Function test_udata 

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

Check whether the value at ud is a full userdata with metatable tname. Returns Some(userdata) if yes, None otherwise.

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