idk, I really feel like I should write a blog post, or docs, or something about this. `static mut` is really footgunny (more so than UnsafeCell IMO), but this is one of those things that:

* IS decently challenging to get right
* WAS often incorrect many times
* Many people THINK they know how it does/should work

I think the change to UnsafeCell is better, it closes a couple doors like "`static mut` makes everything `Sync`", where for different types you can do stuff like `unsafe impl<T: Sync> Sync for Container<T> {}`, or `unsafe impl<T: Send> Sync for Container<T> {}` or so.