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diff --git a/js/public/Id.h b/js/public/Id.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * 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/. */
+
+#ifndef js_Id_h
+#define js_Id_h
+
+// [SMDOC] Property Key / JSID
+//
+// A jsid is an identifier for a property or method of an object which is
+// either a 31-bit unsigned integer, interned string or symbol.
+//
+// Also, there is an additional jsid value, JSID_VOID, which does not occur in
+// JS scripts but may be used to indicate the absence of a valid jsid.  A void
+// jsid is not a valid id and only arises as an exceptional API return value,
+// such as in JS_NextProperty. Embeddings must not pass JSID_VOID into JSAPI
+// entry points expecting a jsid and do not need to handle JSID_VOID in hooks
+// receiving a jsid except when explicitly noted in the API contract.
+//
+// A jsid is not implicitly convertible to or from a Value; JS_ValueToId or
+// JS_IdToValue must be used instead.
+
+#include "mozilla/Maybe.h"
+
+#include "jstypes.h"
+
+#include "js/HeapAPI.h"
+#include "js/RootingAPI.h"
+#include "js/TypeDecls.h"
+#include "js/Utility.h"
+
+// All jsids with the low bit set are integer ids. This means the other type
+// tags must all be even.
+#define JSID_TYPE_INT_BIT 0x1
+
+// Use 0 for JSID_TYPE_STRING to avoid a bitwise op for atom <-> id conversions.
+#define JSID_TYPE_STRING 0x0
+#define JSID_TYPE_VOID 0x2
+#define JSID_TYPE_SYMBOL 0x4
+#define JSID_TYPE_EMPTY 0x6
+#define JSID_TYPE_MASK 0x7
+
+namespace JS {
+
+enum class SymbolCode : uint32_t;
+
+struct PropertyKey {
+  size_t asBits;
+
+  constexpr PropertyKey() : asBits(JSID_TYPE_VOID) {}
+
+  static constexpr MOZ_ALWAYS_INLINE PropertyKey fromRawBits(size_t bits) {
+    PropertyKey id;
+    id.asBits = bits;
+    return id;
+  }
+
+  bool operator==(const PropertyKey& rhs) const { return asBits == rhs.asBits; }
+  bool operator!=(const PropertyKey& rhs) const { return asBits != rhs.asBits; }
+
+  MOZ_ALWAYS_INLINE bool isInt() const {
+    return !!(asBits & JSID_TYPE_INT_BIT);
+  }
+
+  MOZ_ALWAYS_INLINE bool isString() const {
+    return (asBits & JSID_TYPE_MASK) == JSID_TYPE_STRING;
+  }
+
+  MOZ_ALWAYS_INLINE bool isSymbol() const {
+    return (asBits & JSID_TYPE_MASK) == JSID_TYPE_SYMBOL;
+  }
+
+  MOZ_ALWAYS_INLINE bool isGCThing() const { return isString() || isSymbol(); }
+
+  MOZ_ALWAYS_INLINE int32_t toInt() const {
+    MOZ_ASSERT(isInt());
+    uint32_t bits = static_cast<uint32_t>(asBits) >> 1;
+    return static_cast<int32_t>(bits);
+  }
+
+  MOZ_ALWAYS_INLINE JSString* toString() const {
+    MOZ_ASSERT(isString());
+    // Use XOR instead of `& ~JSID_TYPE_MASK` because small immediates can be
+    // encoded more efficiently on some platorms.
+    return reinterpret_cast<JSString*>(asBits ^ JSID_TYPE_STRING);
+  }
+
+  MOZ_ALWAYS_INLINE JS::Symbol* toSymbol() const {
+    MOZ_ASSERT(isSymbol());
+    return reinterpret_cast<JS::Symbol*>(asBits ^ JSID_TYPE_SYMBOL);
+  }
+
+  js::gc::Cell* toGCThing() const {
+    MOZ_ASSERT(isGCThing());
+    return reinterpret_cast<js::gc::Cell*>(asBits & ~(size_t)JSID_TYPE_MASK);
+  }
+
+  GCCellPtr toGCCellPtr() const {
+    js::gc::Cell* thing = toGCThing();
+    if (isString()) {
+      return JS::GCCellPtr(thing, JS::TraceKind::String);
+    }
+    MOZ_ASSERT(isSymbol());
+    return JS::GCCellPtr(thing, JS::TraceKind::Symbol);
+  }
+
+  bool isPrivateName() const;
+
+  bool isWellKnownSymbol(JS::SymbolCode code) const;
+
+  // This API can be used by embedders to convert pinned (aka interned) strings,
+  // as created by JS_AtomizeAndPinJSString, into PropertyKeys.
+  // This means the string does not have to be explicitly rooted.
+  //
+  // Only use this API when absolutely necessary, otherwise use JS_StringToId.
+  static PropertyKey fromPinnedString(JSString* str);
+
+  // Must not be used on atoms that are representable as integer PropertyKey.
+  // Prefer NameToId or AtomToId over this function:
+  //
+  // A PropertyName is an atom that does not contain an integer in the range
+  // [0, UINT32_MAX]. However, PropertyKey can only hold an integer in the range
+  // [0, JSID_INT_MAX] (where JSID_INT_MAX == 2^31-1).  Thus, for the range of
+  // integers (JSID_INT_MAX, UINT32_MAX], to represent as a 'id', it must be
+  // the case id.isString() and id.toString()->isIndex(). In most
+  // cases when creating a PropertyKey, code does not have to care about
+  // this corner case because:
+  //
+  // - When given an arbitrary JSAtom*, AtomToId must be used, which checks for
+  //   integer atoms representable as integer PropertyKey, and does this
+  //   conversion.
+  //
+  // - When given a PropertyName*, NameToId can be used which does not need
+  //   to do any dynamic checks.
+  //
+  // Thus, it is only the rare third case which needs this function, which
+  // handles any JSAtom* that is known not to be representable with an int
+  // PropertyKey.
+  static PropertyKey fromNonIntAtom(JSAtom* atom) {
+    MOZ_ASSERT((size_t(atom) & JSID_TYPE_MASK) == 0);
+    MOZ_ASSERT(PropertyKey::isNonIntAtom(atom));
+    return PropertyKey::fromRawBits(size_t(atom) | JSID_TYPE_STRING);
+  }
+
+  // The JSAtom/JSString type exposed to embedders is opaque.
+  static PropertyKey fromNonIntAtom(JSString* str) {
+    MOZ_ASSERT((size_t(str) & JSID_TYPE_MASK) == 0);
+    MOZ_ASSERT(PropertyKey::isNonIntAtom(str));
+    return PropertyKey::fromRawBits(size_t(str) | JSID_TYPE_STRING);
+  }
+
+ private:
+  static bool isNonIntAtom(JSAtom* atom);
+  static bool isNonIntAtom(JSString* atom);
+} JS_HAZ_GC_POINTER;
+
+}  // namespace JS
+
+using jsid = JS::PropertyKey;
+
+#define JSID_BITS(id) (id.asBits)
+
+static MOZ_ALWAYS_INLINE bool JSID_IS_STRING(jsid id) { return id.isString(); }
+
+static MOZ_ALWAYS_INLINE JSString* JSID_TO_STRING(jsid id) {
+  return id.toString();
+}
+
+static MOZ_ALWAYS_INLINE bool JSID_IS_INT(jsid id) { return id.isInt(); }
+
+static MOZ_ALWAYS_INLINE int32_t JSID_TO_INT(jsid id) { return id.toInt(); }
+
+#define JSID_INT_MIN 0
+#define JSID_INT_MAX INT32_MAX
+
+static MOZ_ALWAYS_INLINE bool INT_FITS_IN_JSID(int32_t i) { return i >= 0; }
+
+static MOZ_ALWAYS_INLINE jsid INT_TO_JSID(int32_t i) {
+  jsid id;
+  MOZ_ASSERT(INT_FITS_IN_JSID(i));
+  uint32_t bits = (static_cast<uint32_t>(i) << 1) | JSID_TYPE_INT_BIT;
+  JSID_BITS(id) = static_cast<size_t>(bits);
+  return id;
+}
+
+static MOZ_ALWAYS_INLINE bool JSID_IS_SYMBOL(jsid id) { return id.isSymbol(); }
+
+static MOZ_ALWAYS_INLINE JS::Symbol* JSID_TO_SYMBOL(jsid id) {
+  return id.toSymbol();
+}
+
+static MOZ_ALWAYS_INLINE jsid SYMBOL_TO_JSID(JS::Symbol* sym) {
+  jsid id;
+  MOZ_ASSERT(sym != nullptr);
+  MOZ_ASSERT((size_t(sym) & JSID_TYPE_MASK) == 0);
+  MOZ_ASSERT(!js::gc::IsInsideNursery(reinterpret_cast<js::gc::Cell*>(sym)));
+  JSID_BITS(id) = (size_t(sym) | JSID_TYPE_SYMBOL);
+  return id;
+}
+
+static MOZ_ALWAYS_INLINE bool JSID_IS_VOID(const jsid id) {
+  MOZ_ASSERT_IF((JSID_BITS(id) & JSID_TYPE_MASK) == JSID_TYPE_VOID,
+                JSID_BITS(id) == JSID_TYPE_VOID);
+  return JSID_BITS(id) == JSID_TYPE_VOID;
+}
+
+static MOZ_ALWAYS_INLINE bool JSID_IS_EMPTY(const jsid id) {
+  MOZ_ASSERT_IF((JSID_BITS(id) & JSID_TYPE_MASK) == JSID_TYPE_EMPTY,
+                JSID_BITS(id) == JSID_TYPE_EMPTY);
+  return JSID_BITS(id) == JSID_TYPE_EMPTY;
+}
+
+constexpr const jsid JSID_VOID;
+constexpr const jsid JSID_EMPTY = jsid::fromRawBits(JSID_TYPE_EMPTY);
+
+extern JS_PUBLIC_DATA const JS::HandleId JSID_VOIDHANDLE;
+extern JS_PUBLIC_DATA const JS::HandleId JSID_EMPTYHANDLE;
+
+namespace JS {
+
+template <>
+struct GCPolicy<jsid> {
+  static void trace(JSTracer* trc, jsid* idp, const char* name) {
+    // It's not safe to trace unbarriered pointers except as part of root
+    // marking.
+    UnsafeTraceRoot(trc, idp, name);
+  }
+  static bool isValid(jsid id) {
+    return !id.isGCThing() ||
+           js::gc::IsCellPointerValid(id.toGCCellPtr().asCell());
+  }
+
+  static bool isTenured(jsid id) {
+    MOZ_ASSERT_IF(id.isGCThing(),
+                  !js::gc::IsInsideNursery(id.toGCCellPtr().asCell()));
+    return true;
+  }
+};
+
+#ifdef DEBUG
+MOZ_ALWAYS_INLINE void AssertIdIsNotGray(jsid id) {
+  if (id.isGCThing()) {
+    AssertCellIsNotGray(id.toGCCellPtr().asCell());
+  }
+}
+#endif
+
+}  // namespace JS
+
+namespace js {
+
+template <>
+struct BarrierMethods<jsid> {
+  static gc::Cell* asGCThingOrNull(jsid id) {
+    if (JSID_IS_STRING(id)) {
+      return reinterpret_cast<gc::Cell*>(JSID_TO_STRING(id));
+    }
+    if (JSID_IS_SYMBOL(id)) {
+      return reinterpret_cast<gc::Cell*>(JSID_TO_SYMBOL(id));
+    }
+    return nullptr;
+  }
+  static void postWriteBarrier(jsid* idp, jsid prev, jsid next) {
+    MOZ_ASSERT_IF(JSID_IS_STRING(next),
+                  !gc::IsInsideNursery(JSID_TO_STRING(next)));
+  }
+  static void exposeToJS(jsid id) {
+    if (id.isGCThing()) {
+      js::gc::ExposeGCThingToActiveJS(id.toGCCellPtr());
+    }
+  }
+};
+
+// If the jsid is a GC pointer type, convert to that type and call |f| with the
+// pointer and return the result wrapped in a Maybe, otherwise return None().
+template <typename F>
+auto MapGCThingTyped(const jsid& id, F&& f) {
+  if (JSID_IS_STRING(id)) {
+    return mozilla::Some(f(JSID_TO_STRING(id)));
+  }
+  if (JSID_IS_SYMBOL(id)) {
+    return mozilla::Some(f(JSID_TO_SYMBOL(id)));
+  }
+  MOZ_ASSERT(!id.isGCThing());
+  using ReturnType = decltype(f(static_cast<JSString*>(nullptr)));
+  return mozilla::Maybe<ReturnType>();
+}
+
+// If the jsid is a GC pointer type, convert to that type and call |f| with the
+// pointer. Return whether this happened.
+template <typename F>
+bool ApplyGCThingTyped(const jsid& id, F&& f) {
+  return MapGCThingTyped(id,
+                         [&f](auto t) {
+                           f(t);
+                           return true;
+                         })
+      .isSome();
+}
+
+template <typename Wrapper>
+class WrappedPtrOperations<JS::PropertyKey, Wrapper> {
+  const JS::PropertyKey& id() const {
+    return static_cast<const Wrapper*>(this)->get();
+  }
+
+ public:
+  bool isInt() const { return id().isInt(); }
+  bool isString() const { return id().isString(); }
+  bool isSymbol() const { return id().isSymbol(); }
+  bool isGCThing() const { return id().isGCThing(); }
+
+  int32_t toInt() const { return id().toInt(); }
+  JSString* toString() const { return id().toString(); }
+  JS::Symbol* toSymbol() const { return id().toSymbol(); }
+
+  bool isPrivateName() const { return id().isPrivateName(); }
+
+  bool isWellKnownSymbol(JS::SymbolCode code) const {
+    return id().isWellKnownSymbol(code);
+  }
+};
+
+}  // namespace js
+
+#endif /* js_Id_h */