static Upvaldesc *allocupvalue (FuncState *fs) {
  Proto *f = fs->f;
  int oldsize = f->sizeupvalues;
  checklimit(fs, fs->nups + 1, MAXUPVAL, "upvalues");
  luaM_growvector(fs->ls->L, f->upvalues, fs->nups, f->sizeupvalues,
                  Upvaldesc, MAXUPVAL, "upvalues");
  while (oldsize < f->sizeupvalues)
    f->upvalues[oldsize++].name = NULL;
  return &f->upvalues[fs->nups++];
}