using ResourceIdSet = llvm::SmallSet<ResourceId, 8>;

// Note that we cannot simply define a `static const llvm::SmallSet` here
// because of missing `initializer_list` support for `llvm::SmallSet`.
const ResourceIdSet& UnknownResourceSet() {
  // clang-format off
  static auto* id_set = new ResourceIdSet();
  id_set->insert(kUnknownResourceId);
  return *id_set;
}

// Helper function to avoid frequent checks for unknown IDs.