Method ProjectKeys

common/values/value_builder.cc:1109–1122

 private:
  const CompatListValue* absl_nonnull ProjectKeys() const {
    absl::call_once(keys_once_, [this]() {
      ListValueBuilderImpl builder(map_.get_allocator().arena());
      builder.Reserve(map_.size());

      for (const auto& entry : map_) {
        builder.UnsafeAdd(entry.first);
      }

      std::move(builder).BuildCompatAt(&keys_[0]);
    });
    return std::launder(
        reinterpret_cast<const CompatListValueImpl*>(&keys_[0]));
  }

  const ValueFlatHashMap map_;
  mutable absl::once_flag keys_once_;