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Flattening Rust’s learning curve

(corrode.dev)
451 points birdculture | 2 comments | 13 May 25 22:25 UTC | HN request time: 0s | source
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dmitrygr ◴[13 May 25 23:24 UTC] No.43978986[source]▶
> Treat the borrow checker as a co-author, not an adversary

Why would I pair-program with someone who doesn’t understand doubly-linked lists?

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mre ◴[13 May 25 23:43 UTC] No.43979123[source]▶
For people who don't get the reference, this might be referring to the notoriously gnarly task of implementing a doubly-linked lists in Rust [1]

It is doable, just not as easy as in other languages because a production-grade linked-list is unsafe because Rust's ownership model fundamentally conflicts with the doubly-linked structure. Each node in a doubly-linked list needs to point to both its next and previous nodes, but Rust's ownership rules don't easily allow for multiple owners of the same data or circular references.

You can implement one in safe Rust using Rc<RefCell<Node>> (reference counting with interior mutability), but that adds runtime overhead and isn't as performant. Or you can use raw pointers with unsafe code, which is what most production implementations do, including the standard library's LinkedList.

https://rust-unofficial.github.io/too-many-lists/

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1. sbrother ◴[14 May 25 03:30 UTC] No.43980462[source]▶
Apologies since I have not taken the time to learn rust yet, but I've written a lot of modern C++. Is the ownership model kind of like std::unique_ptr and std::move, and `Rc<RefCell<Node>>` the same idea as `std::shared_ptr`? But less idiomatic? Or do I have the wrong idea?
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2. khuey ◴[14 May 25 04:28 UTC] No.43980810[source]▶
Not really, because Rust enforces a "many readers or one writer" invariant on everything that has no C++ equivalent. That invariant is precisely what makes the doubly-linked list case hard (because every interior node in the list would be readable from two places, which means it can never be written to).