[xg15]

I'm not convinced that cyclic data structures are a feature that's exotic enough to warrant breaking the language contracts. This thread alone shows that it's a common occurrence in tree structures. There will be likely be other structures as well.

So I'd think a better way than saying "don't do that - or if you have to, you're on your own" would be to analyze use cases and see which scenarios the language can satisfy. (As another poster suggested in making "parent pointers" an explicit language feature(

[Animats]

Agree. The language's memory safety system needs to support more common use cases. At least:

- Back pointers. (They're not owning pointers, and they have a well defined relationship with some other pointer. The language may need some way to explicitly say that.)

- Collections where not all slots are initialized. (This requires simple proofs by induction as you grow a collection into previously allocated unused space.)

[stouset]

You can always use std::rc::Weak if you need a cyclic data structure with weak references. That covers, I suspect, the majority of the use-cases for these type of structures.

For real, honest-to-god, cyclic data structures — just use unsafe Rust. Again, unsafe Rust isn't "bad". It's just unsafe. More care will be expected of the code to ensure that it exports a safe interface, but nothing in the language stops you from doing it. There's quite literally no loss of expressive power.