[config] Add [[nodiscard]] as an attribute to v8config

This allows us to assert at compile time that a class instance is
assigned, which is particularly useful for Guard classes.

Change-Id: Id16b2bb70d29573566e821c908c1169d49ec57af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2552415
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71397}

include/v8config.h

@@ -239,6 +239,7 @@
 //  V8_HAS_ATTRIBUTE_VISIBILITY         - __attribute__((visibility)) supported
 //  V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT - __attribute__((warn_unused_result))
 //                                        supported
+//  V8_HAS_CPP_ATTRIBUTE_NODISCARD      - [[nodiscard]] supported
 //  V8_HAS_BUILTIN_BSWAP16              - __builtin_bswap16() supported
 //  V8_HAS_BUILTIN_BSWAP32              - __builtin_bswap32() supported
 //  V8_HAS_BUILTIN_BSWAP64              - __builtin_bswap64() supported
@@ -262,6 +263,12 @@
 //   ...
 //  #endif
 
+#if defined(__has_cpp_attribute)
+#define V8_HAS_CPP_ATTRIBUTE(FEATURE) __has_cpp_attribute(FEATURE)
+#else
+#define V8_HAS_CPP_ATTRIBUTE(FEATURE) 0
+#endif
+
 #if defined(__clang__)
 
 #if defined(__GNUC__)  // Clang in gcc mode.
@@ -276,6 +283,8 @@
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
     (__has_attribute(warn_unused_result))
 
+# define V8_HAS_CPP_ATTRIBUTE_NODISCARD (V8_HAS_CPP_ATTRIBUTE(nodiscard))
+
 # define V8_HAS_BUILTIN_ASSUME_ALIGNED (__has_builtin(__builtin_assume_aligned))
 # define V8_HAS_BUILTIN_BSWAP16 (__has_builtin(__builtin_bswap16))
 # define V8_HAS_BUILTIN_BSWAP32 (__has_builtin(__builtin_bswap32))
@@ -319,6 +328,7 @@
 # define V8_HAS_ATTRIBUTE_UNUSED 1
 # define V8_HAS_ATTRIBUTE_VISIBILITY 1
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT (!V8_CC_INTEL)
+# define V8_HAS_CPP_ATTRIBUTE_NODISCARD (V8_HAS_CPP_ATTRIBUTE(nodiscard))
 
 # define V8_HAS_BUILTIN_ASSUME_ALIGNED 1
 # define V8_HAS_BUILTIN_CLZ 1
@@ -436,6 +446,20 @@
 #define V8_WARN_UNUSED_RESULT /* NOT SUPPORTED */
 #endif
 
+
+// Annotate a class or constructor indicating the caller must assign the
+// constructed instances.
+// Apply to the whole class like:
+//   class V8_NODISCARD Foo() { ... };
+// or apply to just one constructor like:
+//   V8_NODISCARD Foo() { ... };
+// [[nodiscard]] comes in C++17 but supported in clang with -std >= c++11.
+#if V8_HAS_CPP_ATTRIBUTE_NODISCARD
+#define V8_NODISCARD [[nodiscard]]
+#else
+#define V8_NODISCARD /* NOT SUPPORTED */
+#endif
+
 // Helper macro to define no_sanitize attributes only with clang.
 #if defined(__clang__) && defined(__has_attribute)
 #if __has_attribute(no_sanitize)
@@ -484,4 +508,6 @@ V8 shared library set USING_V8_SHARED.
 
 // clang-format on
 
+#undef V8_HAS_CPP_ATTRIBUTE
+
 #endif  // V8CONFIG_H_

src/base/platform/mutex.h

@@ -282,7 +282,7 @@ class V8_BASE_EXPORT SharedMutex final {
 enum class NullBehavior { kRequireNotNull, kIgnoreIfNull };
 
 template <typename Mutex, NullBehavior Behavior = NullBehavior::kRequireNotNull>
-class LockGuard final {
+class V8_NODISCARD LockGuard final {
  public:
   explicit LockGuard(Mutex* mutex) : mutex_(mutex) {
     if (has_mutex()) mutex_->Lock();
@@ -310,7 +310,7 @@ enum MutexSharedType : bool { kShared = true, kExclusive = false };
 
 template <MutexSharedType kIsShared,
           NullBehavior Behavior = NullBehavior::kRequireNotNull>
-class SharedMutexGuard final {
+class V8_NODISCARD SharedMutexGuard final {
  public:
   explicit SharedMutexGuard(SharedMutex* mutex) : mutex_(mutex) {
     if (!has_mutex()) return;
@@ -343,7 +343,7 @@ class SharedMutexGuard final {
 
 template <MutexSharedType kIsShared,
           NullBehavior Behavior = NullBehavior::kRequireNotNull>
-class SharedMutexGuardIf final {
+class V8_NODISCARD SharedMutexGuardIf final {
  public:
   SharedMutexGuardIf(SharedMutex* mutex, bool enable_mutex) {
     if (enable_mutex) mutex_.emplace(mutex);

src/common/assert-scope.h

@@ -236,7 +236,7 @@ class DisallowHeapAccessIf {
 
 // Like MutexGuard but also asserts that no garbage collection happens while
 // we're holding the mutex.
-class NoGarbageCollectionMutexGuard {
+class V8_NODISCARD NoGarbageCollectionMutexGuard {
  public:
   explicit NoGarbageCollectionMutexGuard(base::Mutex* mutex)
       : guard_(mutex), mutex_(mutex), no_gc_(new DisallowGarbageCollection()) {}

src/execution/stack-guard.h

@@ -21,7 +21,7 @@ class RootVisitor;
 // StackGuard contains the handling of the limits that are used to limit the
 // number of nested invocations of JavaScript and the stack size used in each
 // invocation.
-class V8_EXPORT_PRIVATE StackGuard final {
+class V8_EXPORT_PRIVATE V8_NODISCARD StackGuard final {
  public:
   explicit StackGuard(Isolate* isolate) : isolate_(isolate) {}
 

src/heap/parked-scope.h

@@ -44,7 +44,7 @@ class UnparkedScope {
   LocalHeap* const local_heap_;
 };
 
-class ParkedMutexGuard {
+class V8_NODISCARD ParkedMutexGuard {
   base::Mutex* guard_;
 
  public:

src/libsampler/sampler.h

@@ -97,7 +97,7 @@ using AtomicMutex = std::atomic_bool;
 
 // A helper that uses an std::atomic_bool to create a lock that is obtained on
 // construction and released on destruction.
-class V8_EXPORT_PRIVATE AtomicGuard {
+class V8_EXPORT_PRIVATE V8_NODISCARD AtomicGuard {
  public:
   // Attempt to obtain the lock represented by |atomic|. |is_blocking|
   // determines whether we will block to obtain the lock, or only make one

src/objects/string-inl.h

@@ -26,7 +26,7 @@ namespace internal {
 
 #include "torque-generated/src/objects/string-tq-inl.inc"
 
-class SharedStringAccessGuardIfNeeded {
+class V8_NODISCARD SharedStringAccessGuardIfNeeded {
  public:
   // Creates a SharedMutexGuard<kShared> for the string access if:
   // A) {str} is not a read only string, and