Hm, good point. (It's implemented as a Once (which is an atomic usize) and an UnsafeCell for holding the actual data.) And while using SyncOnceCell normally might incur some small checking overhead on each access, you can use it to obtain a &'static. https://play.rust-lang.org/?version=nightly&mode=debug&editi...

But then, by tialaramex's logic, isn't it just as bad as Box::leak? (Maybe not quite as, because it can leak maximally one thing. You can't do this in a loop.)