Function check_lstring 

pub fn check_lstring(
    state: &mut LuaState,
    arg: i32,
) -> Result<GcRef<LuaString>, LuaError>

Return the string at arg as bytes; raise a type error if not a string.

C: LUALIB_API const char *luaL_checklstring(lua_State *L, int arg, size_t *len)