Once rust-lang/rust#77547 lands, I think the untagged_unions feature can be removed from stdarch. I do not know what the usual process for this is, does stdarch use bootstrap flags like everything else?

I also noticed stdarch tests use unions to transmute things. Is there any particular reason why the tests do not use transmute? Note in particular that stdarch is using unions incorrectly here; without repr(C), there is no guarantee that the union fields all start at the same offset.