Function lua_lightuserdatatag 

pub unsafe extern "C-unwind" fn lua_lightuserdatatag(
    L: *mut lua_State,
    idx: c_int,
) -> c_int

Returns the tag of the light userdata at the given index.

If the value at the given index is not a light userdata, this function will return -1. Otherwise, it will return the tag of the light userdata.