79,10 @@ JSObject* JSAPITest::createGlobal(JSPrincipals* principals) {
   /* Create the global object. */
   JS::RootedObject newGlobal(cx);
   JS::RealmOptions options;
-  options.creationOptions().setStreamsEnabled(true);
-  options.creationOptions().setBigIntEnabled(true);
+  options.creationOptions()
+      .setStreamsEnabled(true)
+      .setBigIntEnabled(true)
+      .setFieldsEnabled(true);
   newGlobal = JS_NewGlobalObject(cx, getGlobalClass(), principals,
                                  JS::FireOnNewGlobalHook, options);
   if (!newGlobal) {

js/src/jsapi.cpp

@@ -3425,6 +3425,7 @@ void JS::TransitiveCompileOptions::copyPODTransitiveOptions(
   isProbablySystemCode = rhs.isProbablySystemCode;
   hideScriptFromDebugger = rhs.hideScriptFromDebugger;
   bigIntEnabledOption = rhs.bigIntEnabledOption;
+  fieldsEnabledOption = rhs.fieldsEnabledOption;
 };
 
 void JS::ReadOnlyCompileOptions::copyPODOptions(
@@ -3552,6 +3553,7 @@ JS::CompileOptions::CompileOptions(JSContext* cx)
   throwOnAsmJSValidationFailureOption =
       cx->options().throwOnAsmJSValidationFailure();
   bigIntEnabledOption = cx->realm()->creationOptions().getBigIntEnabled();
+  fieldsEnabledOption = cx->realm()->creationOptions().getFieldsEnabled();
 }
 
 CompileOptions& CompileOptions::setIntroductionInfoToCaller(

js/src/jsapi.h

@@ -964,6 +964,7 @@ class JS_PUBLIC_API RealmCreationOptions {
         sharedMemoryAndAtomics_(false),
         streams_(false),
         bigint_(false),
+        fields_(false),
         secureContext_(false),
         clampAndJitterTime_(true) {}
 
@@ -1045,6 +1046,12 @@ class JS_PUBLIC_API RealmCreationOptions {
     return *this;
   }
 
+  bool getFieldsEnabled() const { return fields_; }
+  RealmCreationOptions& setFieldsEnabled(bool flag) {
+    fields_ = flag;
+    return *this;
+  }
+
   // This flag doesn't affect JS engine behavior.  It is used by Gecko to
   // mark whether content windows and workers are "Secure Context"s. See
   // https://w3c.github.io/webappsec-secure-contexts/
@@ -1075,6 +1082,7 @@ class JS_PUBLIC_API RealmCreationOptions {
   bool sharedMemoryAndAtomics_;
   bool streams_;
   bool bigint_;
+  bool fields_;
   bool secureContext_;
   bool clampAndJitterTime_;
 };

js/src/shell/js.cpp

@@ -503,6 +503,7 @@ static bool enableTestWasmAwaitTier2 = false;
 static bool enableAsyncStacks = false;
 static bool enableStreams = false;
 static bool enableBigInt = false;
+static bool enableFields = false;
 #ifdef JS_GC_ZEAL
 static uint32_t gZealBits = 0;
 static uint32_t gZealFrequency = 0;
@@ -3763,7 +3764,8 @@ static void SetStandardRealmOptions(JS::RealmOptions& options) {
   options.creationOptions()
       .setSharedMemoryAndAtomicsEnabled(enableSharedMemory)
       .setBigIntEnabled(enableBigInt)
-      .setStreamsEnabled(enableStreams);
+      .setStreamsEnabled(enableStreams)
+      .setFieldsEnabled(enableFields);
 }
 
 static MOZ_MUST_USE bool CheckRealmOptions(JSContext* cx,
@@ -10186,6 +10188,7 @@ static bool SetContextOptions(JSContext* cx, const OptionParser& op) {
   enableAsyncStacks = !op.getBoolOption("no-async-stacks");
   enableStreams = !op.getBoolOption("no-streams");
   enableBigInt = !op.getBoolOption("no-bigint");
+  enableFields = op.getBoolOption("enable-experimental-fields");
 
   JS::ContextOptionsRef(cx)
       .setBaseline(enableBaseline)
@@ -10897,8 +10900,8 @@ int main(int argc, char** argv, char** envp) {
       !op.addBoolOption('\0', "enable-streams",
                         "Enable WHATWG Streams (default)") ||
       !op.addBoolOption('\0', "no-streams", "Disable WHATWG Streams") ||
-      !op.addBoolOption('\0', "no-bigint",
-                        "Disable experimental BigInt support") ||
+      !op.addBoolOption('\0', "no-bigint", "Disable BigInt support") ||
+      !op.addBoolOption('\0', "enable-experimental-fields", "Enable fields in classes") ||
       !op.addStringOption('\0', "shared-memory", "on/off",
                           "SharedArrayBuffer and Atomics "
 #if SHARED_MEMORY_DEFAULT

js/xpconnect/src/XPCJSContext.cpp

@@ -760,12 +760,14 @@ bool xpc::ExtraWarningsForSystemJS() { return false; }
 static mozilla::Atomic<bool> sSharedMemoryEnabled(false);
 static mozilla::Atomic<bool> sStreamsEnabled(false);
 static mozilla::Atomic<bool> sBigIntEnabled(false);
+static mozilla::Atomic<bool> sFieldsEnabled(false);
 
 void xpc::SetPrefableRealmOptions(JS::RealmOptions& options) {
   options.creationOptions()
       .setSharedMemoryAndAtomicsEnabled(sSharedMemoryEnabled)
       .setBigIntEnabled(sBigIntEnabled)
-      .setStreamsEnabled(sStreamsEnabled);
+      .setStreamsEnabled(sStreamsEnabled)
+      .setFieldsEnabled(sFieldsEnabled);
 }
 
 static void ReloadPrefsCallback(const char* pref, XPCJSContext* xpccx) {
@@ -847,6 +849,7 @@ static void ReloadPrefsCallback(const char* pref, XPCJSContext* xpccx) {
   sSharedMemoryEnabled =
       Preferences::GetBool(JS_OPTIONS_DOT_STR "shared_memory");
   sStreamsEnabled = Preferences::GetBool(JS_OPTIONS_DOT_STR "streams");
+  sFieldsEnabled = Preferences::GetBool(JS_OPTIONS_DOT_STR "experimental.fields");
 
 #ifdef DEBUG
   sExtraWarningsForSystemJS =

modules/libpref/init/StaticPrefList.h

@@ -1113,6 +1113,12 @@ VARCACHE_PREF(
   RelaxedAtomicBool, false
 )
 
+VARCACHE_PREF(
+  "javascript.options.experimental.fields",
+   javascript_options_experimental_fields,
+  RelaxedAtomicBool, false
+)
+
 
 //---------------------------------------------------------------------------
 // Media prefs

modules/libpref/init/all.js

@@ -1603,6 +1603,8 @@ pref("javascript.options.streams", true);
 // BigInt API
 pref("javascript.options.bigint", false);
 
+pref("javascript.options.experimental.fields", false);
+
 // Dynamic module import.
 pref("javascript.options.dynamicImport", true);