The borrowchecker is what I like the least about Rust

I don't use Rust much, but I agree with the thrust of the article. However, I do think that the borrowchecker is the only reason Rust actually caught on. In my opinion, it's really hard for a new language to succeed unless you can point to something and say "You literally can't do this in your language"

Without something like that, I think it just would have been impossible for Rust to gain enough momentum, and also attract the sort of people that made its culture what it is.

Otherwise, IMO Rust would have ended up just like D, a language that few people have ever used, but most people who have heard of it will say "apparently it's a better safer C++, but I'm not going to switch because I can technically do all that stuff in C++"

> [The pain of the borrow checker is felt] when your existing project requires a small modification to ownership structure, and the borrowchecker then refuses to compile your code. Then, once you pull at the tiny loose fiber in your code's fabric, you find you have to unspool half your code before the borrowchecker is satisfied.

Probably I just haven't been writing very "advanced" rust programs in the sense of doing complicated things that require advanced usages of lifetimes and references. But having written rust professionally for 3 years now, I haven't encountered this once. Just putting this out there as another data point.

Of course, partial borrows would make things nicer. So would polonius (which I believe is supposed to resolve the "famous" issue the post mentions, and maybe allow self-referential structs a long way down the road). But it's very rare that I encounter a situation where I actually need these. (example: a much more common need for me is more powerful consteval.)

Before writing Rust professionally, I wrote OCaml professionally. To people who wish for "rust, but with a garbage collector", I suggest you use OCaml! The languages are extremely similar.

Regarding Indexes: "When the same borrowchecker makes references unworkable, their solution is to... recommend that I manually manage them, with zero safety and zero language support?!?"

Language support: You can implement extension traits on an integer so you can do things like current_node.next(v) (like if you have an integer named 'current_node' which is an index into a vector v of nodes) and customize how your next() works.

Also, I disagree there is 'zero safety', since the indexes are into a Rust vector, they are bounds checked by default when "dereferencing" the index into the vector (v[i]), and the checking is not that slow for vast majority of use cases. If you go out of bounds, Rust will panic and tell you exactly where it panicked. If panicking is a problem you could theoretically have custom deference code that does something more graceful than panic.

But with using indexes there is no corruption of memory outside of the vector where you are keeping your data, in other words there isn't a buffer overflow attack that allows for machine instructions to be overwritten with data, which is where a huge amount of vulnerabilities and hacks have come from over the past few decades. That's what is meant by 'safety' in general.

I know people stick in 'unsafe' to gain a few percent speed sometimes, but then it's unsafe rust by definition. I agree that unsafe rust is unsafe.

Also you can do silly optimization tricks like if you need to perform a single operation on the entire collection of nodes, you can parallelize it easily by iterating thru the vector without having to iterate through the data structure using next/prev leaf/branch whatever.

I choose a language that is as ergonomic as possible, but as performant as necessary. If e.g. Kotlin is fine, there is no way I will choose Rust.

Many projects are written in Rust that would absolutely be fine in Go, Swift or a JVM language. And I don't understand: it is nicer to write in those other languages, why choose Rust?

On the other hand, Rust is a lot nicer than C/C++, so I see it as a valid alternative there: I'm a lot happier having to make the borrow-checker happy than tracking tricky memory errors in C.

One of his examples of a borrow checker excess:

    struct Id(u32);

    fn main() {
        let id = Id(5);
        let mut v = vec![id];
        println!("{}", id.0);
    }

He does point out two significant problems in Rust. When you need to change a program, re-doing the ownership plumbing can be quite time-consuming. Losing a few days on that is a routine Rust experience. Rust forces you to pay for your technical debt up front in that area.

The other big problem is back references. Rust still lacks a good solution in that area. So often, you want A to own B, and B to be able to reference A. Rust will not allow that directly. There are three workarounds commonly used.

- Put all the items in an array and refer to them by index. Then write run-time code to manage all that. The Bevy game engine is an example of a large Rust system which does this. The trouble is that you've re-created dangling pointers, in the form of indices kept around after they are invalid. Now you have most of the problems of raw pointers. They will at least be an index to some structure of the right type, but that's all the guarantee you get. I've found bugs in that approach in Rust crates.

- Unsafe code with raw pointers. That seldom ends well. Crates which do that are almost the only time I've had to use a debugger on Rust code.

- Rc/RefCell/run-time ".borrow()". This moves all the checking to run time. It's safe, but you panic at run time if two things borrow the same item.

This is a fundamental problem in Rust. I've mentioned this before. What's needed to fix this is an analyzer that checks the scope of explicit .borrow() and .borrow_mut() calls, and determines that all scopes for the same object are disjoint. This is not too hard conceptually if all the .borrow() calls produce locally scoped results. It does mean a full call chain analysis. It's a lot like static detection of deadlock, which is a known area of research [1] but something not seen in production yet.

I've discussed this with some of the Rust developers. The problem is generics. When you call a generic, the calling code has no idea what code the generic is going to generate. You don't know what it's going to borrow. You'd have to do this static analysis after generic expansion. Rust avoids that; generics either compile for all cases, or not at all. Such restricted generic expansion avoids the huge compile error messages from hell associated with C++ template instantiation fails. Post template expansion static analysis is thus considered undesirable.

Fixing that could be done with annotation, along the lines of "this function might borrow 'foo'". That rapidly gets clunky. People hate doing transitive closure by hand. Remember Java checked exceptions.

This is a good PhD topic for somebody in programming language theory. It's a well-known hard problem for which a solution would be useful. There's no easy general fix.

[1] https://dl.acm.org/doi/10.1145/3540250.3549110

This reminds me of something that was popular in some bioinformatics circles years ago. People claimed that Java was faster than C++. To "prove" that, they wrote reasonably efficient Java code for some task, and then rewrote it in C++. Using std::shared_ptr extensively to get something resembling garbage collection. No wonder the real Java code was faster than the Java code written in C++.

I've been writing C++ for almost 30 years, and a few years of Rust. I sometimes struggle with the Rust borrow checker, and it's almost always my fault. I keep trying to write C++ in Rust, because I'm thinking in C++ instead of Rust.

The lesson is always the same. If you want to use language X, you must learn to write X, instead of writing language Y in X.

Using indexes (or node ids or opaque handles) in graph/tree implementations is a good idea both in C++ and in Rust. It makes serialization easier and faster. It allows you to use data structures where you can't have a pointer to a node. And it can also save memory, as pointers and separate memory allocations take a lot of space when you have billions of them. Like when working with human genomes.

For the disjoint field issues raised, it’s not that the borrow checker can’t “reason across functions,” it’s that the field borrows are done through getter functions which themselves borrow the whole struct mutably. This could be avoided by making the fields public so they can be referenced directly, or if the fields needs to be passed to other functions, just pass the the field references rather than passing the whole struct.

There are open ideas for how to handle “view types” that express that you’re only borrowing specific fields of a struct, including Self, but they’re an ergonomic improvement, not a semantic power improvement.

The author's motivation for writing this is well-founded. However, the author doesn't take into account the full spirit of rust and the un-constructive conclusion doesn't really help anyone.

A huge part of the spirit of rust is fearless concurrency. The simple seeming false positive examples become non-trivial in concurrent code.

The author admits they don't write large concurrent - which clearly explains why they don't find much use in the borrow checker. So the problem isn't that the rust doesn't work for them - it's that a central language feature of rust hampers them instead of helping them.

The conclusion for this article should have been: if you're like me and don't write concurrent programs, enums and matches are great. The language would be work better for me if the arc/box syntax spam went away.

As a side note, if your code is a house of cards, it's probably because you prematurely optimized. A good way to get around this problem is to arc/box spam upfront with as little abstraction as possible, then profile, then optimize.

I think there's a lot love for the borrowchecker because a lot of people in the Rust community are working on ecosystems (eg https://github.com/linebender) which means they are building up an api over many years. In that case having a very restrictive language is really great, because it kinda defines the shape the api can have at the language level, meaning that familiarity with Rust also means quick familiarity with your api. In that sense it doesn't matter if the restrictions are "arbitrary" or useful.

The other end of the spectrum is something like gamedev: you write code that pretty explicitly has an end-date, and the actual shape of the program can change drastically during development (because it's a creative thing) so you very much don't want to slowly build up rigidity over time.

This post pretty much completely ignores the advantages of the borrow checker. I'm not talking about memory safety, which is it's original purpose. I'm talking about the fact that code that follows Rust's tree-style ownership pattern and doesn't excessively circumvent the borrow checker is more likely to be correct.

I don't think that was ever the intent behind the borrow checker but it is definitely an outcome.

So yes, the borrow checker makes some code more awkward than it would be in GC languages, but the benefits are easily worth it and they stretch far beyond memory safety.

If you notice a rule for the first time, restricting what you want to do and making you jump through a hoop, it can be hard to see what the rule is actually for. The thrust of the piece is 'there should not be so many rules, let me do whatever I want to do if the code would make sense to me'. This does not survive contact with (say) an Enterprise Java codebase filled with a billion cases of defensive strict immutability and defensive copies, because the language lacks Rust's rules and constructs about shared mutability and without them you have to use constant discipline to prevent bugs. `derive(Copy)` as One More Pointless Thing You Have To Do only makes sense if you haven't spent much time code-reviewing someone's C++.

If you try to write Java in Rust, you will fail. Rust is no different in this regard from Haskell, but method syntax feels so friendly that it doesn't register that this is a language you genuinely have to learn instead of picking up the basics in a couple hours and immediately start implementing `increment_counter`-style interfaces.

And this is an inexperienced take, no matter how eloquently it's written. You can see it immediately from the complaint about CS101 pointer-chasing graph structures, and apoplexy at the thought of index-based structures, when any serious graph should be written with an index-based adjacency list and writing your own nonintrusive collection types is pretty rare in normal code. Just Use Petgraph.

A beginner is told to 'Just' use borrow-splitting functions, and this feels like a hoop to jump through. This is because it's not the real answer. The real answer is that once you have properly learned Rust, once your reflexes are procedural instead of object-oriented, you stop running into the problem altogether; you automatically architect code so it doesn't come up (as often). The article mentions this point and says 'nuh uh', but everyone saying it is personally at this level; 'intermittent' Rust usage is not really a good Learning Environment.

To the author, I would be a borrow checker apologist or perhaps extremist. I will take that mantle gladly: I am very much of the opinion that a systems programming language without a borrow checker[^1] will not find itself holding C++-like hegemony anymore (once/if C++ releases the scepter, that is). I guess I would even be sad if C++ kept the scepter for the rest of my life, or was replaced by another language that didn't have something like a borrow checker.

It doesn't need to be Rust: Rust's borrow checker has (mostly reasonable) limitations that eg. make some interprocedural things impossible while being possible within a single function (eg. &mut Vec<u32> and &mut u32 derived from it, both being used at the same time as shared references, and then one or the other being used as exclusive later). Maybe some other language will come in with a more powerful and omniscient borrow checker[^1], and leave Rust in the dust. It definitely can happen, and if it does then I suppose we'll enjoy that language then.

But: it is my opinion that a borrow checker is an absolutely massive thing in a (non-GC) programming language, and one that cannot be ignored in the future. (Though, Zig is proving me wrong here and it's doing a lot of really cool things. What memory safety vulnerabilities in the Ziglang world end up looking like remains to be seen.) Memory is always owned by some_one_, its validity is always determined by some_one_, and having that validity enforced by the language is absolutely priceless.

Wanting GC for some things is of course totally valid; just reach for a GC library for those cases, or if you think it's the right tool for the job then use a GC language.

[^1]: Or something even better that can replace the borrow checker; maybe Graydon Hoare's original idea of path based aliasing analysis would've been that? Who knows.