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What Rust calls "reference" is a much more specific thing, that is not so general-purpose. (users.rust-lang.org)
submitted 2 years ago by maegul@lemmy.ml to c/learningrustandlemmy@lemmy.ml
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A nice comment in a forum thread (extracted below, but also see the shorter more facetious version below that) about references and their lifetimes in structs. Here is a link to the full thread over on users.rust-lang.org

I feel like I needed to hear or read this, and I feel like this sort of clarification is not put front and centre enough in rust learning material (as others in the thread say too). "The Book" certainly doesn't seem interested in clarifying perspectives like this.

The Comment

Other languages use term "reference" for storing things "by reference" or just referencing any object anywhere in general. It's not like that in Rust.

What Rust calls "reference" is a much more specific thing, that is no so general-purpose. It has a narrower, restrictive usage. Rust references are more like read-only or write-exclusive locks. They make their target unmovable and immutable for entire duration of their existence. They can't exist on their own, only as a counterpart of an owned value.

References in structs also make the whole struct itself temporary, and everything that touches that struct becomes temporary and tied to the scope of the borrowed value that started it.

If these restrictions (that cause you fight with the borrow checker) aren't what you want to achieve, then you don't want temporary references.

99% of the time when you need to store something "by reference", Box (or Arc or String or PathBuf or Vec or some other owned type) is the right answer.

Note that &T and Box<T> have identical representation in memory — a pointer. They differ by ownership.

In Short

From here

You're not allowed to use references in structs until you think Rust is easy. They're the evil-hardmode of Rust that will ruin your day.

😉

Use Box or Arc to store things in structs "by reference". Temporary borrows don't do what you think they do.

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[-] Jayjader@jlai.lu 2 points 2 years ago

I was reading this last evening and ultimately decide against pinging you with it at the time, lol.

It directly riffs off of, and complements, the article you posted. It also speaks to your remarks on an optional garbage collector, as well as "how to think about the lifetime/borrow checker/permissions interplay".

but I think I would have found it better to go from the basic idea of pointers as a way of not taking ownership

small quibble: String, Box, and Vec are all technically pointers that do take ownership (rather, they have ownership of what they're pointing to). It's really only "references" in Rust that don't take ownership. Which, IIRC, is more or less how The Book introduces references in chap 4. So I'm not really sure how what you're describing would differ from the current state of the book. Nonetheless, I understand the confusion that comes from "learning about" lifetimes so "late".

I suspect Rust is so complex, and like a living organism its parts have co-evolved together so much, that there is no linear explanation or introduction that can "truly" do it justice. You sort of have to throw yourself in, and repeat that process from a handful of directions/trajectories, to start to get a cohesive idea of the whole. I say "co-evolved" notably because so much of "modern" Rust seems to me to be syntactic sugar over "core/original/2015" rust.

I haven't gotten to the chapters on explicit lifetime annotation this time around, but I expect The Book to clearly state, then, that everything has a lifetime and the compiler actually infers most of them for you as part of this "syntactic sugar".

[-] maegul@lemmy.ml 1 points 2 years ago

Thanks for the link! Will try to read it.

small quibble: String, Box, and Vec are all technically pointers that do take ownership (rather, they have ownership of what they’re pointing to). It’s really only “references” in Rust that don’t take ownership. Which, IIRC, is more or less how The Book introduces references in chap 4. So I’m not really sure how what you’re describing would differ from the current state of the book. Nonetheless, I understand the confusion that comes from “learning about” lifetimes so “late”.

Yea, I was going to specify (lol) that my first time through I really missed or didn't focus on what Box and Vec were as possibly alternative tools to references or at least interesting objects in terms of combining ownership and pointers to data on the heap ... and how I'm not sure that's really on me given the order of ideas in The Book and the emphasis on references. Again, for me right now, it seems that "lifetimes" is the connecting concept in that it clarifies what's going on and what problems are being solved/created. For me, instead of the section in Ch 4, it was an early section in the Rustonomicon (that I was just casually browsing through ... I know nothing about unsafe rust) that leaned in hard on the centrality of lifetimes.

Nonetheless, I'm a little keen now to get a grip on how a Box (or other owning + pointing type) can be used as an easier/cleaner substitute for references. I don't have a clear image in my mind and I think I just need some application to work with or example to read through (maybe in the blog post you linked).

Thanks again for the link!

this post was submitted on 28 May 2024
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