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. I hope there is someone out there mining research papers for the implementation strategies that were abandoned as unworkable due to bookkeeping issues, which Rust has already paid.
But in the case of Options they tend to be both write-once and short-lived, so that removes a lot of necessity. Options are going to stay mostly on the stack and in async callbacks, unless they go into caches.
But for other data structures where multiples fields need a tag, I suspect Rust could use some bitfields for representing them. You’d need a fairly big win to make it worth implementing however.
> 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
I'm honestly not exactly sure what you're talking about, but the fundamental limit for atomic writes is usually the register-size for a CPU which is usually 64 or 32 bits. Considering enum variants are often larger than a single register in size, I don't see how atomic writes would ever be sane expectation.
I see. Yeah, you would either have to add the tagging to the upper bits of the pointer itself or concede that updates to a tagged type is not atomic. I feel like the latter is fine in most every scenario I've encountered in Rust (thanks to borrow checker rules) but in other languages the same cannot be said.
But in the case of Options they tend to be both write-once and short-lived, so that removes a lot of necessity. Options are going to stay mostly on the stack and in async callbacks, unless they go into caches.
But for other data structures where multiples fields need a tag, I suspect Rust could use some bitfields for representing them. You’d need a fairly big win to make it worth implementing however.