It seems like one of Const-me's objections is that Rust data structures like HashMap don't document a lot of guarantees about when they would and wouldn't invalidate unsafe interior pointers. That said, for Vec in particular, Rust actually makes a ton of guarantees about its layout (more that C++ std::vector I think): https://doc.rust-lang.org/std/vec/struct.Vec.html
While vectors are comparable, C++ also guarantees a lot about the rest of the containers. E.g. unordered associative containers never expire pointers to keys or values. Linked lists never expire pointers nor iterators.
In C++ I can create an efficient LRU cache in a dozen lines of code, combining list<const tKey* > with unordered_map<tKey, struct{tValue, list<const tKey* >::iterator}> (this implies tKey is not an int, otherwise list<tKey> is more efficient). Rust’s built-in LinkedHashMap had to reimplement a linked list instead.
Thanks for the example -- it was hard for me to picture what was going on before. I wonder if the unstable "Placer" APIs will make the standard LinkedList more capable of this sort of thing, but I'm not really familiar with them. Still, I suspect it'll always require `unsafe`.
