Yeah, `unsafe` is the developer signing a contract to uphold all the guarantees that safe rustc provided (at least from the perspective of the public interface, the invariants can be broken in private code).

That's why Rust developers don't take kindly to unnecessary unsafe usage, because the audit surface area (and interaction complexity) increases.

Rust takes const with pointers very seriously. It is undefined behavior to mutate anything non-mut (pointer or reference) unless it is through an UnsafeCell. While you can break compiler guarantees through pointers in Rust, dereferencing this pointers must still obey the above guarantee (basically think of it as you need to go back to references to actually use the aliased pointers which would be undefined due to the aliasing).

Therefore, the optimizer can always assume mutations don't alias. Even UnsafeCell isn't allowed to break the alias rules, it just provides flexibility going from immutable to mutable.

I mean obviously there are people who write incorrect unsafe code, but it is certainly something people take seriously.