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diff --git a/js/public/CallArgs.h b/js/public/CallArgs.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/. */
+
+/*
+ * [SMDOC] JS::CallArgs API
+ *
+ * Helper classes encapsulating access to the callee, |this| value, arguments,
+ * and argument count for a call/construct operation.
+ *
+ * JS::CallArgs encapsulates access to a JSNative's un-abstracted
+ * |unsigned argc, Value* vp| arguments.  The principal way to create a
+ * JS::CallArgs is using JS::CallArgsFromVp:
+ *
+ *   // If provided no arguments or a non-numeric first argument, return zero.
+ *   // Otherwise return |this| exactly as given, without boxing.
+ *   static bool
+ *   Func(JSContext* cx, unsigned argc, JS::Value* vp)
+ *   {
+ *       JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+ *
+ *       // Guard against no arguments or a non-numeric arg0.
+ *       if (args.length() == 0 || !args[0].isNumber()) {
+ *           args.rval().setInt32(0);
+ *           return true;
+ *       }
+ *
+ *       // Access to the callee must occur before accessing/setting
+ *       // the return value.
+ *       JSObject& callee = args.callee();
+ *       args.rval().setObject(callee);
+ *
+ *       // callee() and calleev() will now assert.
+ *
+ *       // It's always fine to access thisv().
+ *       HandleValue thisv = args.thisv();
+ *       args.rval().set(thisv);
+ *
+ *       // As the return value was last set to |this|, returns |this|.
+ *       return true;
+ *   }
+ *
+ * CallArgs is exposed publicly and used internally.  Not all parts of its
+ * public interface are meant to be used by embedders!  See inline comments to
+ * for details.
+ *
+ * It's possible (albeit deprecated) to manually index into |vp| to access the
+ * callee, |this|, and arguments of a function, and to set its return value.
+ * This does not have the error-handling or moving-GC correctness of CallArgs.
+ * New code should use CallArgs instead whenever possible.
+ *
+ * The eventual plan is to change JSNative to take |const CallArgs&| directly,
+ * for automatic assertion of correct use and to make calling functions more
+ * efficient.  Embedders should start internally switching away from using
+ * |argc| and |vp| directly, except to create a |CallArgs|.  Then, when an
+ * eventual release making that change occurs, porting efforts will require
+ * changing methods' signatures but won't require invasive changes to the
+ * methods' implementations, potentially under time pressure.
+ */
+
+#ifndef js_CallArgs_h
+#define js_CallArgs_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+
+#include <type_traits>
+
+#include "jstypes.h"
+
+#include "js/RootingAPI.h"
+#include "js/Value.h"
+
+/* Typedef for native functions called by the JS VM. */
+using JSNative = bool (*)(JSContext* cx, unsigned argc, JS::Value* vp);
+
+namespace JS {
+
+extern JS_PUBLIC_DATA const HandleValue UndefinedHandleValue;
+
+namespace detail {
+
+/*
+ * Compute |this| for the |vp| inside a JSNative, either boxing primitives or
+ * replacing with the global object as necessary.
+ */
+extern JS_PUBLIC_API bool ComputeThis(JSContext* cx, JS::Value* vp,
+                                      MutableHandleObject thisObject);
+
+#ifdef JS_DEBUG
+extern JS_PUBLIC_API void CheckIsValidConstructible(const Value& v);
+#endif
+
+class MOZ_STACK_CLASS IncludeUsedRval {
+  mutable bool usedRval_;
+
+ public:
+  bool usedRval() const { return usedRval_; }
+  void setUsedRval() const { usedRval_ = true; }
+  void clearUsedRval() const { usedRval_ = false; }
+  void assertUnusedRval() const { MOZ_ASSERT(!usedRval_); }
+};
+
+class MOZ_STACK_CLASS NoUsedRval {
+ public:
+  bool usedRval() const { return false; }
+  void setUsedRval() const {}
+  void clearUsedRval() const {}
+  void assertUnusedRval() const {}
+};
+
+template <class WantUsedRval>
+class MOZ_STACK_CLASS CallArgsBase {
+  static_assert(std::is_same_v<WantUsedRval, IncludeUsedRval> ||
+                    std::is_same_v<WantUsedRval, NoUsedRval>,
+                "WantUsedRval can only be IncludeUsedRval or NoUsedRval");
+
+ protected:
+  Value* argv_;
+  unsigned argc_;
+  bool constructing_ : 1;
+
+  // True if the caller does not use the return value.
+  bool ignoresReturnValue_ : 1;
+
+#ifdef JS_DEBUG
+  WantUsedRval wantUsedRval_;
+  bool usedRval() const { return wantUsedRval_.usedRval(); }
+  void setUsedRval() const { wantUsedRval_.setUsedRval(); }
+  void clearUsedRval() const { wantUsedRval_.clearUsedRval(); }
+  void assertUnusedRval() const { wantUsedRval_.assertUnusedRval(); }
+#else
+  bool usedRval() const { return false; }
+  void setUsedRval() const {}
+  void clearUsedRval() const {}
+  void assertUnusedRval() const {}
+#endif
+
+ public:
+  // CALLEE ACCESS
+
+  /*
+   * Returns the function being called, as a value.  Must not be called after
+   * rval() has been used!
+   */
+  HandleValue calleev() const {
+    this->assertUnusedRval();
+    return HandleValue::fromMarkedLocation(&argv_[-2]);
+  }
+
+  /*
+   * Returns the function being called, as an object.  Must not be called
+   * after rval() has been used!
+   */
+  JSObject& callee() const { return calleev().toObject(); }
+
+  // CALLING/CONSTRUCTING-DIFFERENTIATIONS
+
+  bool isConstructing() const {
+    if (!argv_[-1].isMagic()) {
+      return false;
+    }
+
+#ifdef JS_DEBUG
+    if (!this->usedRval()) {
+      CheckIsValidConstructible(calleev());
+    }
+#endif
+
+    return true;
+  }
+
+  bool ignoresReturnValue() const { return ignoresReturnValue_; }
+
+  MutableHandleValue newTarget() const {
+    MOZ_ASSERT(constructing_);
+    return MutableHandleValue::fromMarkedLocation(&this->argv_[argc_]);
+  }
+
+  /*
+   * Returns the |this| value passed to the function.  This method must not
+   * be called when the function is being called as a constructor via |new|.
+   * The value may or may not be an object: it is the individual function's
+   * responsibility to box the value if needed.
+   */
+  HandleValue thisv() const {
+    // Some internal code uses thisv() in constructing cases, so don't do
+    // this yet.
+    // MOZ_ASSERT(!argv_[-1].isMagic(JS_IS_CONSTRUCTING));
+    return HandleValue::fromMarkedLocation(&argv_[-1]);
+  }
+
+  bool computeThis(JSContext* cx, MutableHandleObject thisObject) const {
+    if (thisv().isObject()) {
+      thisObject.set(&thisv().toObject());
+      return true;
+    }
+
+    return ComputeThis(cx, base(), thisObject);
+  }
+
+  // ARGUMENTS
+
+  /* Returns the number of arguments. */
+  unsigned length() const { return argc_; }
+
+  /* Returns the i-th zero-indexed argument. */
+  MutableHandleValue operator[](unsigned i) const {
+    MOZ_ASSERT(i < argc_);
+    return MutableHandleValue::fromMarkedLocation(&this->argv_[i]);
+  }
+
+  /*
+   * Returns the i-th zero-indexed argument, or |undefined| if there's no
+   * such argument.
+   */
+  HandleValue get(unsigned i) const {
+    return i < length() ? HandleValue::fromMarkedLocation(&this->argv_[i])
+                        : UndefinedHandleValue;
+  }
+
+  /*
+   * Returns true if the i-th zero-indexed argument is present and is not
+   * |undefined|.
+   */
+  bool hasDefined(unsigned i) const {
+    return i < argc_ && !this->argv_[i].isUndefined();
+  }
+
+  // RETURN VALUE
+
+  /*
+   * Returns the currently-set return value.  The initial contents of this
+   * value are unspecified.  Once this method has been called, callee() and
+   * calleev() can no longer be used.  (If you're compiling against a debug
+   * build of SpiderMonkey, these methods will assert to aid debugging.)
+   *
+   * If the method you're implementing succeeds by returning true, you *must*
+   * set this.  (SpiderMonkey doesn't currently assert this, but it will do
+   * so eventually.)  You don't need to use or change this if your method
+   * fails.
+   */
+  MutableHandleValue rval() const {
+    this->setUsedRval();
+    return MutableHandleValue::fromMarkedLocation(&argv_[-2]);
+  }
+
+  /*
+   * Returns true if there are at least |required| arguments passed in. If
+   * false, it reports an error message on the context.
+   */
+  JS_PUBLIC_API inline bool requireAtLeast(JSContext* cx, const char* fnname,
+                                           unsigned required) const;
+
+ public:
+  // These methods are publicly exposed, but they are *not* to be used when
+  // implementing a JSNative method and encapsulating access to |vp| within
+  // it.  You probably don't want to use these!
+
+  void setCallee(const Value& aCalleev) const {
+    this->clearUsedRval();
+    argv_[-2] = aCalleev;
+  }
+
+  void setThis(const Value& aThisv) const { argv_[-1] = aThisv; }
+
+  MutableHandleValue mutableThisv() const {
+    return MutableHandleValue::fromMarkedLocation(&argv_[-1]);
+  }
+
+ public:
+  // These methods are publicly exposed, but we're unsure of the interfaces
+  // (because they're hackish and drop assertions).  Avoid using these if you
+  // can.
+
+  Value* array() const { return argv_; }
+  Value* end() const { return argv_ + argc_ + constructing_; }
+
+ public:
+  // These methods are only intended for internal use.  Embedders shouldn't
+  // use them!
+
+  Value* base() const { return argv_ - 2; }
+
+  Value* spAfterCall() const {
+    this->setUsedRval();
+    return argv_ - 1;
+  }
+};
+
+}  // namespace detail
+
+class MOZ_STACK_CLASS CallArgs
+    : public detail::CallArgsBase<detail::IncludeUsedRval> {
+ private:
+  friend CallArgs CallArgsFromVp(unsigned argc, Value* vp);
+  friend CallArgs CallArgsFromSp(unsigned stackSlots, Value* sp,
+                                 bool constructing, bool ignoresReturnValue);
+
+  static CallArgs create(unsigned argc, Value* argv, bool constructing,
+                         bool ignoresReturnValue = false) {
+    CallArgs args;
+    args.clearUsedRval();
+    args.argv_ = argv;
+    args.argc_ = argc;
+    args.constructing_ = constructing;
+    args.ignoresReturnValue_ = ignoresReturnValue;
+#ifdef DEBUG
+    AssertValueIsNotGray(args.thisv());
+    AssertValueIsNotGray(args.calleev());
+    for (unsigned i = 0; i < argc; ++i) {
+      AssertValueIsNotGray(argv[i]);
+    }
+#endif
+    return args;
+  }
+
+ public:
+  /*
+   * Helper for requireAtLeast to report the actual exception.  Public
+   * so we can call it from CallArgsBase and not need multiple
+   * per-template instantiations of it.
+   */
+  static JS_PUBLIC_API void reportMoreArgsNeeded(JSContext* cx,
+                                                 const char* fnname,
+                                                 unsigned required,
+                                                 unsigned actual);
+};
+
+namespace detail {
+template <class WantUsedRval>
+JS_PUBLIC_API inline bool CallArgsBase<WantUsedRval>::requireAtLeast(
+    JSContext* cx, const char* fnname, unsigned required) const {
+  if (MOZ_LIKELY(required <= length())) {
+    return true;
+  }
+
+  CallArgs::reportMoreArgsNeeded(cx, fnname, required, length());
+  return false;
+}
+}  // namespace detail
+
+MOZ_ALWAYS_INLINE CallArgs CallArgsFromVp(unsigned argc, Value* vp) {
+  return CallArgs::create(argc, vp + 2, vp[1].isMagic(JS_IS_CONSTRUCTING));
+}
+
+// This method is only intended for internal use in SpiderMonkey.  We may
+// eventually move it to an internal header.  Embedders should use
+// JS::CallArgsFromVp!
+MOZ_ALWAYS_INLINE CallArgs CallArgsFromSp(unsigned stackSlots, Value* sp,
+                                          bool constructing = false,
+                                          bool ignoresReturnValue = false) {
+  return CallArgs::create(stackSlots - constructing, sp - stackSlots,
+                          constructing, ignoresReturnValue);
+}
+
+}  // namespace JS
+
+#endif /* js_CallArgs_h */