Bug 1851045 - Part 4: Add a NeverDestroyed helper type to MFBT for static local members, r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D189248

mfbt/NeverDestroyed.h

@@ -0,0 +1,66 @@
+/* -*- 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 mozilla_NeverDestroyed_h
+#define mozilla_NeverDestroyed_h
+
+#include <new>
+#include <type_traits>
+#include <utility>
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+// Helper type for creating a local static member of type `T` when `T` has a
+// non-trivial static destructor. When used for the local static value, this
+// type will avoid introducing a static destructor for these types, as they
+// will survive until shutdown.
+//
+// This can be very useful to avoid static destructors, which are heavily
+// discouraged. Using this type is unnecessary if `T` already has a trivial
+// destructor, and may introduce unnecessary extra overhead.
+//
+// This type must only be used with static local members within a function,
+// which will be enforced by the clang static analysis.
+template <typename T>
+class MOZ_STATIC_LOCAL_CLASS MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS
+    NeverDestroyed {
+ public:
+  static_assert(
+      !std::is_trivially_destructible_v<T>,
+      "NeverDestroyed is unnecessary for trivially destructable types");
+
+  // Allow constructing the inner type.
+  // This isn't constexpr, as it requires invoking placement-new. See the
+  // comment on `mStorage`.
+  template <typename... U>
+  explicit NeverDestroyed(U&&... aArgs) {
+    new (mStorage) T(std::forward<U>(aArgs)...);
+  }
+
+  const T& operator*() const { return *get(); }
+  T& operator*() { return *get(); }
+
+  const T* operator->() const { return get(); }
+  T* operator->() { return get(); }
+
+  const T* get() const { return reinterpret_cast<T*>(mStorage); }
+  T* get() { return reinterpret_cast<T*>(mStorage); }
+
+  // Block copy & move constructor, as the type is not safe to copy.
+  NeverDestroyed(const NeverDestroyed&) = delete;
+  NeverDestroyed& operator=(const NeverDestroyed&) = delete;
+
+ private:
+  // Correctly aligned storage for the type. We unfortunately can't use a union
+  // for alignment & constexpr initialization as that would require an explicit
+  // destructor declaration, making `NeverDestroyed` non-trivially destructable.
+  alignas(T) char mStorage[sizeof(T)];
+};
+
+};  // namespace mozilla
+
+#endif  // mozilla_NeverDestroyed_h

mfbt/moz.build

@@ -73,6 +73,7 @@ EXPORTS.mozilla = [
     "MemoryReporting.h",
     "MoveOnlyFunction.h",
     "MruCache.h",
+    "NeverDestroyed.h",
     "NonDereferenceable.h",
     "NotNull.h",
     "Opaque.h",