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Rust questoin, cyclic data structures (lemmy.world)

submitted 1 week ago* (last edited 1 week ago) by [email protected] to c/[email protected]

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Question is how to do these in Rust. An example might be a browser DOM: each node has a parent pointer, a list of child pointers, left and right sibling pointers, maybe a CSS node pointer, etc. Inserting or deleting nodes has to repair the pointers to and from the neighboring nodes as needed.

I know this is doable since obviously Servo (Rust's initial driving application) has to do it. I hope the answer doesn't involve the word "unsafe". But I am quite unclear about how to create such a structure under Rust's rules of pointer ownership, and also how to reliably reclaim storage when nodes and trees/subtrees are deleted. Plus there will be thread safety rules that should be statically enforced if possible.

I've heard that doubly linked lists in Rust are handled by an unsafe library, yet this seems even worse. Thanks.

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[–] [email protected] 2 points 1 week ago (1 children)

Doubly linked list is one of std's collections. And safety in Rust is built on top of unsafely, because there is no way around that.

Did you try to actually look up literally anything before asking?! Because simply checking out std::collections docs would have given you some answers.

[–] [email protected] 4 points 1 week ago* (last edited 1 week ago)

I don't see anything in the std::collections doc relevant to this question. Yes I knew about the presence of doubly linked lists in that library. Obviously you could implement the DOM structure unsafely with C-style pointers but then what are you getting from using Rust instead of C? So I hoped to hear how to do it safely, maybe using a library object as a building block. But, I don't think the stuff in std::collections suffice for the purpose.

Rust safety isn't necessarily built on unsafety, though apparently it is in the case of doubly linked lists. I think singly linked lists can be made safely in Rust, similar to using C++ std::unique_ptr. It's possible, though very complicated, to eliminate all the unsafety everywhere. Rust doesn't attempt this, but (for example) ATS does (ats-lang.org).

Anyway your scolding is unhelpful. If you've got a usable and clear answer to my question I'd be happy to try to digest it. Otherwise I don't see any point in continuing to respond.