Function unpack 

pub fn unpack(state: &mut LuaState) -> Result<usize, LuaError>

Pushes t[i], t[i+1], …, t[j] and returns the count.

static int tunpack (lua_State *L) {
  lua_Unsigned n;
  lua_Integer i = luaL_optinteger(L, 2, 1);
  lua_Integer e = luaL_opt(L, luaL_checkinteger, 3, luaL_len(L, 1));
  if (i > e) return 0;
  n = (lua_Unsigned)e - i;
  if (l_unlikely(n >= (unsigned int)INT_MAX ||
                 !lua_checkstack(L, (int)(++n))))
    return luaL_error(L, "too many results to unpack");
  for (; i < e; i++) lua_geti(L, 1, i);
  lua_geti(L, 1, e);
  return (int)n;
}