flag_trick.h

  // This is reserved space for an absl::Mutex to guard flag data. It will be
  // initialized in FlagImpl::Init via placement new.
  // We can't use "absl::Mutex data_guard_", since this class is not literal.
  // We do not want to use "absl::Mutex* data_guard_", since this would require
  // heap allocation during initialization, which is both slows program startup
  // and can fail. Using reserved space + placement new allows us to avoid both
  // problems.
  alignas(absl::Mutex) mutable char data_guard_[sizeof(absl::Mutex)];