[Georgelemental]

> In a lot of languages space optimizing Optional Types without using a reserved enum value or pointer tags would lead to memory model problems with atomically writing two values at once which might be more easily solved in a borrow semantics world.

Yes, Rust suppresses the niche optimization for values wrapped in an `UnsafeCell` (which is how you signal to the compiler that “atomically writing two values at once” might happen). https://github.com/rust-lang/rust/pull/68491