path: root/mfbt/Assertions.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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 runtime and static assertion macros for C and C++. */

#ifndef mozilla_Assertions_h
#define mozilla_Assertions_h

#if defined(MOZILLA_INTERNAL_API) && defined(__cplusplus)
#define MOZ_DUMP_ASSERTION_STACK
#endif

#include "mozilla/Attributes.h"
#include "mozilla/Compiler.h"
#include "mozilla/Likely.h"
#include "mozilla/MacroArgs.h"
#include "mozilla/StaticAnalysisFunctions.h"
#include "mozilla/Types.h"
#ifdef MOZ_DUMP_ASSERTION_STACK
#include "nsTraceRefcnt.h"
#endif

#if defined(MOZ_HAS_MOZGLUE) || defined(MOZILLA_INTERNAL_API)
/*
 * The crash reason set by MOZ_CRASH_ANNOTATE is consumed by the crash reporter
 * if present. It is declared here (and defined in Assertions.cpp) to make it
 * available to all code, even libraries that don't link with the crash reporter
 * directly.
 */
MOZ_BEGIN_EXTERN_C
extern MFBT_DATA const char* gMozCrashReason;
MOZ_END_EXTERN_C

static inline void
AnnotateMozCrashReason(const char* reason)
{
  gMozCrashReason = reason;
}
#  define MOZ_CRASH_ANNOTATE(...) AnnotateMozCrashReason(__VA_ARGS__)
#else
#  define MOZ_CRASH_ANNOTATE(...) do { /* nothing */ } while (0)
#endif

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#ifdef WIN32
   /*
    * TerminateProcess and GetCurrentProcess are defined in <winbase.h>, which
    * further depends on <windef.h>.  We hardcode these few definitions manually
    * because those headers clutter the global namespace with a significant
    * number of undesired macros and symbols.
    */
#  ifdef __cplusplus
extern "C" {
#  endif
__declspec(dllimport) int __stdcall
TerminateProcess(void* hProcess, unsigned int uExitCode);
__declspec(dllimport) void* __stdcall GetCurrentProcess(void);
#  ifdef __cplusplus
}
#  endif
#else
#  include <signal.h>
#endif

/*
 * MOZ_STATIC_ASSERT may be used to assert a condition *at compile time* in C.
 * In C++11, static_assert is provided by the compiler to the same effect.
 * This can be useful when you make certain assumptions about what must hold for
 * optimal, or even correct, behavior.  For example, you might assert that the
 * size of a struct is a multiple of the target architecture's word size:
 *
 *   struct S { ... };
 *   // C
 *   MOZ_STATIC_ASSERT(sizeof(S) % sizeof(size_t) == 0,
 *                     "S should be a multiple of word size for efficiency");
 *   // C++11
 *   static_assert(sizeof(S) % sizeof(size_t) == 0,
 *                 "S should be a multiple of word size for efficiency");
 *
 * This macro can be used in any location where both an extern declaration and a
 * typedef could be used.
 */
#ifndef __cplusplus
   /*
    * Some of the definitions below create an otherwise-unused typedef.  This
    * triggers compiler warnings with some versions of gcc, so mark the typedefs
    * as permissibly-unused to disable the warnings.
    */
#  if defined(__GNUC__)
#    define MOZ_STATIC_ASSERT_UNUSED_ATTRIBUTE __attribute__((unused))
#  else
#    define MOZ_STATIC_ASSERT_UNUSED_ATTRIBUTE /* nothing */
#  endif
#  define MOZ_STATIC_ASSERT_GLUE1(x, y)          x##y
#  define MOZ_STATIC_ASSERT_GLUE(x, y)           MOZ_STATIC_ASSERT_GLUE1(x, y)
#  if defined(__SUNPRO_CC)
     /*
      * The Sun Studio C++ compiler is buggy when declaring, inside a function,
      * another extern'd function with an array argument whose length contains a
      * sizeof, triggering the error message "sizeof expression not accepted as
      * size of array parameter".  This bug (6688515, not public yet) would hit
      * defining moz_static_assert as a function, so we always define an extern
      * array for Sun Studio.
      *
      * We include the line number in the symbol name in a best-effort attempt
      * to avoid conflicts (see below).
      */
#    define MOZ_STATIC_ASSERT(cond, reason) \
       extern char MOZ_STATIC_ASSERT_GLUE(moz_static_assert, __LINE__)[(cond) ? 1 : -1]
#  elif defined(__COUNTER__)
     /*
      * If there was no preferred alternative, use a compiler-agnostic version.
      *
      * Note that the non-__COUNTER__ version has a bug in C++: it can't be used
      * in both |extern "C"| and normal C++ in the same translation unit.  (Alas
      * |extern "C"| isn't allowed in a function.)  The only affected compiler
      * we really care about is gcc 4.2.  For that compiler and others like it,
      * we include the line number in the function name to do the best we can to
      * avoid conflicts.  These should be rare: a conflict would require use of
      * MOZ_STATIC_ASSERT on the same line in separate files in the same
      * translation unit, *and* the uses would have to be in code with
      * different linkage, *and* the first observed use must be in C++-linkage
      * code.
      */
#    define MOZ_STATIC_ASSERT(cond, reason) \
       typedef int MOZ_STATIC_ASSERT_GLUE(moz_static_assert, __COUNTER__)[(cond) ? 1 : -1] MOZ_STATIC_ASSERT_UNUSED_ATTRIBUTE
#  else
#    define MOZ_STATIC_ASSERT(cond, reason) \
       extern void MOZ_STATIC_ASSERT_GLUE(moz_static_assert, __LINE__)(int arg[(cond) ? 1 : -1]) MOZ_STATIC_ASSERT_UNUSED_ATTRIBUTE
#  endif

#define MOZ_STATIC_ASSERT_IF(cond, expr, reason)  MOZ_STATIC_ASSERT(!(cond) || (expr), reason)
#else
#define MOZ_STATIC_ASSERT_IF(cond, expr, reason)  static_assert(!(cond) || (expr), reason)
#endif

#ifdef __cplusplus
extern "C" {
#endif

/*
 * Prints |aStr| as an assertion failure (using aFilename and aLine as the
 * location of the assertion) to the standard debug-output channel.
 *
 * Usually you should use MOZ_ASSERT or MOZ_CRASH instead of this method.  This
 * method is primarily for internal use in this header, and only secondarily
 * for use in implementing release-build assertions.
 */
static MOZ_COLD MOZ_ALWAYS_INLINE void
MOZ_ReportAssertionFailure(const char* aStr, const char* aFilename, int aLine)
  MOZ_PRETEND_NORETURN_FOR_STATIC_ANALYSIS
{
  fprintf(stderr, "Assertion failure: %s, at %s:%d\n", aStr, aFilename, aLine);
#if defined (MOZ_DUMP_ASSERTION_STACK)
  nsTraceRefcnt::WalkTheStack(stderr);
#endif
  fflush(stderr);
}

static MOZ_COLD MOZ_ALWAYS_INLINE void
MOZ_ReportCrash(const char* aStr, const char* aFilename, int aLine)
  MOZ_PRETEND_NORETURN_FOR_STATIC_ANALYSIS
{
  fprintf(stderr, "Hit MOZ_CRASH(%s) at %s:%d\n", aStr, aFilename, aLine);
#if defined(MOZ_DUMP_ASSERTION_STACK)
  nsTraceRefcnt::WalkTheStack(stderr);
#endif
  fflush(stderr);
}

/**
 * MOZ_REALLY_CRASH is used in the implementation of MOZ_CRASH().  You should
 * call MOZ_CRASH instead.
 */
#if defined(_MSC_VER)
   /*
    * On MSVC use the __debugbreak compiler intrinsic, which produces an inline
    * (not nested in a system function) breakpoint.  This distinctively invokes
    * Breakpad without requiring system library symbols on all stack-processing
    * machines, as a nested breakpoint would require.
    *
    * We use TerminateProcess with the exit code aborting would generate
    * because we don't want to invoke atexit handlers, destructors, library
    * unload handlers, and so on when our process might be in a compromised
    * state.
    *
    * We don't use abort() because it'd cause Windows to annoyingly pop up the
    * process error dialog multiple times.  See bug 345118 and bug 426163.
    *
    * We follow TerminateProcess() with a call to MOZ_NoReturn() so that the
    * compiler doesn't hassle us to provide a return statement after a
    * MOZ_REALLY_CRASH() call.
    *
    * (Technically these are Windows requirements, not MSVC requirements.  But
    * practically you need MSVC for debugging, and we only ship builds created
    * by MSVC, so doing it this way reduces complexity.)
    */

__declspec(noreturn) __inline void MOZ_NoReturn() {}

#  ifdef __cplusplus
#    define MOZ_REALLY_CRASH(line) \
       do { \
         ::__debugbreak(); \
         *((volatile int*) NULL) = line; \
         ::TerminateProcess(::GetCurrentProcess(), 3); \
         ::MOZ_NoReturn(); \
       } while (0)
#  else
#    define MOZ_REALLY_CRASH(line) \
       do { \
         __debugbreak(); \
         *((volatile int*) NULL) = line; \
         TerminateProcess(GetCurrentProcess(), 3); \
         MOZ_NoReturn(); \
       } while (0)
#  endif
#else
#  ifdef __cplusplus
#    define MOZ_REALLY_CRASH(line) \
       do { \
         *((volatile int*) NULL) = line; \
         ::abort(); \
       } while (0)
#  else
#    define MOZ_REALLY_CRASH(line) \
       do { \
         *((volatile int*) NULL) = line; \
         abort(); \
       } while (0)
#  endif
#endif

/*
 * MOZ_CRASH([explanation-string]) crashes the program, plain and simple, in a
 * Breakpad-compatible way, in both debug and release builds.
 *
 * MOZ_CRASH is a good solution for "handling" failure cases when you're
 * unwilling or unable to handle them more cleanly -- for OOM, for likely memory
 * corruption, and so on.  It's also a good solution if you need safe behavior
 * in release builds as well as debug builds.  But if the failure is one that
 * should be debugged and fixed, MOZ_ASSERT is generally preferable.
 *
 * The optional explanation-string, if provided, must be a string literal
 * explaining why we're crashing.  This argument is intended for use with
 * MOZ_CRASH() calls whose rationale is non-obvious; don't use it if it's
 * obvious why we're crashing.
 *
 * If we're a DEBUG build and we crash at a MOZ_CRASH which provides an
 * explanation-string, we print the string to stderr.  Otherwise, we don't
 * print anything; this is because we want MOZ_CRASH to be 100% safe in release
 * builds, and it's hard to print to stderr safely when memory might have been
 * corrupted.
 */
#ifndef DEBUG
#  define MOZ_CRASH(...) \
     do { \
       MOZ_CRASH_ANNOTATE("MOZ_CRASH(" __VA_ARGS__ ")"); \
       MOZ_REALLY_CRASH(__LINE__); \
     } while (0)
#else
#  define MOZ_CRASH(...) \
     do { \
       MOZ_ReportCrash("" __VA_ARGS__, __FILE__, __LINE__); \
       MOZ_CRASH_ANNOTATE("MOZ_CRASH(" __VA_ARGS__ ")"); \
       MOZ_REALLY_CRASH(__LINE__); \
     } while (0)
#endif

/*
 * MOZ_CRASH_UNSAFE_OOL(explanation-string) can be used if the explanation
 * string cannot be a string literal (but no other processing needs to be done
 * on it). A regular MOZ_CRASH() is preferred wherever possible, as passing
 * arbitrary strings from a potentially compromised process is not without risk.
 * If the string being passed is the result of a printf-style function,
 * consider using MOZ_CRASH_UNSAFE_PRINTF instead.
 */
#ifndef DEBUG
MFBT_API MOZ_COLD MOZ_NORETURN MOZ_NEVER_INLINE void
MOZ_CrashOOL(int aLine, const char* aReason);
#  define MOZ_CRASH_UNSAFE_OOL(reason) MOZ_CrashOOL(__LINE__, reason)
#else
MFBT_API MOZ_COLD MOZ_NORETURN MOZ_NEVER_INLINE void
MOZ_CrashOOL(const char* aFilename, int aLine, const char* aReason);
#  define MOZ_CRASH_UNSAFE_OOL(reason) MOZ_CrashOOL(__FILE__, __LINE__, reason)
#endif

static const size_t sPrintfMaxArgs = 4;
static const size_t sPrintfCrashReasonSize = 1024;

#ifndef DEBUG
MFBT_API MOZ_COLD MOZ_NORETURN MOZ_NEVER_INLINE MOZ_FORMAT_PRINTF(2, 3) void
MOZ_CrashPrintf(int aLine, const char* aFormat, ...);
#  define MOZ_CALL_CRASH_PRINTF(format, ...) \
     MOZ_CrashPrintf(__LINE__, format, __VA_ARGS__)
#else
MFBT_API MOZ_COLD MOZ_NORETURN MOZ_NEVER_INLINE MOZ_FORMAT_PRINTF(3, 4) void
MOZ_CrashPrintf(const char* aFilename, int aLine, const char* aFormat, ...);
#  define MOZ_CALL_CRASH_PRINTF(format, ...) \
     MOZ_CrashPrintf(__FILE__, __LINE__, format, __VA_ARGS__)
#endif

/*
 * MOZ_CRASH_UNSAFE_PRINTF(format, arg1 [, args]) can be used when more
 * information is desired than a string literal can supply. The caller provides
 * a printf-style format string, which must be a string literal and between
 * 1 and 4 additional arguments. A regular MOZ_CRASH() is preferred wherever
 * possible, as passing arbitrary strings to printf from a potentially
 * compromised process is not without risk.
 */
#define MOZ_CRASH_UNSAFE_PRINTF(format, ...) \
   do { \
     MOZ_STATIC_ASSERT_VALID_ARG_COUNT(__VA_ARGS__); \
     static_assert( \
       MOZ_PASTE_PREFIX_AND_ARG_COUNT(, __VA_ARGS__) <= sPrintfMaxArgs, \
       "Only up to 4 additional arguments are allowed!"); \
     static_assert(sizeof(format) <= sPrintfCrashReasonSize, \
       "The supplied format string is too long!"); \
     MOZ_CALL_CRASH_PRINTF("" format, __VA_ARGS__); \
   } while (0)

#ifdef __cplusplus
} /* extern "C" */
#endif

/*
 * MOZ_ASSERT(expr [, explanation-string]) asserts that |expr| must be truthy in
 * debug builds.  If it is, execution continues.  Otherwise, an error message
 * including the expression and the explanation-string (if provided) is printed,
 * an attempt is made to invoke any existing debugger, and execution halts.
 * MOZ_ASSERT is fatal: no recovery is possible.  Do not assert a condition
 * which can correctly be falsy.
 *
 * The optional explanation-string, if provided, must be a string literal
 * explaining the assertion.  It is intended for use with assertions whose
 * correctness or rationale is non-obvious, and for assertions where the "real"
 * condition being tested is best described prosaically.  Don't provide an
 * explanation if it's not actually helpful.
 *
 *   // No explanation needed: pointer arguments often must not be NULL.
 *   MOZ_ASSERT(arg);
 *
 *   // An explanation can be helpful to explain exactly how we know an
 *   // assertion is valid.
 *   MOZ_ASSERT(state == WAITING_FOR_RESPONSE,
 *              "given that <thingA> and <thingB>, we must have...");
 *
 *   // Or it might disambiguate multiple identical (save for their location)
 *   // assertions of the same expression.
 *   MOZ_ASSERT(getSlot(PRIMITIVE_THIS_SLOT).isUndefined(),
 *              "we already set [[PrimitiveThis]] for this Boolean object");
 *   MOZ_ASSERT(getSlot(PRIMITIVE_THIS_SLOT).isUndefined(),
 *              "we already set [[PrimitiveThis]] for this String object");
 *
 * MOZ_ASSERT has no effect in non-debug builds.  It is designed to catch bugs
 * *only* during debugging, not "in the field". If you want the latter, use
 * MOZ_RELEASE_ASSERT, which applies to non-debug builds as well.
 *
 * MOZ_DIAGNOSTIC_ASSERT works like MOZ_RELEASE_ASSERT in Nightly/Aurora and
 * MOZ_ASSERT in Beta/Release - use this when a condition is potentially rare
 * enough to require real user testing to hit, but is not security-sensitive.
 * This can cause user pain, so use it sparingly. If a MOZ_DIAGNOSTIC_ASSERT
 * is firing, it should promptly be converted to a MOZ_ASSERT while the failure
 * is being investigated, rather than letting users suffer.
 */

/*
 * Implement MOZ_VALIDATE_ASSERT_CONDITION_TYPE, which is used to guard against
 * accidentally passing something unintended in lieu of an assertion condition.
 */

#ifdef __cplusplus
#  include "mozilla/TypeTraits.h"
namespace mozilla {
namespace detail {

template<typename T>
struct AssertionConditionType
{
  typedef typename RemoveReference<T>::Type ValueT;
  static_assert(!IsArray<ValueT>::value,
                "Expected boolean assertion condition, got an array or a "
                "string!");
  static_assert(!IsFunction<ValueT>::value,
                "Expected boolean assertion condition, got a function! Did "
                "you intend to call that function?");
  static_assert(!IsFloatingPoint<ValueT>::value,
                "It's often a bad idea to assert that a floating-point number "
                "is nonzero, because such assertions tend to intermittently "
                "fail. Shouldn't your code gracefully handle this case instead "
                "of asserting? Anyway, if you really want to do that, write an "
                "explicit boolean condition, like !!x or x!=0.");

  static const bool isValid = true;
};

} // namespace detail
} // namespace mozilla
#  define MOZ_VALIDATE_ASSERT_CONDITION_TYPE(x) \
     static_assert(mozilla::detail::AssertionConditionType<decltype(x)>::isValid, \
                   "invalid assertion condition")
#else
#  define MOZ_VALIDATE_ASSERT_CONDITION_TYPE(x)
#endif

/* First the single-argument form. */
#define MOZ_ASSERT_HELPER1(expr) \
  do { \
    MOZ_VALIDATE_ASSERT_CONDITION_TYPE(expr); \
    if (MOZ_UNLIKELY(!MOZ_CHECK_ASSERT_ASSIGNMENT(expr))) { \
      MOZ_ReportAssertionFailure(#expr, __FILE__, __LINE__); \
      MOZ_CRASH_ANNOTATE("MOZ_RELEASE_ASSERT(" #expr ")"); \
      MOZ_REALLY_CRASH(__LINE__); \
    } \
  } while (0)
/* Now the two-argument form. */
#define MOZ_ASSERT_HELPER2(expr, explain) \
  do { \
    MOZ_VALIDATE_ASSERT_CONDITION_TYPE(expr); \
    if (MOZ_UNLIKELY(!MOZ_CHECK_ASSERT_ASSIGNMENT(expr))) { \
      MOZ_ReportAssertionFailure(#expr " (" explain ")", __FILE__, __LINE__); \
      MOZ_CRASH_ANNOTATE("MOZ_RELEASE_ASSERT(" #expr ") (" explain ")"); \
      MOZ_REALLY_CRASH(__LINE__); \
    } \
  } while (0)

#define MOZ_RELEASE_ASSERT_GLUE(a, b) a b
#define MOZ_RELEASE_ASSERT(...) \
  MOZ_RELEASE_ASSERT_GLUE( \
    MOZ_PASTE_PREFIX_AND_ARG_COUNT(MOZ_ASSERT_HELPER, __VA_ARGS__), \
    (__VA_ARGS__))

#ifdef DEBUG
#  define MOZ_ASSERT(...) MOZ_RELEASE_ASSERT(__VA_ARGS__)
#else
#  define MOZ_ASSERT(...) do { } while (0)
#endif /* DEBUG */

#ifdef RELEASE_OR_BETA
#  define MOZ_DIAGNOSTIC_ASSERT MOZ_ASSERT
#else
#  define MOZ_DIAGNOSTIC_ASSERT MOZ_RELEASE_ASSERT
#endif

/*
 * MOZ_ASSERT_IF(cond1, cond2) is equivalent to MOZ_ASSERT(cond2) if cond1 is
 * true.
 *
 *   MOZ_ASSERT_IF(isPrime(num), num == 2 || isOdd(num));
 *
 * As with MOZ_ASSERT, MOZ_ASSERT_IF has effect only in debug builds.  It is
 * designed to catch bugs during debugging, not "in the field".
 */
#ifdef DEBUG
#  define MOZ_ASSERT_IF(cond, expr) \
     do { \
       if (cond) { \
         MOZ_ASSERT(expr); \
       } \
     } while (0)
#else
#  define MOZ_ASSERT_IF(cond, expr)  do { } while (0)
#endif

/*
 * MOZ_ASSUME_UNREACHABLE_MARKER() expands to an expression which states that
 * it is undefined behavior for execution to reach this point.  No guarantees
 * are made about what will happen if this is reached at runtime.  Most code
 * should use MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE because it has extra
 * asserts.
 */
#if defined(__clang__) || defined(__GNUC__)
#  define MOZ_ASSUME_UNREACHABLE_MARKER() __builtin_unreachable()
#elif defined(_MSC_VER)
#  define MOZ_ASSUME_UNREACHABLE_MARKER() __assume(0)
#else
#  ifdef __cplusplus
#    define MOZ_ASSUME_UNREACHABLE_MARKER() ::abort()
#  else
#    define MOZ_ASSUME_UNREACHABLE_MARKER() abort()
#  endif
#endif

/*
 * MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE([reason]) tells the compiler that it
 * can assume that the macro call cannot be reached during execution.  This lets
 * the compiler generate better-optimized code under some circumstances, at the
 * expense of the program's behavior being undefined if control reaches the
 * MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE.
 *
 * In Gecko, you probably should not use this macro outside of performance- or
 * size-critical code, because it's unsafe.  If you don't care about code size
 * or performance, you should probably use MOZ_ASSERT or MOZ_CRASH.
 *
 * SpiderMonkey is a different beast, and there it's acceptable to use
 * MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE more widely.
 *
 * Note that MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE is noreturn, so it's valid
 * not to return a value following a MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE
 * call.
 *
 * Example usage:
 *
 *   enum ValueType {
 *     VALUE_STRING,
 *     VALUE_INT,
 *     VALUE_FLOAT
 *   };
 *
 *   int ptrToInt(ValueType type, void* value) {
 *   {
 *     // We know for sure that type is either INT or FLOAT, and we want this
 *     // code to run as quickly as possible.
 *     switch (type) {
 *     case VALUE_INT:
 *       return *(int*) value;
 *     case VALUE_FLOAT:
 *       return (int) *(float*) value;
 *     default:
 *       MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE("Unexpected ValueType");
 *     }
 *   }
 */

/*
 * Unconditional assert in debug builds for (assumed) unreachable code paths
 * that have a safe return without crashing in release builds.
 */
#define MOZ_ASSERT_UNREACHABLE(reason) \
   MOZ_ASSERT(false, "MOZ_ASSERT_UNREACHABLE: " reason)

#define MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE(reason) \
   do { \
     MOZ_ASSERT_UNREACHABLE(reason); \
     MOZ_ASSUME_UNREACHABLE_MARKER(); \
   } while (0)

/**
 * MOZ_FALLTHROUGH_ASSERT is an annotation to suppress compiler warnings about
 * switch cases that MOZ_ASSERT(false) (or its alias MOZ_ASSERT_UNREACHABLE) in
 * debug builds, but intentionally fall through in release builds to handle
 * unexpected values.
 *
 * Why do we need MOZ_FALLTHROUGH_ASSERT in addition to MOZ_FALLTHROUGH? In
 * release builds, the MOZ_ASSERT(false) will expand to `do { } while (0)`,
 * requiring a MOZ_FALLTHROUGH annotation to suppress a -Wimplicit-fallthrough
 * warning. In debug builds, the MOZ_ASSERT(false) will expand to something like
 * `if (true) { MOZ_CRASH(); }` and the MOZ_FALLTHROUGH annotation will cause
 * a -Wunreachable-code warning. The MOZ_FALLTHROUGH_ASSERT macro breaks this
 * warning stalemate.
 *
 * // Example before MOZ_FALLTHROUGH_ASSERT:
 * switch (foo) {
 *   default:
 *     // This case wants to assert in debug builds, fall through in release.
 *     MOZ_ASSERT(false); // -Wimplicit-fallthrough warning in release builds!
 *     MOZ_FALLTHROUGH;   // but -Wunreachable-code warning in debug builds!
 *   case 5:
 *     return 5;
 * }
 *
 * // Example with MOZ_FALLTHROUGH_ASSERT:
 * switch (foo) {
 *   default:
 *     // This case asserts in debug builds, falls through in release.
 *     MOZ_FALLTHROUGH_ASSERT("Unexpected foo value?!");
 *   case 5:
 *     return 5;
 * }
 */
#ifdef DEBUG
#  define MOZ_FALLTHROUGH_ASSERT(reason) MOZ_CRASH("MOZ_FALLTHROUGH_ASSERT: " reason)
#else
#  define MOZ_FALLTHROUGH_ASSERT(...) MOZ_FALLTHROUGH
#endif

/*
 * MOZ_ALWAYS_TRUE(expr) and MOZ_ALWAYS_FALSE(expr) always evaluate the provided
 * expression, in debug builds and in release builds both.  Then, in debug
 * builds only, the value of the expression is asserted either true or false
 * using MOZ_ASSERT.
 */
#ifdef DEBUG
#  define MOZ_ALWAYS_TRUE(expr) \
     do { \
       if ((expr)) { \
         /* Do nothing. */ \
       } else { \
         MOZ_ASSERT(false, #expr); \
       } \
     } while (0)
#  define MOZ_ALWAYS_FALSE(expr) \
     do { \
       if ((expr)) { \
         MOZ_ASSERT(false, #expr); \
       } else { \
         /* Do nothing. */ \
       } \
     } while (0)
#else
#  define MOZ_ALWAYS_TRUE(expr) \
     do { \
       if ((expr)) { \
          /* Silence MOZ_MUST_USE. */ \
       } \
     } while (0)
#  define MOZ_ALWAYS_FALSE(expr) \
     do { \
       if ((expr)) { \
         /* Silence MOZ_MUST_USE. */ \
       } \
     } while (0)
#endif

#undef MOZ_DUMP_ASSERTION_STACK
#undef MOZ_CRASH_CRASHREPORT

#endif /* mozilla_Assertions_h */