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by pizlonator 265 days ago
Doubly linked lists. Any cyclic data structure.
5 comments

While I think this is Rust's biggest flaw, this doesn't stem from any particular hatred of C/C++. This is related to memory safety, as it is very difficult to reason about memory lifetimes of object graphs with cycles.
There are doubly-linked list libraries in Rust. They are safe and well-designed. It's rare for a project to actually need a doubly-linked list. When you need it, you should use those instead of reimplementing your own.
Both of those are a direct consequence of Rust's memory model, not a result of animus against C++.
That's not at all a "we don't like C++" problem. That's "the chosen borrow-checking model makes cyclic structures hard, you have to do Rc/Arc and some manual management, or use unsafe and raw pointers". (Frankly, the latter is easier.)
Ref counting doesn’t work for cyclic data structures
Sure it does. Make the backreferences weak, or use a library that provides cycle detection (or even full-on tracing GC if you really want).
That changes the semantics. It won’t work for a graph for example.
Right. The comment you replied to said "Rc/Arc and some manual management" -- "some manual management" implying that you need to account for the change to semantics in the manner appropriate for your application.
(Note that I edited my comment for clarity since the original reply. My original comment just said "reference counting".)
It doesn't fully handle the memory management, but it handles having shared references. You can use weak references or manually break the links when you want to free something. Or you can use unsafe raw pointers, which frankly seem simpler. Either way you're going to wrap it in a data structure that encapsulates all the details.
It works well. One easy trick is to have one reference count for the whole graph.
Rc has downgrade and the Weak type explicitly for this purpose.
The easiest way to implement these is with Weak references in one of the directions. This requires some checking and/or locks, but the fact it is not trivial to do is kinda the point: the integrity of a cyclic data structure depends on managing multiple pointers in multiple structs simultaneously, and with that comes concurrency dragons.