[myrrlyn]

the main problem here is that C has the capability to declare mmap regions correctly: `volatile char[]` and Rust does not (`[Cell<u8>]` is close but not exactly right, and annoying)

most rust folks who use mmap don't mark the region as Celled, which means they risk UB in the form of incorrect behavior because the compiler assumes that the memory region is untouchable outside the single Rust program, and that's not true

(it's also not true generally b/c /dev/mem and /proc/pid/mem exist, but it's beyond Rust's scope that the OS allows intrusion like that)