Function check_udata 

pub fn check_udata(
    state: &mut LuaState,
    ud: i32,
    tname: &[u8],
) -> Result<GcRef<LuaUserData>, LuaError>

Like test_udata but raises a type error if the check fails.

C: LUALIB_API void *luaL_checkudata(lua_State *L, int ud, const char *tname)