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diff --git a/mfbt/OperatorNewExtensions.h b/mfbt/OperatorNewExtensions.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/. */
+
+/* A version of |operator new| that eschews mandatory null-checks. */
+
+#ifndef mozilla_OperatorNewExtensions_h
+#define mozilla_OperatorNewExtensions_h
+
+#include "mozilla/Assertions.h"
+
+// Credit goes to WebKit for this implementation, cf.
+// https://bugs.webkit.org/show_bug.cgi?id=74676
+namespace mozilla {
+enum NotNullTag {
+  KnownNotNull,
+};
+}  // namespace mozilla
+
+/*
+ * The logic here is a little subtle.  [expr.new] states that if the allocation
+ * function being called returns null, then object initialization must not be
+ * done, and the entirety of the new expression must return null.  Non-throwing
+ * (noexcept) functions are defined to return null to indicate failure.  The
+ * standard placement operator new is defined in such a way, and so it requires
+ * a null check, even when that null check would be extraneous.  Functions
+ * declared without such a specification are defined to throw std::bad_alloc if
+ * they fail, and return a non-null pointer otherwise.  We compile without
+ * exceptions, so any placement new overload we define that doesn't declare
+ * itself as noexcept must therefore avoid generating a null check.  Below is
+ * just such an overload.
+ *
+ * You might think that MOZ_NONNULL might perform the same function, but
+ * MOZ_NONNULL isn't supported on all of our compilers, and even when it is
+ * supported, doesn't work on all the versions we support.  And even keeping
+ * those limitations in mind, we can't put MOZ_NONNULL on the global,
+ * standardized placement new function in any event.
+ *
+ * We deliberately don't add MOZ_NONNULL(3) to tag |p| as non-null, to benefit
+ * hypothetical static analyzers.  Doing so makes |MOZ_ASSERT(p)|'s internal
+ * test vacuous, and some compilers warn about such vacuous tests.
+ */
+inline void* operator new(size_t, mozilla::NotNullTag, void* p) {
+  MOZ_ASSERT(p);
+  return p;
+}
+
+#endif  // mozilla_OperatorNewExtensions_h