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diff --git a/js/public/PropertySpec.h b/js/public/PropertySpec.h
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+++ b/js/public/PropertySpec.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Property descriptors and flags. */
+
+#ifndef js_PropertySpec_h
+#define js_PropertySpec_h
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT{,_IF}
+
+#include <stddef.h>     // size_t
+#include <stdint.h>     // uint8_t, uint16_t, int32_t, uint32_t, uintptr_t
+#include <type_traits>  // std::enable_if
+
+#include "jstypes.h"  // JS_PUBLIC_API
+
+#include "js/CallArgs.h"            // JSNative
+#include "js/PropertyDescriptor.h"  // JSPROP_*
+#include "js/RootingAPI.h"          // JS::MutableHandle
+#include "js/Symbol.h"              // JS::SymbolCode, PropertySpecNameIsSymbol
+#include "js/Value.h"               // JS::Value
+
+struct JS_PUBLIC_API JSContext;
+class JSJitInfo;
+
+/**
+ * Wrapper to relace JSNative for JSPropertySpecs and JSFunctionSpecs. This will
+ * allow us to pass one JSJitInfo per function with the property/function spec,
+ * without additional field overhead.
+ */
+struct JSNativeWrapper {
+  JSNative op = nullptr;
+  const JSJitInfo* info = nullptr;
+
+  JSNativeWrapper() = default;
+
+  JSNativeWrapper(const JSNativeWrapper& other) = default;
+
+  constexpr JSNativeWrapper(JSNative op, const JSJitInfo* info)
+      : op(op), info(info) {}
+};
+
+/**
+ * Description of a property. JS_DefineProperties and JS_InitClass take arrays
+ * of these and define many properties at once. JS_PSG, JS_PSGS and JS_PS_END
+ * are helper macros for defining such arrays.
+ */
+struct JSPropertySpec {
+  struct SelfHostedWrapper {
+    // The same type as JSNativeWrapper's first field, so that the access in
+    // JSPropertySpec::checkAccessorsAreSelfHosted become valid.
+    JSNative unused = nullptr;
+
+    const char* funname;
+
+    SelfHostedWrapper() = delete;
+
+    explicit constexpr SelfHostedWrapper(const char* funname)
+        : funname(funname) {}
+  };
+
+  struct ValueWrapper {
+    enum class Type : uint8_t { String, Int32, Double };
+    Type type;
+    union {
+      const char* string;
+      int32_t int32;
+      double double_;
+    };
+
+   private:
+    ValueWrapper() = delete;
+
+    explicit constexpr ValueWrapper(int32_t n) : type(Type::Int32), int32(n) {}
+
+    explicit constexpr ValueWrapper(const char* s)
+        : type(Type::String), string(s) {}
+
+    explicit constexpr ValueWrapper(double d)
+        : type(Type::Double), double_(d) {}
+
+   public:
+    ValueWrapper(const ValueWrapper& other) = default;
+
+    static constexpr ValueWrapper int32Value(int32_t n) {
+      return ValueWrapper(n);
+    }
+
+    static constexpr ValueWrapper stringValue(const char* s) {
+      return ValueWrapper(s);
+    }
+
+    static constexpr ValueWrapper doubleValue(double d) {
+      return ValueWrapper(d);
+    }
+  };
+
+  union Accessor {
+    JSNativeWrapper native;
+    SelfHostedWrapper selfHosted;
+
+   private:
+    Accessor() = delete;
+
+    constexpr Accessor(JSNative op, const JSJitInfo* info) : native(op, info) {}
+
+    explicit constexpr Accessor(const char* funname) : selfHosted(funname) {}
+
+   public:
+    Accessor(const Accessor& other) = default;
+
+    static constexpr Accessor nativeAccessor(JSNative op,
+                                             const JSJitInfo* info = nullptr) {
+      return Accessor(op, info);
+    }
+
+    static constexpr Accessor selfHostedAccessor(const char* funname) {
+      return Accessor(funname);
+    }
+
+    static constexpr Accessor noAccessor() {
+      return Accessor(nullptr, nullptr);
+    }
+  };
+
+  union AccessorsOrValue {
+    struct Accessors {
+      Accessor getter;
+      Accessor setter;
+
+      constexpr Accessors(Accessor getter, Accessor setter)
+          : getter(getter), setter(setter) {}
+    } accessors;
+    ValueWrapper value;
+
+   private:
+    AccessorsOrValue() = delete;
+
+    constexpr AccessorsOrValue(Accessor getter, Accessor setter)
+        : accessors(getter, setter) {}
+
+    explicit constexpr AccessorsOrValue(ValueWrapper value) : value(value) {}
+
+   public:
+    AccessorsOrValue(const AccessorsOrValue& other) = default;
+
+    static constexpr AccessorsOrValue fromAccessors(Accessor getter,
+                                                    Accessor setter) {
+      return AccessorsOrValue(getter, setter);
+    }
+
+    static constexpr AccessorsOrValue fromValue(ValueWrapper value) {
+      return AccessorsOrValue(value);
+    }
+  };
+
+  union Name {
+   private:
+    const char* string_;
+    uintptr_t symbol_;
+
+   public:
+    Name() = delete;
+
+    explicit constexpr Name(const char* str) : string_(str) {}
+    explicit constexpr Name(JS::SymbolCode symbol)
+        : symbol_(uint32_t(symbol) + 1) {}
+
+    explicit operator bool() const { return !!symbol_; }
+
+    bool isSymbol() const { return JS::PropertySpecNameIsSymbol(symbol_); }
+    JS::SymbolCode symbol() const {
+      MOZ_ASSERT(isSymbol());
+      return JS::SymbolCode(symbol_ - 1);
+    }
+
+    bool isString() const { return !isSymbol(); }
+    const char* string() const {
+      MOZ_ASSERT(isString());
+      return string_;
+    }
+  };
+
+  Name name;
+
+ private:
+  // JSPROP_* property attributes as defined in PropertyDescriptor.h.
+  uint8_t attributes_;
+
+  // Whether AccessorsOrValue below stores a value, JSNative accessors, or
+  // self-hosted accessors.
+  enum class Kind : uint8_t { Value, SelfHostedAccessor, NativeAccessor };
+  Kind kind_;
+
+ public:
+  AccessorsOrValue u;
+
+ private:
+  JSPropertySpec() = delete;
+
+  constexpr JSPropertySpec(const char* name, uint8_t attributes, Kind kind,
+                           AccessorsOrValue u)
+      : name(name), attributes_(attributes), kind_(kind), u(u) {}
+  constexpr JSPropertySpec(JS::SymbolCode name, uint8_t attributes, Kind kind,
+                           AccessorsOrValue u)
+      : name(name), attributes_(attributes), kind_(kind), u(u) {}
+
+ public:
+  JSPropertySpec(const JSPropertySpec& other) = default;
+
+  static constexpr JSPropertySpec nativeAccessors(
+      const char* name, uint8_t attributes, JSNative getter,
+      const JSJitInfo* getterInfo, JSNative setter = nullptr,
+      const JSJitInfo* setterInfo = nullptr) {
+    return JSPropertySpec(
+        name, attributes, Kind::NativeAccessor,
+        AccessorsOrValue::fromAccessors(
+            JSPropertySpec::Accessor::nativeAccessor(getter, getterInfo),
+            JSPropertySpec::Accessor::nativeAccessor(setter, setterInfo)));
+  }
+
+  static constexpr JSPropertySpec nativeAccessors(
+      JS::SymbolCode name, uint8_t attributes, JSNative getter,
+      const JSJitInfo* getterInfo, JSNative setter = nullptr,
+      const JSJitInfo* setterInfo = nullptr) {
+    return JSPropertySpec(
+        name, attributes, Kind::NativeAccessor,
+        AccessorsOrValue::fromAccessors(
+            JSPropertySpec::Accessor::nativeAccessor(getter, getterInfo),
+            JSPropertySpec::Accessor::nativeAccessor(setter, setterInfo)));
+  }
+
+  static constexpr JSPropertySpec selfHostedAccessors(
+      const char* name, uint8_t attributes, const char* getterName,
+      const char* setterName = nullptr) {
+    return JSPropertySpec(
+        name, attributes, Kind::SelfHostedAccessor,
+        AccessorsOrValue::fromAccessors(
+            JSPropertySpec::Accessor::selfHostedAccessor(getterName),
+            setterName
+                ? JSPropertySpec::Accessor::selfHostedAccessor(setterName)
+                : JSPropertySpec::Accessor::noAccessor()));
+  }
+
+  static constexpr JSPropertySpec selfHostedAccessors(
+      JS::SymbolCode name, uint8_t attributes, const char* getterName,
+      const char* setterName = nullptr) {
+    return JSPropertySpec(
+        name, attributes, Kind::SelfHostedAccessor,
+        AccessorsOrValue::fromAccessors(
+            JSPropertySpec::Accessor::selfHostedAccessor(getterName),
+            setterName
+                ? JSPropertySpec::Accessor::selfHostedAccessor(setterName)
+                : JSPropertySpec::Accessor::noAccessor()));
+  }
+
+  static constexpr JSPropertySpec int32Value(const char* name,
+                                             uint8_t attributes, int32_t n) {
+    return JSPropertySpec(name, attributes, Kind::Value,
+                          AccessorsOrValue::fromValue(
+                              JSPropertySpec::ValueWrapper::int32Value(n)));
+  }
+
+  static constexpr JSPropertySpec int32Value(JS::SymbolCode name,
+                                             uint8_t attributes, int32_t n) {
+    return JSPropertySpec(name, attributes, Kind::Value,
+                          AccessorsOrValue::fromValue(
+                              JSPropertySpec::ValueWrapper::int32Value(n)));
+  }
+
+  static constexpr JSPropertySpec stringValue(const char* name,
+                                              uint8_t attributes,
+                                              const char* s) {
+    return JSPropertySpec(name, attributes, Kind::Value,
+                          AccessorsOrValue::fromValue(
+                              JSPropertySpec::ValueWrapper::stringValue(s)));
+  }
+
+  static constexpr JSPropertySpec stringValue(JS::SymbolCode name,
+                                              uint8_t attributes,
+                                              const char* s) {
+    return JSPropertySpec(name, attributes, Kind::Value,
+                          AccessorsOrValue::fromValue(
+                              JSPropertySpec::ValueWrapper::stringValue(s)));
+  }
+
+  static constexpr JSPropertySpec doubleValue(const char* name,
+                                              uint8_t attributes, double d) {
+    return JSPropertySpec(name, attributes, Kind::Value,
+                          AccessorsOrValue::fromValue(
+                              JSPropertySpec::ValueWrapper::doubleValue(d)));
+  }
+
+  static constexpr JSPropertySpec sentinel() {
+    return JSPropertySpec(nullptr, 0, Kind::NativeAccessor,
+                          AccessorsOrValue::fromAccessors(
+                              JSPropertySpec::Accessor::noAccessor(),
+                              JSPropertySpec::Accessor::noAccessor()));
+  }
+
+  unsigned attributes() const { return attributes_; }
+
+  bool isAccessor() const {
+    return (kind_ == Kind::NativeAccessor || kind_ == Kind::SelfHostedAccessor);
+  }
+
+  JS_PUBLIC_API bool getValue(JSContext* cx,
+                              JS::MutableHandle<JS::Value> value) const;
+
+  bool isSelfHosted() const {
+    MOZ_ASSERT(isAccessor());
+#ifdef DEBUG
+    // Verify that our accessors match our Kind.
+    if (kind_ == Kind::SelfHostedAccessor) {
+      checkAccessorsAreSelfHosted();
+    } else {
+      checkAccessorsAreNative();
+    }
+#endif
+    return kind_ == Kind::SelfHostedAccessor;
+  }
+
+  static_assert(sizeof(SelfHostedWrapper) == sizeof(JSNativeWrapper),
+                "JSPropertySpec::getter/setter must be compact");
+  static_assert(offsetof(SelfHostedWrapper, unused) ==
+                        offsetof(JSNativeWrapper, op) &&
+                    offsetof(SelfHostedWrapper, funname) ==
+                        offsetof(JSNativeWrapper, info),
+                "checkAccessorsAreNative below require that "
+                "SelfHostedWrapper::funname overlay "
+                "JSNativeWrapper::info and "
+                "SelfHostedWrapper::unused overlay "
+                "JSNativeWrapper::op");
+
+ private:
+  void checkAccessorsAreNative() const {
+    // We may have a getter or a setter or both.  And whichever ones we have
+    // should not have a SelfHostedWrapper for the accessor.
+    MOZ_ASSERT_IF(u.accessors.getter.native.info, u.accessors.getter.native.op);
+    MOZ_ASSERT_IF(u.accessors.setter.native.info, u.accessors.setter.native.op);
+  }
+
+  void checkAccessorsAreSelfHosted() const {
+    MOZ_ASSERT(!u.accessors.getter.selfHosted.unused);
+    MOZ_ASSERT(!u.accessors.setter.selfHosted.unused);
+  }
+};
+
+// There can be many JSPropertySpec instances so verify the size is what we
+// expect:
+//
+// - Name (1 word)
+// - attributes_ + isAccessor_ (1 word)
+// - AccessorsOrValue (4 words, native + JSJitInfo for both getter and setter)
+static_assert(sizeof(JSPropertySpec) == 6 * sizeof(uintptr_t));
+
+template <unsigned Attributes>
+constexpr uint8_t CheckAccessorAttrs() {
+  static_assert((Attributes & ~(JSPROP_ENUMERATE | JSPROP_PERMANENT)) == 0,
+                "Unexpected flag (not JSPROP_ENUMERATE or JSPROP_PERMANENT)");
+  return uint8_t(Attributes);
+}
+
+#define JS_PSG(name, getter, attributes)                                  \
+  JSPropertySpec::nativeAccessors(name, CheckAccessorAttrs<attributes>(), \
+                                  getter, nullptr)
+#define JS_PSGS(name, getter, setter, attributes)                         \
+  JSPropertySpec::nativeAccessors(name, CheckAccessorAttrs<attributes>(), \
+                                  getter, nullptr, setter, nullptr)
+#define JS_SYM_GET(symbol, getter, attributes)                              \
+  JSPropertySpec::nativeAccessors(::JS::SymbolCode::symbol,                 \
+                                  CheckAccessorAttrs<attributes>(), getter, \
+                                  nullptr)
+#define JS_SELF_HOSTED_GET(name, getterName, attributes)                      \
+  JSPropertySpec::selfHostedAccessors(name, CheckAccessorAttrs<attributes>(), \
+                                      getterName)
+#define JS_SELF_HOSTED_GETSET(name, getterName, setterName, attributes)       \
+  JSPropertySpec::selfHostedAccessors(name, CheckAccessorAttrs<attributes>(), \
+                                      getterName, setterName)
+#define JS_SELF_HOSTED_SYM_GET(symbol, getterName, attributes) \
+  JSPropertySpec::selfHostedAccessors(                         \
+      ::JS::SymbolCode::symbol, CheckAccessorAttrs<attributes>(), getterName)
+#define JS_STRING_PS(name, string, attributes) \
+  JSPropertySpec::stringValue(name, attributes, string)
+#define JS_STRING_SYM_PS(symbol, string, attributes) \
+  JSPropertySpec::stringValue(::JS::SymbolCode::symbol, attributes, string)
+#define JS_INT32_PS(name, value, attributes) \
+  JSPropertySpec::int32Value(name, attributes, value)
+#define JS_DOUBLE_PS(name, value, attributes) \
+  JSPropertySpec::doubleValue(name, attributes, value)
+#define JS_PS_END JSPropertySpec::sentinel()
+
+/**
+ * To define a native function, set call to a JSNativeWrapper. To define a
+ * self-hosted function, set selfHostedName to the name of a function
+ * compiled during JSRuntime::initSelfHosting.
+ */
+struct JSFunctionSpec {
+  using Name = JSPropertySpec::Name;
+
+  Name name;
+  JSNativeWrapper call;
+  uint16_t nargs;
+  uint16_t flags;
+  const char* selfHostedName;
+
+  // JSPROP_* property attributes as defined in PropertyDescriptor.h
+  unsigned attributes() const { return flags; }
+};
+
+/*
+ * Terminating sentinel initializer to put at the end of a JSFunctionSpec array
+ * that's passed to JS_DefineFunctions or JS_InitClass.
+ */
+#define JS_FS_END JS_FN(nullptr, nullptr, 0, 0)
+
+/*
+ * Initializer macros for a JSFunctionSpec array element. JS_FNINFO allows the
+ * simple adding of JSJitInfos. JS_SELF_HOSTED_FN declares a self-hosted
+ * function. JS_INLINABLE_FN allows specifying an InlinableNative enum value for
+ * natives inlined or specialized by the JIT. Finally JS_FNSPEC has slots for
+ * all the fields.
+ *
+ * The _SYM variants allow defining a function with a symbol key rather than a
+ * string key. For example, use JS_SYM_FN(iterator, ...) to define an
+ * @@iterator method.
+ */
+#define JS_FN(name, call, nargs, flags) \
+  JS_FNSPEC(name, call, nullptr, nargs, flags, nullptr)
+#define JS_INLINABLE_FN(name, call, nargs, flags, native) \
+  JS_FNSPEC(name, call, &js::jit::JitInfo_##native, nargs, flags, nullptr)
+#define JS_SYM_FN(symbol, call, nargs, flags) \
+  JS_SYM_FNSPEC(symbol, call, nullptr, nargs, flags, nullptr)
+#define JS_FNINFO(name, call, info, nargs, flags) \
+  JS_FNSPEC(name, call, info, nargs, flags, nullptr)
+#define JS_SELF_HOSTED_FN(name, selfHostedName, nargs, flags) \
+  JS_FNSPEC(name, nullptr, nullptr, nargs, flags, selfHostedName)
+#define JS_SELF_HOSTED_SYM_FN(symbol, selfHostedName, nargs, flags) \
+  JS_SYM_FNSPEC(symbol, nullptr, nullptr, nargs, flags, selfHostedName)
+#define JS_SYM_FNSPEC(symbol, call, info, nargs, flags, selfHostedName) \
+  JS_FNSPEC(::JS::SymbolCode::symbol, call, info, nargs, flags, selfHostedName)
+#define JS_FNSPEC(name, call, info, nargs, flags, selfHostedName) \
+  { JSFunctionSpec::Name(name), {call, info}, nargs, flags, selfHostedName }
+
+#endif  // js_PropertySpec_h