path: root/mfbt/Attributes.h

/* -*- 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/. */

/* Implementations of various class and method modifier attributes. */

#ifndef mozilla_Attributes_h
#define mozilla_Attributes_h

#include "mozilla/Compiler.h"

/*
 * MOZ_ALWAYS_INLINE is a macro which expands to tell the compiler that the
 * method decorated with it must be inlined, even if the compiler thinks
 * otherwise.  This is only a (much) stronger version of the inline hint:
 * compilers are not guaranteed to respect it (although they're much more likely
 * to do so).
 *
 * The MOZ_ALWAYS_INLINE_EVEN_DEBUG macro is yet stronger. It tells the
 * compiler to inline even in DEBUG builds. It should be used very rarely.
 */
#if defined(_MSC_VER)
#  define MOZ_ALWAYS_INLINE_EVEN_DEBUG __forceinline
#elif defined(__GNUC__)
#  define MOZ_ALWAYS_INLINE_EVEN_DEBUG __attribute__((always_inline)) inline
#else
#  define MOZ_ALWAYS_INLINE_EVEN_DEBUG inline
#endif

#if !defined(DEBUG)
#  define MOZ_ALWAYS_INLINE MOZ_ALWAYS_INLINE_EVEN_DEBUG
#elif defined(_MSC_VER) && !defined(__cplusplus)
#  define MOZ_ALWAYS_INLINE __inline
#else
#  define MOZ_ALWAYS_INLINE inline
#endif

#if defined(_MSC_VER)
/*
 * g++ requires -std=c++0x or -std=gnu++0x to support C++11 functionality
 * without warnings (functionality used by the macros below).  These modes are
 * detectable by checking whether __GXX_EXPERIMENTAL_CXX0X__ is defined or, more
 * standardly, by checking whether __cplusplus has a C++11 or greater value.
 * Current versions of g++ do not correctly set __cplusplus, so we check both
 * for forward compatibility.
 */
#  define MOZ_HAVE_NEVER_INLINE __declspec(noinline)
#  define MOZ_HAVE_NORETURN __declspec(noreturn)
#elif defined(__clang__)
/*
 * Per Clang documentation, "Note that marketing version numbers should not
 * be used to check for language features, as different vendors use different
 * numbering schemes. Instead, use the feature checking macros."
 */
#  ifndef __has_extension
#    define __has_extension \
      __has_feature /* compatibility, for older versions of clang */
#  endif
#  if __has_attribute(noinline)
#    define MOZ_HAVE_NEVER_INLINE __attribute__((noinline))
#  endif
#  if __has_attribute(noreturn)
#    define MOZ_HAVE_NORETURN __attribute__((noreturn))
#  endif
#elif defined(__GNUC__)
#  define MOZ_HAVE_NEVER_INLINE __attribute__((noinline))
#  define MOZ_HAVE_NORETURN __attribute__((noreturn))
#  define MOZ_HAVE_NORETURN_PTR __attribute__((noreturn))
#endif

#if defined(__clang__)
#  if __has_attribute(no_stack_protector)
#    define MOZ_HAVE_NO_STACK_PROTECTOR __attribute__((no_stack_protector))
#  endif
#elif defined(__GNUC__)
#  define MOZ_HAVE_NO_STACK_PROTECTOR __attribute__((no_stack_protector))
#endif

/*
 * When built with clang analyzer (a.k.a scan-build), define MOZ_HAVE_NORETURN
 * to mark some false positives
 */
#ifdef __clang_analyzer__
#  if __has_extension(attribute_analyzer_noreturn)
#    define MOZ_HAVE_ANALYZER_NORETURN __attribute__((analyzer_noreturn))
#  endif
#endif

/*
 * MOZ_NEVER_INLINE is a macro which expands to tell the compiler that the
 * method decorated with it must never be inlined, even if the compiler would
 * otherwise choose to inline the method.  Compilers aren't absolutely
 * guaranteed to support this, but most do.
 */
#if defined(MOZ_HAVE_NEVER_INLINE)
#  define MOZ_NEVER_INLINE MOZ_HAVE_NEVER_INLINE
#else
#  define MOZ_NEVER_INLINE /* no support */
#endif

/*
 * MOZ_NEVER_INLINE_DEBUG is a macro which expands to MOZ_NEVER_INLINE
 * in debug builds, and nothing in opt builds.
 */
#if defined(DEBUG)
#  define MOZ_NEVER_INLINE_DEBUG MOZ_NEVER_INLINE
#else
#  define MOZ_NEVER_INLINE_DEBUG /* don't inline in opt builds */
#endif
/*
 * MOZ_NORETURN, specified at the start of a function declaration, indicates
 * that the given function does not return.  (The function definition does not
 * need to be annotated.)
 *
 *   MOZ_NORETURN void abort(const char* msg);
 *
 * This modifier permits the compiler to optimize code assuming a call to such a
 * function will never return.  It also enables the compiler to avoid spurious
 * warnings about not initializing variables, or about any other seemingly-dodgy
 * operations performed after the function returns.
 *
 * There are two variants. The GCC version of NORETURN may be applied to a
 * function pointer, while for MSVC it may not.
 *
 * This modifier does not affect the corresponding function's linking behavior.
 */
#if defined(MOZ_HAVE_NORETURN)
#  define MOZ_NORETURN MOZ_HAVE_NORETURN
#else
#  define MOZ_NORETURN /* no support */
#endif
#if defined(MOZ_HAVE_NORETURN_PTR)
#  define MOZ_NORETURN_PTR MOZ_HAVE_NORETURN_PTR
#else
#  define MOZ_NORETURN_PTR /* no support */
#endif

/**
 * MOZ_COLD tells the compiler that a function is "cold", meaning infrequently
 * executed. This may lead it to optimize for size more aggressively than speed,
 * or to allocate the body of the function in a distant part of the text segment
 * to help keep it from taking up unnecessary icache when it isn't in use.
 *
 * Place this attribute at the very beginning of a function definition. For
 * example, write
 *
 *   MOZ_COLD int foo();
 *
 * or
 *
 *   MOZ_COLD int foo() { return 42; }
 */
#if defined(__GNUC__) || defined(__clang__)
#  define MOZ_COLD __attribute__((cold))
#else
#  define MOZ_COLD
#endif

/**
 * MOZ_NONNULL tells the compiler that some of the arguments to a function are
 * known to be non-null. The arguments are a list of 1-based argument indexes
 * identifying arguments which are known to be non-null.
 *
 * Place this attribute at the very beginning of a function definition. For
 * example, write
 *
 *   MOZ_NONNULL(1, 2) int foo(char *p, char *q);
 */
#if defined(__GNUC__) || defined(__clang__)
#  define MOZ_NONNULL(...) __attribute__((nonnull(__VA_ARGS__)))
#else
#  define MOZ_NONNULL(...)
#endif

/**
 * MOZ_NONNULL_RETURN tells the compiler that the function's return value is
 * guaranteed to be a non-null pointer, which may enable the compiler to
 * optimize better at call sites.
 *
 * Place this attribute at the end of a function declaration. For example,
 *
 *   char* foo(char *p, char *q) MOZ_NONNULL_RETURN;
 */
#if defined(__GNUC__) || defined(__clang__)
#  define MOZ_NONNULL_RETURN __attribute__((returns_nonnull))
#else
#  define MOZ_NONNULL_RETURN
#endif

/*
 * MOZ_PRETEND_NORETURN_FOR_STATIC_ANALYSIS, specified at the end of a function
 * declaration, indicates that for the purposes of static analysis, this
 * function does not return.  (The function definition does not need to be
 * annotated.)
 *
 * MOZ_ReportCrash(const char* s, const char* file, int ln)
 *   MOZ_PRETEND_NORETURN_FOR_STATIC_ANALYSIS
 *
 * Some static analyzers, like scan-build from clang, can use this information
 * to eliminate false positives.  From the upstream documentation of scan-build:
 * "This attribute is useful for annotating assertion handlers that actually
 * can return, but for the purpose of using the analyzer we want to pretend
 * that such functions do not return."
 *
 */
#if defined(MOZ_HAVE_ANALYZER_NORETURN)
#  define MOZ_PRETEND_NORETURN_FOR_STATIC_ANALYSIS MOZ_HAVE_ANALYZER_NORETURN
#else
#  define MOZ_PRETEND_NORETURN_FOR_STATIC_ANALYSIS /* no support */
#endif

/*
 * MOZ_ASAN_IGNORE is a macro to tell AddressSanitizer (a compile-time
 * instrumentation shipped with Clang and GCC) to not instrument the annotated
 * function. Furthermore, it will prevent the compiler from inlining the
 * function because inlining currently breaks the blocklisting mechanism of
 * AddressSanitizer.
 */
#if defined(__has_feature)
#  if __has_feature(address_sanitizer)
#    define MOZ_HAVE_ASAN_IGNORE
#  endif
#elif defined(__GNUC__)
#  if defined(__SANITIZE_ADDRESS__)
#    define MOZ_HAVE_ASAN_IGNORE
#  endif
#endif

#if defined(MOZ_HAVE_ASAN_IGNORE)
#  define MOZ_ASAN_IGNORE MOZ_NEVER_INLINE __attribute__((no_sanitize_address))
#else
#  define MOZ_ASAN_IGNORE /* nothing */
#endif

/*
 * MOZ_TSAN_IGNORE is a macro to tell ThreadSanitizer (a compile-time
 * instrumentation shipped with Clang) to not instrument the annotated function.
 * Furthermore, it will prevent the compiler from inlining the function because
 * inlining currently breaks the blocklisting mechanism of ThreadSanitizer.
 */
#if defined(__has_feature)
#  if __has_feature(thread_sanitizer)
#    define MOZ_TSAN_IGNORE MOZ_NEVER_INLINE __attribute__((no_sanitize_thread))
#  else
#    define MOZ_TSAN_IGNORE /* nothing */
#  endif
#else
#  define MOZ_TSAN_IGNORE /* nothing */
#endif

#if defined(__has_attribute)
#  if __has_attribute(no_sanitize)
#    define MOZ_HAVE_NO_SANITIZE_ATTR
#  endif
#endif

#ifdef __clang__
#  ifdef MOZ_HAVE_NO_SANITIZE_ATTR
#    define MOZ_HAVE_UNSIGNED_OVERFLOW_SANITIZE_ATTR
#    define MOZ_HAVE_SIGNED_OVERFLOW_SANITIZE_ATTR
#  endif
#endif

/*
 * MOZ_NO_SANITIZE_UNSIGNED_OVERFLOW disables *un*signed integer overflow
 * checking on the function it annotates, in builds configured to perform it.
 * (Currently this is only Clang using -fsanitize=unsigned-integer-overflow, or
 * via --enable-unsigned-overflow-sanitizer in Mozilla's build system.)  It has
 * no effect in other builds.
 *
 * Place this attribute at the very beginning of a function declaration.
 *
 * Unsigned integer overflow isn't *necessarily* a bug.  It's well-defined in
 * C/C++, and code may reasonably depend upon it.  For example,
 *
 *   MOZ_NO_SANITIZE_UNSIGNED_OVERFLOW inline bool
 *   IsDecimal(char aChar)
 *   {
 *     // For chars less than '0', unsigned integer underflow occurs, to a value
 *     // much greater than 10, so the overall test is false.
 *     // For chars greater than '0', no overflow occurs, and only '0' to '9'
 *     // pass the overall test.
 *     return static_cast<unsigned int>(aChar) - '0' < 10;
 *   }
 *
 * But even well-defined unsigned overflow often causes bugs when it occurs, so
 * it should be restricted to functions annotated with this attribute.
 *
 * The compiler instrumentation to detect unsigned integer overflow has costs
 * both at compile time and at runtime.  Functions that are repeatedly inlined
 * at compile time will also implicitly inline the necessary instrumentation,
 * increasing compile time.  Similarly, frequently-executed functions that
 * require large amounts of instrumentation will also notice significant runtime
 * slowdown to execute that instrumentation.  Use this attribute to eliminate
 * those costs -- but only after carefully verifying that no overflow can occur.
 */
#ifdef MOZ_HAVE_UNSIGNED_OVERFLOW_SANITIZE_ATTR
#  define MOZ_NO_SANITIZE_UNSIGNED_OVERFLOW \
    __attribute__((no_sanitize("unsigned-integer-overflow")))
#else
#  define MOZ_NO_SANITIZE_UNSIGNED_OVERFLOW /* nothing */
#endif

/*
 * MOZ_NO_SANITIZE_SIGNED_OVERFLOW disables *signed* integer overflow checking
 * on the function it annotates, in builds configured to perform it.  (Currently
 * this is only Clang using -fsanitize=signed-integer-overflow, or via
 * --enable-signed-overflow-sanitizer in Mozilla's build system.  GCC support
 * will probably be added in the future.)  It has no effect in other builds.
 *
 * Place this attribute at the very beginning of a function declaration.
 *
 * Signed integer overflow is undefined behavior in C/C++: *anything* can happen
 * when it occurs.  *Maybe* wraparound behavior will occur, but maybe also the
 * compiler will assume no overflow happens and will adversely optimize the rest
 * of your code.  Code that contains signed integer overflow needs to be fixed.
 *
 * The compiler instrumentation to detect signed integer overflow has costs both
 * at compile time and at runtime.  Functions that are repeatedly inlined at
 * compile time will also implicitly inline the necessary instrumentation,
 * increasing compile time.  Similarly, frequently-executed functions that
 * require large amounts of instrumentation will also notice significant runtime
 * slowdown to execute that instrumentation.  Use this attribute to eliminate
 * those costs -- but only after carefully verifying that no overflow can occur.
 */
#ifdef MOZ_HAVE_SIGNED_OVERFLOW_SANITIZE_ATTR
#  define MOZ_NO_SANITIZE_SIGNED_OVERFLOW \
    __attribute__((no_sanitize("signed-integer-overflow")))
#else
#  define MOZ_NO_SANITIZE_SIGNED_OVERFLOW /* nothing */
#endif

#undef MOZ_HAVE_NO_SANITIZE_ATTR

/**
 * MOZ_ALLOCATOR tells the compiler that the function it marks returns either a
 * "fresh", "pointer-free" block of memory, or nullptr. "Fresh" means that the
 * block is not pointed to by any other reachable pointer in the program.
 * "Pointer-free" means that the block contains no pointers to any valid object
 * in the program. It may be initialized with other (non-pointer) values.
 *
 * Placing this attribute on appropriate functions helps GCC analyze pointer
 * aliasing more accurately in their callers.
 *
 * GCC warns if a caller ignores the value returned by a function marked with
 * MOZ_ALLOCATOR: it is hard to imagine cases where dropping the value returned
 * by a function that meets the criteria above would be intentional.
 *
 * Place this attribute after the argument list and 'this' qualifiers of a
 * function definition. For example, write
 *
 *   void *my_allocator(size_t) MOZ_ALLOCATOR;
 *
 * or
 *
 *   void *my_allocator(size_t bytes) MOZ_ALLOCATOR { ... }
 */
#if defined(__GNUC__) || defined(__clang__)
#  define MOZ_ALLOCATOR __attribute__((malloc, warn_unused_result))
#  define MOZ_INFALLIBLE_ALLOCATOR \
    __attribute__((malloc, warn_unused_result, returns_nonnull))
#else
#  define MOZ_ALLOCATOR
#  define MOZ_INFALLIBLE_ALLOCATOR
#endif

/**
 * MOZ_MAYBE_UNUSED suppresses compiler warnings about functions that are
 * never called (in this build configuration, at least).
 *
 * Place this attribute at the very beginning of a function declaration. For
 * example, write
 *
 *   MOZ_MAYBE_UNUSED int foo();
 *
 * or
 *
 *   MOZ_MAYBE_UNUSED int foo() { return 42; }
 */
#if defined(__GNUC__) || defined(__clang__)
#  define MOZ_MAYBE_UNUSED __attribute__((__unused__))
#elif defined(_MSC_VER)
#  define MOZ_MAYBE_UNUSED __pragma(warning(suppress : 4505))
#else
#  define MOZ_MAYBE_UNUSED
#endif

/*
 * MOZ_NO_STACK_PROTECTOR, specified at the start of a function declaration,
 * indicates that the given function should *NOT* be instrumented to detect
 * stack buffer overflows at runtime. (The function definition does not need to
 * be annotated.)
 *
 *   MOZ_NO_STACK_PROTECTOR int foo();
 *
 * Detecting stack buffer overflows at runtime is a security feature. This
 * modifier should thus only be used on functions which are provably exempt of
 * stack buffer overflows, for example because they do not use stack buffers.
 *
 * This modifier does not affect the corresponding function's linking behavior.
 */
#if defined(MOZ_HAVE_NO_STACK_PROTECTOR)
#  define MOZ_NO_STACK_PROTECTOR MOZ_HAVE_NO_STACK_PROTECTOR
#else
#  define MOZ_NO_STACK_PROTECTOR /* no support */
#endif

#ifdef __cplusplus

/**
 * C++11 lets unions contain members that have non-trivial special member
 * functions (default/copy/move constructor, copy/move assignment operator,
 * destructor) if the user defines the corresponding functions on the union.
 * (Such user-defined functions must rely on external knowledge about which arm
 * is active to be safe.  Be extra-careful defining these functions!)
 *
 * MSVC unfortunately warns/errors for this bog-standard C++11 pattern.  Use
 * these macro-guards around such member functions to disable the warnings:
 *
 *   union U
 *   {
 *     std::string s;
 *     int x;
 *
 *     MOZ_PUSH_DISABLE_NONTRIVIAL_UNION_WARNINGS
 *
 *     // |U| must have a user-defined default constructor because |std::string|
 *     // has a non-trivial default constructor.
 *     U() ... { ... }
 *
 *     // |U| must have a user-defined destructor because |std::string| has a
 *     // non-trivial destructor.
 *     ~U() { ... }
 *
 *     MOZ_POP_DISABLE_NONTRIVIAL_UNION_WARNINGS
 *   };
 */
#  if defined(_MSC_VER)
#    define MOZ_PUSH_DISABLE_NONTRIVIAL_UNION_WARNINGS          \
      __pragma(warning(push)) __pragma(warning(disable : 4582)) \
          __pragma(warning(disable : 4583))
#    define MOZ_POP_DISABLE_NONTRIVIAL_UNION_WARNINGS __pragma(warning(pop))
#  else
#    define MOZ_PUSH_DISABLE_NONTRIVIAL_UNION_WARNINGS /* nothing */
#    define MOZ_POP_DISABLE_NONTRIVIAL_UNION_WARNINGS  /* nothing */
#  endif

/*
 * The following macros are attributes that support the static analysis plugin
 * included with Mozilla, and will be implemented (when such support is enabled)
 * as C++11 attributes. Since such attributes are legal pretty much everywhere
 * and have subtly different semantics depending on their placement, the
 * following is a guide on where to place the attributes.
 *
 * Attributes that apply to a struct or class precede the name of the class:
 * (Note that this is different from the placement of final for classes!)
 *
 *   class MOZ_CLASS_ATTRIBUTE SomeClass {};
 *
 * Attributes that apply to functions follow the parentheses and const
 * qualifiers but precede final, override and the function body:
 *
 *   void DeclaredFunction() MOZ_FUNCTION_ATTRIBUTE;
 *   void SomeFunction() MOZ_FUNCTION_ATTRIBUTE {}
 *   void PureFunction() const MOZ_FUNCTION_ATTRIBUTE = 0;
 *   void OverriddenFunction() MOZ_FUNCTION_ATTIRBUTE override;
 *
 * Attributes that apply to variables or parameters follow the variable's name:
 *
 *   int variable MOZ_VARIABLE_ATTRIBUTE;
 *
 * Attributes that apply to types follow the type name:
 *
 *   typedef int MOZ_TYPE_ATTRIBUTE MagicInt;
 *   int MOZ_TYPE_ATTRIBUTE someVariable;
 *   int* MOZ_TYPE_ATTRIBUTE magicPtrInt;
 *   int MOZ_TYPE_ATTRIBUTE* ptrToMagicInt;
 *
 * Attributes that apply to statements precede the statement:
 *
 *   MOZ_IF_ATTRIBUTE if (x == 0)
 *   MOZ_DO_ATTRIBUTE do { } while (0);
 *
 * Attributes that apply to labels precede the label:
 *
 *   MOZ_LABEL_ATTRIBUTE target:
 *     goto target;
 *   MOZ_CASE_ATTRIBUTE case 5:
 *   MOZ_DEFAULT_ATTRIBUTE default:
 *
 * The static analyses that are performed by the plugin are as follows:
 *
 * MOZ_CAN_RUN_SCRIPT: Applies to functions which can run script. Callers of
 *   this function must also be marked as MOZ_CAN_RUN_SCRIPT, and all refcounted
 *   arguments must be strongly held in the caller. Note that MOZ_CAN_RUN_SCRIPT
 *   should only be applied to function declarations, not definitions. If you
 *   need to apply it to a definition (eg because both are generated by a macro)
 *   use MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION.
 *
 *   MOZ_CAN_RUN_SCRIPT can be applied to XPIDL-generated declarations by
 *   annotating the method or attribute as [can_run_script] in the .idl file.
 *
 * MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION: Same as MOZ_CAN_RUN_SCRIPT, but usable on
 *   a definition. If the declaration is in a header file, users of that header
 *   file may not see the annotation.
 * MOZ_CAN_RUN_SCRIPT_BOUNDARY: Applies to functions which need to call
 *   MOZ_CAN_RUN_SCRIPT functions, but should not themselves be considered
 *   MOZ_CAN_RUN_SCRIPT. This should generally be avoided but can be used in
 *   two cases:
 *     1) As a temporary measure to limit the scope of changes when adding
 *        MOZ_CAN_RUN_SCRIPT.  Such a use must be accompanied by a follow-up bug
 *        to replace the MOZ_CAN_RUN_SCRIPT_BOUNDARY with MOZ_CAN_RUN_SCRIPT and
 *        a comment linking to that bug.
 *     2) If we can reason that the MOZ_CAN_RUN_SCRIPT callees of the function
 *        do not in fact run script (for example, because their behavior depends
 *        on arguments and we pass the arguments that don't allow script
 *        execution).  Such a use must be accompanied by a comment that explains
 *        why it's OK to have the MOZ_CAN_RUN_SCRIPT_BOUNDARY, as well as
 *        comments in the callee pointing out that if its behavior changes the
 *        caller might need adjusting.  And perhaps also a followup bug to
 *        refactor things so the "script" and "no script" codepaths do not share
 *        a chokepoint.
 *   Importantly, any use MUST be accompanied by a comment explaining why it's
 *   there, and should ideally have an action plan for getting rid of the
 *   MOZ_CAN_RUN_SCRIPT_BOUNDARY annotation.
 * MOZ_MUST_OVERRIDE: Applies to all C++ member functions. All immediate
 *   subclasses must provide an exact override of this method; if a subclass
 *   does not override this method, the compiler will emit an error. This
 *   attribute is not limited to virtual methods, so if it is applied to a
 *   nonvirtual method and the subclass does not provide an equivalent
 *   definition, the compiler will emit an error.
 * MOZ_STATIC_CLASS: Applies to all classes. Any class with this annotation is
 *   expected to live in static memory, so it is a compile-time error to use
 *   it, or an array of such objects, as the type of a variable declaration, or
 *   as a temporary object, or as the type of a new expression (unless
 *   placement new is being used). If a member of another class uses this
 *   class, or if another class inherits from this class, then it is considered
 *   to be a static class as well, although this attribute need not be provided
 *   in such cases.
 * MOZ_STATIC_LOCAL_CLASS: Applies to all classes. Any class with this
 *   annotation is expected to be a static local variable, so it is
 *   a compile-time error to use it, or an array of such objects, or as a
 *   temporary object, or as the type of a new expression. If another class
 *   inherits from this class then it is considered to be a static local
 *   class as well, although this attribute need not be provided in such cases.
 *   It is also a compile-time error for any class with this annotation to have
 *   a non-trivial destructor.
 * MOZ_STACK_CLASS: Applies to all classes. Any class with this annotation is
 *   expected to live on the stack, so it is a compile-time error to use it, or
 *   an array of such objects, as a global or static variable, or as the type of
 *   a new expression (unless placement new is being used). If a member of
 *   another class uses this class, or if another class inherits from this
 *   class, then it is considered to be a stack class as well, although this
 *   attribute need not be provided in such cases.
 * MOZ_NONHEAP_CLASS: Applies to all classes. Any class with this annotation is
 *   expected to live on the stack or in static storage, so it is a compile-time
 *   error to use it, or an array of such objects, as the type of a new
 *   expression. If a member of another class uses this class, or if another
 *   class inherits from this class, then it is considered to be a non-heap
 *   class as well, although this attribute need not be provided in such cases.
 * MOZ_HEAP_CLASS: Applies to all classes. Any class with this annotation is
 *   expected to live on the heap, so it is a compile-time error to use it, or
 *   an array of such objects, as the type of a variable declaration, or as a
 *   temporary object. If a member of another class uses this class, or if
 *   another class inherits from this class, then it is considered to be a heap
 *   class as well, although this attribute need not be provided in such cases.
 * MOZ_NON_TEMPORARY_CLASS: Applies to all classes. Any class with this
 *   annotation is expected not to live in a temporary. If a member of another
 *   class uses this class or if another class inherits from this class, then it
 *   is considered to be a non-temporary class as well, although this attribute
 *   need not be provided in such cases.
 * MOZ_TEMPORARY_CLASS: Applies to all classes. Any class with this annotation
 *   is expected to only live in a temporary. If another class inherits from
 *   this class, then it is considered to be a non-temporary class as well,
 *   although this attribute need not be provided in such cases.
 * MOZ_RAII: Applies to all classes. Any class with this annotation is assumed
 *   to be a RAII guard, which is expected to live on the stack in an automatic
 *   allocation. It is prohibited from being allocated in a temporary, static
 *   storage, or on the heap. This is a combination of MOZ_STACK_CLASS and
 *   MOZ_NON_TEMPORARY_CLASS.
 * MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS: Applies to all classes that are
 *   intended to prevent introducing static initializers.  This attribute
 *   currently makes it a compile-time error to instantiate these classes
 *   anywhere other than at the global scope, or as a static member of a class.
 *   In non-debug mode, it also prohibits non-trivial constructors and
 *   destructors.
 * MOZ_TRIVIAL_CTOR_DTOR: Applies to all classes that must have both a trivial
 *   or constexpr constructor and a trivial destructor. Setting this attribute
 *   on a class makes it a compile-time error for that class to get a
 *   non-trivial constructor or destructor for any reason.
 * MOZ_ALLOW_TEMPORARY: Applies to constructors. This indicates that using the
 *   constructor is allowed in temporary expressions, if it would have otherwise
 *   been forbidden by the type being a MOZ_NON_TEMPORARY_CLASS. Useful for
 *   constructors like Maybe(Nothing).
 * MOZ_HEAP_ALLOCATOR: Applies to any function. This indicates that the return
 *   value is allocated on the heap, and will as a result check such allocations
 *   during MOZ_STACK_CLASS and MOZ_NONHEAP_CLASS annotation checking.
 * MOZ_IMPLICIT: Applies to constructors. Implicit conversion constructors
 *   are disallowed by default unless they are marked as MOZ_IMPLICIT. This
 *   attribute must be used for constructors which intend to provide implicit
 *   conversions.
 * MOZ_IS_REFPTR: Applies to class declarations of ref pointer to mark them as
 *   such for use with static-analysis.
 *   A ref pointer is an object wrapping a pointer and automatically taking care
 *   of its refcounting upon construction/destruction/transfer of ownership.
 *   This annotation implies MOZ_IS_SMARTPTR_TO_REFCOUNTED.
 * MOZ_IS_SMARTPTR_TO_REFCOUNTED: Applies to class declarations of smart
 *   pointers to ref counted classes to mark them as such for use with
 *   static-analysis.
 * MOZ_NO_ARITHMETIC_EXPR_IN_ARGUMENT: Applies to functions. Makes it a compile
 *   time error to pass arithmetic expressions on variables to the function.
 * MOZ_OWNING_REF: Applies to declarations of pointers to reference counted
 *   types.  This attribute tells the compiler that the raw pointer is a strong
 *   reference, where ownership through methods such as AddRef and Release is
 *   managed manually.  This can make the compiler ignore these pointers when
 *   validating the usage of pointers otherwise.
 *
 *   Example uses include owned pointers inside of unions, and pointers stored
 *   in POD types where a using a smart pointer class would make the object
 *   non-POD.
 * MOZ_NON_OWNING_REF: Applies to declarations of pointers to reference counted
 *   types.  This attribute tells the compiler that the raw pointer is a weak
 *   reference, which is ensured to be valid by a guarantee that the reference
 *   will be nulled before the pointer becomes invalid.  This can make the
 *   compiler ignore these pointers when validating the usage of pointers
 *   otherwise.
 *
 *   Examples include an mOwner pointer, which is nulled by the owning class's
 *   destructor, and is null-checked before dereferencing.
 * MOZ_UNSAFE_REF: Applies to declarations of pointers to reference counted
 *   types.  Occasionally there are non-owning references which are valid, but
 *   do not take the form of a MOZ_NON_OWNING_REF.  Their safety may be
 *   dependent on the behaviour of API consumers.  The string argument passed
 *   to this macro documents the safety conditions.  This can make the compiler
 *   ignore these pointers when validating the usage of pointers elsewhere.
 *
 *   Examples include an nsAtom* member which is known at compile time to point
 *   to a static atom which is valid throughout the lifetime of the program, or
 *   an API which stores a pointer, but doesn't take ownership over it, instead
 *   requiring the API consumer to correctly null the value before it becomes
 *   invalid.
 *
 *   Use of this annotation is discouraged when a strong reference or one of
 *   the above two annotations can be used instead.
 * MOZ_NO_ADDREF_RELEASE_ON_RETURN: Applies to function declarations.  Makes it
 *   a compile time error to call AddRef or Release on the return value of a
 *   function.  This is intended to be used with operator->() of our smart
 *   pointer classes to ensure that the refcount of an object wrapped in a
 *   smart pointer is not manipulated directly.
 * MOZ_NEEDS_NO_VTABLE_TYPE: Applies to template class declarations.  Makes it
 *   a compile time error to instantiate this template with a type parameter
 *   which has a VTable.
 * MOZ_NON_MEMMOVABLE: Applies to class declarations for types that are not safe
 *   to be moved in memory using memmove().
 * MOZ_NEEDS_MEMMOVABLE_TYPE: Applies to template class declarations where the
 *   template arguments are required to be safe to move in memory using
 *   memmove().  Passing MOZ_NON_MEMMOVABLE types to these templates is a
 *   compile time error.
 * MOZ_NEEDS_MEMMOVABLE_MEMBERS: Applies to class declarations where each member
 *   must be safe to move in memory using memmove().  MOZ_NON_MEMMOVABLE types
 *   used in members of these classes are compile time errors.
 * MOZ_NO_DANGLING_ON_TEMPORARIES: Applies to method declarations which return
 *   a pointer that is freed when the destructor of the class is called. This
 *   prevents these methods from being called on temporaries of the class,
 *   reducing risks of use-after-free.
 *   This attribute cannot be applied to && methods.
 *   In some cases, adding a deleted &&-qualified overload is too restrictive as
 *   this method should still be callable as a non-escaping argument to another
 *   function. This annotation can be used in those cases.
 * MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS: Applies to template class
 *   declarations where an instance of the template should be considered, for
 *   static analysis purposes, to inherit any type annotations (such as
 *   MOZ_STACK_CLASS) from its template arguments.
 * MOZ_INIT_OUTSIDE_CTOR: Applies to class member declarations. Occasionally
 *   there are class members that are not initialized in the constructor,
 *   but logic elsewhere in the class ensures they are initialized prior to use.
 *   Using this attribute on a member disables the check that this member must
 *   be initialized in constructors via list-initialization, in the constructor
 *   body, or via functions called from the constructor body.
 * MOZ_IS_CLASS_INIT: Applies to class method declarations. Occasionally the
 *   constructor doesn't initialize all of the member variables and another
 *   function is used to initialize the rest. This marker is used to make the
 *   static analysis tool aware that the marked function is part of the
 *   initialization process and to include the marked function in the scan
 *   mechanism that determines which member variables still remain
 *   uninitialized.
 * MOZ_NON_PARAM: Applies to types. Makes it compile time error to use the type
 *   in parameter without pointer or reference.
 * MOZ_NON_AUTOABLE: Applies to class declarations. Makes it a compile time
 *   error to use `auto` in place of this type in variable declarations.  This
 *   is intended to be used with types which are intended to be implicitly
 *   constructed into other other types before being assigned to variables.
 * MOZ_REQUIRED_BASE_METHOD: Applies to virtual class method declarations.
 *   Sometimes derived classes override methods that need to be called by their
 *   overridden counterparts. This marker indicates that the marked method must
 *   be called by the method that it overrides.
 * MOZ_MUST_RETURN_FROM_CALLER_IF_THIS_IS_ARG: Applies to method declarations.
 *   Callers of the annotated method must return from that function within the
 *   calling block using an explicit `return` statement if the "this" value for
 *   the call is a parameter of the caller.  Only calls to Constructors,
 *   references to local and member variables, and calls to functions or
 *   methods marked as MOZ_MAY_CALL_AFTER_MUST_RETURN may be made after the
 *   MOZ_MUST_RETURN_FROM_CALLER_IF_THIS_IS_ARG call.
 * MOZ_MAY_CALL_AFTER_MUST_RETURN: Applies to function or method declarations.
 *   Calls to these methods may be made in functions after calls a
 *   MOZ_MUST_RETURN_FROM_CALLER_IF_THIS_IS_ARG method.
 * MOZ_LIFETIME_BOUND: Applies to method declarations.
 *   The result of calling these functions on temporaries may not be returned as
 *   a reference or bound to a reference variable.
 * MOZ_UNANNOTATED/MOZ_ANNOTATED: Applies to Mutexes/Monitors and variations on
 *   them. MOZ_UNANNOTATED indicates that the Mutex/Monitor/etc hasn't been
 *   examined and annotated using macros from mfbt/ThreadSafety --
 *   MOZ_GUARDED_BY()/REQUIRES()/etc. MOZ_ANNOTATED is used in rare cases to
 *   indicate that is has been looked at, but it did not need any
 *   MOZ_GUARDED_BY()/REQUIRES()/etc (and thus static analysis knows it can
 * ignore this Mutex/Monitor/etc)
 */

// gcc emits a nuisance warning -Wignored-attributes because attributes do not
// affect mangled names, and therefore template arguments do not propagate
// their attributes. It is rare that this would affect anything in practice,
// and most compilers are silent about it. Similarly, -Wattributes complains
// about attributes being ignored during template instantiation.
//
// Be conservative and only suppress the warning when running in a
// configuration where it would be emitted, namely when compiling with the
// XGILL_PLUGIN for the rooting hazard analysis (which runs under gcc.) If we
// end up wanting these attributes in general GCC builds, change this to
// something like
//
//     #if defined(__GNUC__) && ! defined(__clang__)
//
#  ifdef XGILL_PLUGIN
#    pragma GCC diagnostic ignored "-Wignored-attributes"
#    pragma GCC diagnostic ignored "-Wattributes"
#  endif

#  if defined(MOZ_CLANG_PLUGIN) || defined(XGILL_PLUGIN)
#    define MOZ_CAN_RUN_SCRIPT __attribute__((annotate("moz_can_run_script")))
#    define MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION         \
      __attribute__((annotate("moz_can_run_script"))) \
      __attribute__((annotate("moz_can_run_script_for_definition")))
#    define MOZ_CAN_RUN_SCRIPT_BOUNDARY \
      __attribute__((annotate("moz_can_run_script_boundary")))
#    define MOZ_MUST_OVERRIDE __attribute__((annotate("moz_must_override")))
#    define MOZ_STATIC_CLASS __attribute__((annotate("moz_global_class")))
#    define MOZ_STATIC_LOCAL_CLASS                        \
      __attribute__((annotate("moz_static_local_class"))) \
      __attribute__((annotate("moz_trivial_dtor")))
#    define MOZ_STACK_CLASS __attribute__((annotate("moz_stack_class")))
#    define MOZ_NONHEAP_CLASS __attribute__((annotate("moz_nonheap_class")))
#    define MOZ_HEAP_CLASS __attribute__((annotate("moz_heap_class")))
#    define MOZ_NON_TEMPORARY_CLASS \
      __attribute__((annotate("moz_non_temporary_class")))
#    define MOZ_TEMPORARY_CLASS __attribute__((annotate("moz_temporary_class")))
#    define MOZ_TRIVIAL_CTOR_DTOR \
      __attribute__((annotate("moz_trivial_ctor_dtor")))
#    define MOZ_ALLOW_TEMPORARY __attribute__((annotate("moz_allow_temporary")))
#    ifdef DEBUG
/* in debug builds, these classes do have non-trivial constructors. */
#      define MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS \
        __attribute__((annotate("moz_global_class")))
#    else
#      define MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS \
        __attribute__((annotate("moz_global_class"))) MOZ_TRIVIAL_CTOR_DTOR
#    endif
#    define MOZ_IMPLICIT __attribute__((annotate("moz_implicit")))
#    define MOZ_IS_SMARTPTR_TO_REFCOUNTED \
      __attribute__((annotate("moz_is_smartptr_to_refcounted")))
#    define MOZ_IS_REFPTR MOZ_IS_SMARTPTR_TO_REFCOUNTED
#    define MOZ_NO_ARITHMETIC_EXPR_IN_ARGUMENT \
      __attribute__((annotate("moz_no_arith_expr_in_arg")))
#    define MOZ_OWNING_REF __attribute__((annotate("moz_owning_ref")))
#    define MOZ_NON_OWNING_REF __attribute__((annotate("moz_non_owning_ref")))
#    define MOZ_UNSAFE_REF(reason) __attribute__((annotate("moz_unsafe_ref")))
#    define MOZ_NO_ADDREF_RELEASE_ON_RETURN \
      __attribute__((annotate("moz_no_addref_release_on_return")))
#    define MOZ_NEEDS_NO_VTABLE_TYPE \
      __attribute__((annotate("moz_needs_no_vtable_type")))
#    define MOZ_NON_MEMMOVABLE __attribute__((annotate("moz_non_memmovable")))
#    define MOZ_NEEDS_MEMMOVABLE_TYPE \
      __attribute__((annotate("moz_needs_memmovable_type")))
#    define MOZ_NEEDS_MEMMOVABLE_MEMBERS \
      __attribute__((annotate("moz_needs_memmovable_members")))
#    define MOZ_NO_DANGLING_ON_TEMPORARIES \
      __attribute__((annotate("moz_no_dangling_on_temporaries")))
#    define MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS \
      __attribute__((                                       \
          annotate("moz_inherit_type_annotations_from_template_args")))
#    define MOZ_NON_AUTOABLE __attribute__((annotate("moz_non_autoable")))
#    define MOZ_INIT_OUTSIDE_CTOR
#    define MOZ_IS_CLASS_INIT
#    define MOZ_NON_PARAM __attribute__((annotate("moz_non_param")))
#    define MOZ_REQUIRED_BASE_METHOD \
      __attribute__((annotate("moz_required_base_method")))
#    define MOZ_MUST_RETURN_FROM_CALLER_IF_THIS_IS_ARG \
      __attribute__((annotate("moz_must_return_from_caller_if_this_is_arg")))
#    define MOZ_MAY_CALL_AFTER_MUST_RETURN \
      __attribute__((annotate("moz_may_call_after_must_return")))
#    define MOZ_LIFETIME_BOUND __attribute__((annotate("moz_lifetime_bound")))
#    define MOZ_KNOWN_LIVE __attribute__((annotate("moz_known_live")))
#    ifndef XGILL_PLUGIN
#      define MOZ_UNANNOTATED __attribute__((annotate("moz_unannotated")))
#      define MOZ_ANNOTATED __attribute__((annotate("moz_annotated")))
#    else
#      define MOZ_UNANNOTATED /* nothing */
#      define MOZ_ANNOTATED   /* nothing */
#    endif

/*
 * It turns out that clang doesn't like void func() __attribute__ {} without a
 * warning, so use pragmas to disable the warning.
 */
#    ifdef __clang__
#      define MOZ_HEAP_ALLOCATOR                                 \
        _Pragma("clang diagnostic push")                         \
            _Pragma("clang diagnostic ignored \"-Wgcc-compat\"") \
                __attribute__((annotate("moz_heap_allocator")))  \
                _Pragma("clang diagnostic pop")
#    else
#      define MOZ_HEAP_ALLOCATOR __attribute__((annotate("moz_heap_allocator")))
#    endif
#  else
#    define MOZ_CAN_RUN_SCRIPT                              /* nothing */
#    define MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION               /* nothing */
#    define MOZ_CAN_RUN_SCRIPT_BOUNDARY                     /* nothing */
#    define MOZ_MUST_OVERRIDE                               /* nothing */
#    define MOZ_STATIC_CLASS                                /* nothing */
#    define MOZ_STATIC_LOCAL_CLASS                          /* nothing */
#    define MOZ_STACK_CLASS                                 /* nothing */
#    define MOZ_NONHEAP_CLASS                               /* nothing */
#    define MOZ_HEAP_CLASS                                  /* nothing */
#    define MOZ_NON_TEMPORARY_CLASS                         /* nothing */
#    define MOZ_TEMPORARY_CLASS                             /* nothing */
#    define MOZ_TRIVIAL_CTOR_DTOR                           /* nothing */
#    define MOZ_ALLOW_TEMPORARY                             /* nothing */
#    define MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS      /* nothing */
#    define MOZ_IMPLICIT                                    /* nothing */
#    define MOZ_IS_SMARTPTR_TO_REFCOUNTED                   /* nothing */
#    define MOZ_IS_REFPTR                                   /* nothing */
#    define MOZ_NO_ARITHMETIC_EXPR_IN_ARGUMENT              /* nothing */
#    define MOZ_HEAP_ALLOCATOR                              /* nothing */
#    define MOZ_OWNING_REF                                  /* nothing */
#    define MOZ_NON_OWNING_REF                              /* nothing */
#    define MOZ_UNSAFE_REF(reason)                          /* nothing */
#    define MOZ_NO_ADDREF_RELEASE_ON_RETURN                 /* nothing */
#    define MOZ_NEEDS_NO_VTABLE_TYPE                        /* nothing */
#    define MOZ_NON_MEMMOVABLE                              /* nothing */
#    define MOZ_NEEDS_MEMMOVABLE_TYPE                       /* nothing */
#    define MOZ_NEEDS_MEMMOVABLE_MEMBERS                    /* nothing */
#    define MOZ_NO_DANGLING_ON_TEMPORARIES                  /* nothing */
#    define MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS /* nothing */
#    define MOZ_INIT_OUTSIDE_CTOR                           /* nothing */
#    define MOZ_IS_CLASS_INIT                               /* nothing */
#    define MOZ_NON_PARAM                                   /* nothing */
#    define MOZ_NON_AUTOABLE                                /* nothing */
#    define MOZ_REQUIRED_BASE_METHOD                        /* nothing */
#    define MOZ_MUST_RETURN_FROM_CALLER_IF_THIS_IS_ARG      /* nothing */
#    define MOZ_MAY_CALL_AFTER_MUST_RETURN                  /* nothing */
#    define MOZ_LIFETIME_BOUND                              /* nothing */
#    define MOZ_KNOWN_LIVE                                  /* nothing */
#    define MOZ_UNANNOTATED                                 /* nothing */
#    define MOZ_ANNOTATED                                   /* nothing */
#  endif /* defined(MOZ_CLANG_PLUGIN) || defined(XGILL_PLUGIN) */

#  define MOZ_RAII MOZ_NON_TEMPORARY_CLASS MOZ_STACK_CLASS

// XGILL_PLUGIN is used for the GC rooting hazard analysis, which compiles with
// gcc. gcc has different rules governing __attribute__((...)) placement, so
// some attributes will error out when used in the source code where clang
// expects them to be. Remove the problematic annotations when needed.
//
// The placement of c++11 [[...]] attributes is more flexible and defined by a
// spec, so it would be nice to switch to those for the problematic
// cases. Unfortunately, the official spec provides *no* way to annotate a
// lambda function, which is one source of the difficulty here. It appears that
// this will be fixed in c++23: https://github.com/cplusplus/papers/issues/882

#  ifdef XGILL_PLUGIN

#    undef MOZ_MUST_OVERRIDE
#    undef MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION
#    undef MOZ_CAN_RUN_SCRIPT
#    undef MOZ_CAN_RUN_SCRIPT_BOUNDARY
#    define MOZ_MUST_OVERRIDE                 /* nothing */
#    define MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION /* nothing */
#    define MOZ_CAN_RUN_SCRIPT                /* nothing */
#    define MOZ_CAN_RUN_SCRIPT_BOUNDARY       /* nothing */

#  endif

#endif /* __cplusplus */

/**
 * Printf style formats.  MOZ_FORMAT_PRINTF and MOZ_FORMAT_WPRINTF can be used
 * to annotate a function or method that is "printf/wprintf-like"; this will let
 * (some) compilers check that the arguments match the template string.
 *
 * This macro takes two arguments.  The first argument is the argument
 * number of the template string.  The second argument is the argument
 * number of the '...' argument holding the arguments.
 *
 * Argument numbers start at 1.  Note that the implicit "this"
 * argument of a non-static member function counts as an argument.
 *
 * So, for a simple case like:
 *   void print_something (int whatever, const char *fmt, ...);
 * The corresponding annotation would be
 *   MOZ_FORMAT_PRINTF(2, 3)
 * However, if "print_something" were a non-static member function,
 * then the annotation would be:
 *   MOZ_FORMAT_PRINTF(3, 4)
 *
 * The second argument should be 0 for vprintf-like functions; that
 * is, those taking a va_list argument.
 *
 * Note that the checking is limited to standards-conforming
 * printf-likes, and in particular this should not be used for
 * PR_snprintf and friends, which are "printf-like" but which assign
 * different meanings to the various formats.
 *
 * MinGW requires special handling due to different format specifiers
 * on different platforms. The macro __MINGW_PRINTF_FORMAT maps to
 * either gnu_printf or ms_printf depending on where we are compiling
 * to avoid warnings on format specifiers that are legal.
 *
 * At time of writing MinGW has no wide equivalent to __MINGW_PRINTF_FORMAT;
 * therefore __MINGW_WPRINTF_FORMAT has been implemented following the same
 * pattern seen in MinGW's source.
 */
#ifdef __MINGW32__
#  define MOZ_FORMAT_PRINTF(stringIndex, firstToCheck) \
    __attribute__((format(__MINGW_PRINTF_FORMAT, stringIndex, firstToCheck)))
#  ifndef __MINGW_WPRINTF_FORMAT
#    if defined(__clang__)
#      define __MINGW_WPRINTF_FORMAT wprintf
#    elif defined(_UCRT) || __USE_MINGW_ANSI_STDIO
#      define __MINGW_WPRINTF_FORMAT gnu_wprintf
#    else
#      define __MINGW_WPRINTF_FORMAT ms_wprintf
#    endif
#  endif
#  define MOZ_FORMAT_WPRINTF(stringIndex, firstToCheck) \
    __attribute__((format(__MINGW_WPRINTF_FORMAT, stringIndex, firstToCheck)))
#elif __GNUC__ || __clang__
#  define MOZ_FORMAT_PRINTF(stringIndex, firstToCheck) \
    __attribute__((format(printf, stringIndex, firstToCheck)))
#  define MOZ_FORMAT_WPRINTF(stringIndex, firstToCheck) \
    __attribute__((format(wprintf, stringIndex, firstToCheck)))
#else
#  define MOZ_FORMAT_PRINTF(stringIndex, firstToCheck)
#  define MOZ_FORMAT_WPRINTF(stringIndex, firstToCheck)
#endif

/**
 * To manually declare an XPCOM ABI-compatible virtual function, the following
 * macros can be used to handle the non-standard ABI used on Windows for COM
 * compatibility. E.g.:
 *
 *   virtual ReturnType MOZ_XPCOM_ABI foo();
 */
#if defined(XP_WIN)
#  define MOZ_XPCOM_ABI __stdcall
#else
#  define MOZ_XPCOM_ABI
#endif

/**
 * MSVC / clang-cl don't optimize empty bases correctly unless we explicitly
 * tell it to, see:
 *
 * https://stackoverflow.com/questions/12701469/why-is-the-empty-base-class-optimization-ebo-is-not-working-in-msvc
 * https://devblogs.microsoft.com/cppblog/optimizing-the-layout-of-empty-base-classes-in-vs2015-update-2-3/
 */
#if defined(_MSC_VER)
#  define MOZ_EMPTY_BASES __declspec(empty_bases)
#else
#  define MOZ_EMPTY_BASES
#endif

#endif /* mozilla_Attributes_h */