That's not my point. A general built-in checking scheme that handles all the variations of linked lists and trees will likely have to have quite a few nobs to t

That's not my point. A general built-in checking scheme that handles all the variations of linked lists and trees will likely have to have quite a few nobs to tweak for the various trade-offs one might want to make, and will likely have to be very general to handle all the variations, making that general infrastructure significantly more complicated (both to implement in the unsafe block that is the compiler, and, likely, for users to understand) than focused `unsafe` blocks, which can, and often are, packaged up into safe interfaces (like reference counting with weak pointers). > Use of back references is going to have to be restricted so you can't get two mutable handles to the same object. The trick is making that work through restrictions which can be checked locally. Talk is cheap, and I'm not sure this talk actually says anything of note. Those "ideas" seem to be the most obvious first considerations for working out how this feature might even work: - disallowing two mutable handles is the fundamental rule of Rust, - the ability to check locally is a strong convention for programming language implementations, and a very strong one for Rust. Having some concrete idea that addresses just those two points (no need to worry about syntax or anything like that) would be an improvement on the current situation: I haven't see any for this other than "runtime checking" (already exists in Rust) and "no back references". --- In other words, vague assertions like "Rust should support back pointers" with syntax ideas are jumping way ahead. The language typically tries to provide building blocks to allow things to be built in libraries, rather than lumping opinionated features into the language. These features will often then require unsafe to create safe abstractions, but this isn't bad: the compiler itself is essentially one large unsafe block. The place to start would be trying to create a safe back-pointer interface in a library, and seeing if there's anything that is too hard to make truly safe. This has in fact already been done, with Rc/Weak and Arc/Weak, but for zero-overhead/non-reference-counted scenarios, I think a good place to start would be a smart pointer pair like Rc but with a constructor `fn make<T>(val: T) -> (Forward<T>, Back<T>)`.