Another would be static mutable variables, which are perfectly safe in a single-threaded environment, doubly so if you map that memory on a per-core basis to ensure each kernel has a unique variable value. Esp if you need lazy_static then there aren't many options other than wrapping the data in a mutex for no good reason.
`static mut` is not always safe even in a single-threaded environment, because of reentrance - see also https://github.com/rust-lang/rust/issues/53639.
Reentrance is not the issue here, I guard this inside the data structure, interrupts will be worst case reentrance most of the time.
But without guarding mutability behind some way to indicate interrupts are disabled, or an outright lock, and without guaranteeing no data races (Sync), it's not actually safe.
And the compiler can't make `static mut` safe to use as any of those requirements could be broken and then it's not safe at all anymore.