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diff --git a/mfbt/AlreadyAddRefed.h b/mfbt/AlreadyAddRefed.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/. */
+
+/* Typed temporary pointers for reference-counted smart pointers. */
+
+#ifndef AlreadyAddRefed_h
+#define AlreadyAddRefed_h
+
+#include <utility>
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+struct unused_t;
+
+}  // namespace mozilla
+
+/**
+ * already_AddRefed cooperates with reference counting smart pointers to enable
+ * you to assign in a pointer _without_ |AddRef|ing it.  You might want to use
+ * this as a return type from a function that returns an already |AddRef|ed
+ * pointer.  Or, you might want to use this as a parameter type in a function
+ * that wants to force a transfer-of-ownership from a RefPtr in the caller (for
+ * example, if the function expects callers to pass in a newly-created object,
+ * which the function then takes ownership of).
+ *
+ * TODO Move already_AddRefed to namespace mozilla.  This has not yet been done
+ * because of the sheer number of usages of already_AddRefed.
+ *
+ * When should you use already_AddRefed<>?
+ * * Ensure a consumer takes ownership of a reference
+ * * Pass ownership without calling AddRef/Release (sometimes required in
+ *   off-main-thread code)
+ * * The ref pointer type you're using doesn't support move construction
+ *
+ * Otherwise, use std::move(RefPtr/nsCOMPtr/etc).
+ */
+template <class T>
+struct
+#if !defined(MOZ_CLANG_PLUGIN) && !defined(XGILL_PLUGIN)
+    [[nodiscard]]
+#endif
+    MOZ_NON_AUTOABLE already_AddRefed {
+  already_AddRefed() : mRawPtr(nullptr) {}
+
+  // For simplicity, allow returning nullptr from functions returning
+  // already_AddRefed<T>. Don't permit returning raw T*, though; it's preferred
+  // to create already_AddRefed<T> from a reference-counting smart pointer.
+  MOZ_IMPLICIT already_AddRefed(decltype(nullptr)) : mRawPtr(nullptr) {}
+  explicit already_AddRefed(T* aRawPtr) : mRawPtr(aRawPtr) {}
+
+  // Disallow copy constructor and copy assignment operator: move semantics used
+  // instead.
+  already_AddRefed(const already_AddRefed<T>& aOther) = delete;
+  already_AddRefed<T>& operator=(const already_AddRefed<T>& aOther) = delete;
+
+  // WARNING: sketchiness ahead.
+  //
+  // The x86-64 ABI for Unix-like operating systems requires structures to be
+  // returned via invisible reference if they are non-trivial for the purposes
+  // of calls according to the C++ ABI[1].  For our consideration here, that
+  // means that if we have a non-trivial move constructor or destructor,
+  // already_AddRefed must be returned by invisible reference.  But
+  // already_AddRefed is small enough and so commonly used that it would be
+  // beneficial to return it via registers instead.  So we need to figure out
+  // a way to make the move constructor and the destructor trivial.
+  //
+  // Our destructor is normally non-trivial, because it asserts that the
+  // stored pointer has been taken by somebody else prior to destruction.
+  // However, since the assert in question is compiled only for DEBUG builds,
+  // we can make the destructor trivial in non-DEBUG builds by simply defining
+  // it with `= default`.
+  //
+  // We now have to make the move constructor trivial as well.  It is normally
+  // non-trivial, because the incoming object has its pointer null-ed during
+  // the move. This null-ing is done to satisfy the assert in the destructor.
+  // But since that destructor has no assert in non-DEBUG builds, the clearing
+  // is unnecessary in such builds; all we really need to perform is a copy of
+  // the pointer from the incoming object.  So we can let the compiler define
+  // a trivial move constructor for us, and already_AddRefed can now be
+  // returned in registers rather than needing to allocate a stack slot for
+  // an invisible reference.
+  //
+  // The above considerations apply to Unix-like operating systems only; the
+  // conditions for the same optimization to apply on x86-64 Windows are much
+  // more strigent and are basically impossible for already_AddRefed to
+  // satisfy[2].  But we do get some benefit from this optimization on Windows
+  // because we removed the nulling of the pointer during the move, so that's
+  // a codesize win.
+  //
+  // [1] https://itanium-cxx-abi.github.io/cxx-abi/abi.html#non-trivial
+  // [2] https://docs.microsoft.com/en-us/cpp/build/return-values-cpp
+
+  already_AddRefed(already_AddRefed<T>&& aOther)
+#ifdef DEBUG
+      : mRawPtr(aOther.take()){}
+#else
+      = default;
+#endif
+
+        already_AddRefed<T> &
+        operator=(already_AddRefed<T>&& aOther) {
+    mRawPtr = aOther.take();
+    return *this;
+  }
+
+  /**
+   * This helper is useful in cases like
+   *
+   *  already_AddRefed<BaseClass>
+   *  Foo()
+   *  {
+   *    RefPtr<SubClass> x = ...;
+   *    return x.forget();
+   *  }
+   *
+   * The autoconversion allows one to omit the idiom
+   *
+   *    RefPtr<BaseClass> y = x.forget();
+   *    return y.forget();
+   *
+   * Note that nsRefPtr is the XPCOM reference counting smart pointer class.
+   */
+  template <typename U>
+  MOZ_IMPLICIT already_AddRefed(already_AddRefed<U>&& aOther)
+      : mRawPtr(aOther.take()) {}
+
+  ~already_AddRefed()
+#ifdef DEBUG
+  {
+    MOZ_ASSERT(!mRawPtr);
+  }
+#else
+      = default;
+#endif
+
+  // Specialize the unused operator<< for already_AddRefed, to allow
+  // nsCOMPtr<nsIFoo> foo;
+  // Unused << foo.forget();
+  // Note that nsCOMPtr is the XPCOM reference counting smart pointer class.
+  friend void operator<<(const mozilla::unused_t& aUnused,
+                         const already_AddRefed<T>& aRhs) {
+    auto mutableAlreadyAddRefed = const_cast<already_AddRefed<T>*>(&aRhs);
+    aUnused << mutableAlreadyAddRefed->take();
+  }
+
+  [[nodiscard]] T* take() {
+    T* rawPtr = mRawPtr;
+    mRawPtr = nullptr;
+    return rawPtr;
+  }
+
+  /**
+   * This helper provides a static_cast replacement for already_AddRefed, so
+   * if you have
+   *
+   *   already_AddRefed<Parent> F();
+   *
+   * you can write
+   *
+   *   already_AddRefed<Child>
+   *   G()
+   *   {
+   *     return F().downcast<Child>();
+   *   }
+   */
+  template <class U>
+  already_AddRefed<U> downcast() {
+    U* tmp = static_cast<U*>(mRawPtr);
+    mRawPtr = nullptr;
+    return already_AddRefed<U>(tmp);
+  }
+
+ private:
+  T* MOZ_OWNING_REF mRawPtr;
+};
+
+#endif  // AlreadyAddRefed_h