That might indeed be the case, I did not look very closely, but mlua has gotten quite some coverage (and I do like lua quite a bit), so I wanted it mentioned :)

Hmm, right. I think it still might be warranted in niche cases, but trying to think of such a case made it pretty protracted in my head... maybe when functions can also be called for side effects, and the into conversion is costly and the caller might not care about the return value?