Extirpate FOLLY_NODISCARD

Summary: `[[nodiscard]]` is now available in all of our compilers. This diff changes the code to rely on it directly.

Reviewed By: yfeldblum

Differential Revision: D87101329

fbshipit-source-id: 29a60e1cbd3be5103414d9b6bbe52b3bde545bf4

Author: r-barnes

folly/io/async/DelayedDestructionBase.h

@@ -62,7 +62,7 @@ class DelayedDestructionBase {
    * object, causing problems when the callback function returns and the
    * guarded object's method resumes execution.
    */
-  class FOLLY_NODISCARD DestructorGuard {
+  class [[nodiscard]] DestructorGuard {
    public:
     explicit DestructorGuard(DelayedDestructionBase* dd) : dd_(dd) {
       if (dd_ != nullptr) {

folly/io/async/EventBaseAtomicNotificationQueue.h

@@ -92,7 +92,7 @@ class EventBaseAtomicNotificationQueue
    * Returns true iff the task was queued.
    * Can be called from any thread.
    */
-  FOLLY_NODISCARD bool tryPutMessage(Task&& task, uint32_t maxSize);
+  [[nodiscard]] bool tryPutMessage(Task&& task, uint32_t maxSize);
 
   /*
    * Detaches the queue from an EventBase.

folly/io/async/Request.cpp

@@ -235,7 +235,7 @@ RequestContext::State::~State() {
   }
 }
 
-class FOLLY_NODISCARD RequestContext::State::LockGuard {
+class [[nodiscard]] RequestContext::State::LockGuard {
  public:
   explicit LockGuard(RequestContext::State& state)
       : state_(state), lock_(state.mutex_) {}

folly/io/async/observer/AsyncSocketObserverInterface.h

@@ -144,7 +144,7 @@ class AsyncSocketObserverInterface {
      *   and ACK flags, and the other requesting just the SCHED flag, then we
      *   will add the TX, ACK, and SCHED flags to the request.
      */
-    FOLLY_NODISCARD const PrewriteRequest& getMergedRequest() const {
+    [[nodiscard]] const PrewriteRequest& getMergedRequest() const {
       return mergedRequest_;
     }
 
@@ -197,7 +197,7 @@ class AsyncSocketObserverInterface {
     /**
      * For WRITE events, returns if SCHED timestamp requested.
      */
-    FOLLY_NODISCARD bool schedTimestampRequestedOnWrite() const {
+    [[nodiscard]] bool schedTimestampRequestedOnWrite() const {
       CHECK_EQ(Type::WRITE, type);
       CHECK(maybeWriteFlags.has_value());
       return isSet(*maybeWriteFlags, WriteFlags::TIMESTAMP_SCHED);
@@ -206,7 +206,7 @@ class AsyncSocketObserverInterface {
     /**
      * For WRITE events, returns if TX timestamp requested.
      */
-    FOLLY_NODISCARD bool txTimestampRequestedOnWrite() const {
+    [[nodiscard]] bool txTimestampRequestedOnWrite() const {
       CHECK_EQ(Type::WRITE, type);
       CHECK(maybeWriteFlags.has_value());
       return isSet(*maybeWriteFlags, WriteFlags::TIMESTAMP_TX);
@@ -215,7 +215,7 @@ class AsyncSocketObserverInterface {
     /**
      * For WRITE events, returns if ACK timestamp requested.
      */
-    FOLLY_NODISCARD bool ackTimestampRequestedOnWrite() const {
+    [[nodiscard]] bool ackTimestampRequestedOnWrite() const {
       CHECK_EQ(Type::WRITE, type);
       CHECK(maybeWriteFlags.has_value());
       return isSet(*maybeWriteFlags, WriteFlags::TIMESTAMP_ACK);

folly/lang/Bits.h

@@ -241,7 +241,7 @@ template <class T>
 struct n_least_significant_bits_fn {
   static_assert(detail::supported_in_bits_operations_v<T>);
 
-  FOLLY_NODISCARD constexpr T operator()(std::uint32_t n) const {
+  [[nodiscard]] constexpr T operator()(std::uint32_t n) const {
     if (!folly::is_constant_evaluated_or(true)) {
       compiler_may_unsafely_assume(n <= sizeof(T) * 8);
 
@@ -272,7 +272,7 @@ template <class T>
 struct n_most_significant_bits_fn {
   static_assert(detail::supported_in_bits_operations_v<T>);
 
-  FOLLY_NODISCARD constexpr T operator()(std::uint32_t n) const {
+  [[nodiscard]] constexpr T operator()(std::uint32_t n) const {
     if (!folly::is_constant_evaluated_or(true)) {
       compiler_may_unsafely_assume(n <= sizeof(T) * 8);
 
@@ -303,7 +303,7 @@ inline constexpr n_most_significant_bits_fn<T> n_most_significant_bits;
 /// Clears n least significant (right) bits. Other bits stay the same.
 struct clear_n_least_significant_bits_fn {
   template <typename T>
-  FOLLY_NODISCARD constexpr T operator()(T x, std::uint32_t n) const {
+  [[nodiscard]] constexpr T operator()(T x, std::uint32_t n) const {
     static_assert(detail::supported_in_bits_operations_v<T>);
 
     // alternative is to do two shifts but that has
@@ -321,7 +321,7 @@ inline constexpr clear_n_least_significant_bits_fn
 /// Sets n least significant (right) bits. Other bits stay the same.
 struct set_n_least_significant_bits_fn {
   template <typename T>
-  FOLLY_NODISCARD constexpr T operator()(T x, std::uint32_t n) const {
+  [[nodiscard]] constexpr T operator()(T x, std::uint32_t n) const {
     static_assert(detail::supported_in_bits_operations_v<T>);
 
     // alternative is to do two shifts but that has
@@ -338,7 +338,7 @@ inline constexpr set_n_least_significant_bits_fn set_n_least_significant_bits;
 /// Clears n most significant (left) bits. Other bits stay the same.
 struct clear_n_most_significant_bits_fn {
   template <typename T>
-  FOLLY_NODISCARD constexpr T operator()(T x, std::uint32_t n) const {
+  [[nodiscard]] constexpr T operator()(T x, std::uint32_t n) const {
     static_assert(detail::supported_in_bits_operations_v<T>);
 
     if (!folly::is_constant_evaluated_or(true)) {
@@ -366,7 +366,7 @@ inline constexpr clear_n_most_significant_bits_fn clear_n_most_significant_bits;
 /// Sets n most significant (left) bits. Other bits stay the same.
 struct set_n_most_significant_bits_fn {
   template <typename T>
-  FOLLY_NODISCARD constexpr T operator()(T x, std::uint32_t n) const {
+  [[nodiscard]] constexpr T operator()(T x, std::uint32_t n) const {
     static_assert(detail::supported_in_bits_operations_v<T>);
     return x | n_most_significant_bits<T>(n);
   }
@@ -485,7 +485,7 @@ class Endian {
 ///
 struct get_bit_at_fn {
   template <typename Uint>
-  FOLLY_NODISCARD constexpr bool operator()(
+  [[nodiscard]] constexpr bool operator()(
       const Uint* ptr, std::size_t idx) const noexcept {
     static_assert(std::is_unsigned_v<std::remove_cv_t<Uint>>);
     static_assert(!std::is_same_v<std::remove_cv_t<Uint>, bool>);