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Professional Rust developer here.
First of all, congratulations on getting paid to write Rust code without actually liking it. In my experience this was quite difficult to achieve so everyone around me really likes Rust a lot.
Borrow checker is honestly one of the hot topics about Rust that always confuses me, because I almost never have problems with it apart from some very small edge cases. Meanwhile people online seem to base half their Rust experience on the borrow checker. May I ask you what sort of programming did you do?
I will fight you to death about the mut keyword. It is absolutely fantastic and it pushes you in a more functional way of programming.
But this can be achieved easily with like a 10 line wrapper type around Mutexes in your codebase?
Anyway I used to write C code and switched to Rust without brushing much with C++ so it has been a gigantic improvement in all ways. But honestly I fail to see why anyone would prefer C++ at this point for new projects (except for gamedev where shipping something buggy and fun trumps correctness by a lot).
Ok, I guess we're getting into the weeds now. Anybody who's not a programmer, feel free to bow out. :)
Backend infra work. Efficiency was important, so we ended up having to use pointers to bypass the Rust borrow checker in cases where it just didn't work (e.g. for caching a map lookup). Concurrency was also important, and Rust's concurrency features were kind of tacked on later in its design. (Actually, Rust relies on third-party libraries to solve a LOT of its design flaws, like tokio for async stuff.) I do admit that the Send/Sync traits are nice for catching a few common concurrency bugs. But screw Rust for intentionally not implementing them for the pointer type because they want to discourage its use.
I don't think you should compare "const" and "mut" if you're not a C++ developer and you've never used "const" before. "const" is "absolutely fantastic" and pushes you to a "more functional way of programming" - it lets you clearly spell out what side effects all your functions and APIs have. With Rust's version of "&mut" - which despite the lying name (and horrible syntax) actually just means "exclusive reference", something that is correlated with but not the same as "mutable" - a shared non-mut object might change, it might not, you'll just have to check the implementation for details.
Here's a C++ object:
MyStruct. Here's the read-only version of that C++ object:const MyStruct. You'll forgive me for not being impressed that this common safety pattern can be done in "like 10 lines" in Rust by implementing your own Mutex wrapper.Yeah, don't get me wrong, C++ has plenty of issues, and I wish there was a better alternative that kept its strengths too. Rust, unfortunately, decided not to. If you're going directly from C to Rust, you might even think it's normal that you need third-party libraries to, say, put floats into a heap (heh).
EDIT: Actually, while I'm on the topic, and since you're a Rust pro, let me ask... does putting wrappers around everything feel natural to Rust developers? The type inference makes it so you don't have to see all the nested wrapping. Maybe that's just a paradigm I don't grok. Because I find Rust's heap to be spectacularly and uniquely badly designed. Rather than treating the comparator as a separate parameter from the thing being compared - like in every other language in existence - Rust instead has you create a wrapper object to override the object's comparisons. Which means you need to do type conversions for every get and put operation. And if you pull some objects out of a max-heap and you forget to convert them back, then do some comparisons on them yourself... the code (because its types are hidden) will look innocuous, but you'll get exactly the wrong result.
This is wild for me. I work in HFT and almost never have to touch pointers even in cases where latency/performance is important to absurd levels. I know you said you don't work anymore, but passing pointers around threads like what you are describing is not even a good C++ practice: https://floooh.github.io/2018/06/17/handles-vs-pointers.html. Pointers are too big on 64 bit systems and will mess up your cache locality and you will often get horrible assembly because aliasing (more details on that below).
What design flaw is tokio solving? It is quite inherent in Rust that the language doesn't ship with a default runtime, so there is experimentation around this. And tokio is a really fantastic piece of software. Maybe you are thinking of libraries like async-trait? It can be annoying to find gaps like this in the language that takes years to close, but they have been going down in numbers rapidly, and it is quite nice that different solutions can be tested as macro libraries before being baked into language design. Definitely beats a committee hashing out nonsense which then cannot be changed forever because muh backwards compatibility.
Yes and this is fantastic design because aliasing. If you ever stared at absolutely horrible assembly because gcc couldn't prove two pieces of memory are not overlapping, you understand why this language design exists. It is not for political reasons or giving an illusion of progress over C++. Godbolt guy's recent compiler optimizations video is a decent primer: https://youtube.com/watch?v=PPJtJzT2U04?si=kZ3CFZKlzDCeSRxX. mut is much superior to const for this reason alone. Crazy that C has the reserved keyword to address this problem, but C++ doesn't even have this. (also it is a correct choice to make the default non-mut, makes reasoning about code much easier). I think you have a point about interior mutability confusing this relationship though. You win some you lose some.
I am not very knowledgable about this subject, but if const MyStruct contains a pointer in it, you can simply mutate the pointed data right? Chatgpt seems to think so. Then this is the exact same problem you are complaining about with Rust Refcell/Mutexes. Your const MyStruct doesn't guarantee anything at all not already guaranteed by Rust shared reference or Rc/Arc without interior mutability.
Yes the newtype pattern is extremely standard in Rust and I do it all the time.
Usually when you newtype something, you are doing so because you don't want the inside type to be accessible as it is anymore, but only through a certain limited interface. In most use cases you should be writing the newtype in a way that the user can do everything they need through the newtype. It is pretty trivial to define arithmetic operations on your newtype for example
So, you have some good responses and I want to repeat that this is all just my opinion and personal experience. Thanks for the reply.
I won't address your criticism of our design because you don't know the details (and I'm not going into them). But we're not stupid people. Not collectively, at least... :) This attitude of "I've thought for three whole minutes and I can't see why anybody would ever want this, so the people telling me they want this are wrong" is kind of emblematic of Rust's design vs C++'s.
I agree it's nice that Rust solves the aliasing problem. They didn't have to solve it by confusing mutability and exclusivity, though! It's like if a plumber comes and fixes my toilet, but breaks some windows on the way out. The correct response to "hey, why did you break the windows" isn't "look, the toilet's fixed!"
You can if you can access it, yeah, but it'd be weird for the C++ object to give a user access to raw internal pointers. Note that in most cases you're also limited on what methods (including getters/setters) you can call on a "const MyStruct" - whether a function is read-only or not can be explicitly declared in its signature. Now, you don't have to use const at all in a library. Or you could use it but then have the same "internal mutability" as Rust - but why would you do that? The point of the keyword's existence is to allow programmers to positively assert "this is what a read-only version of this object looks like". Which is powerful and important and should be easier than it is in Rust.
If your complaint is that "const" doesn't force the C++ programmer to be safe, you're right about that. C++ gives options, not restrictions, and for bad programmers it is indeed full of footguns. My praise of "const" is due to what it can be in the hands of good programmers - and I would have loved it if Rust introduced the keyword but applied its stringent safety guarantees to it.
But the heap example I give is the exact opposite of this. I want access to the inside type (the number) - I never want to think about the heap's operator except at initialization - but instead I'm being forced to wrap it everywhere. Now, it's not entirely fair to compare Rust's library with C++'s STL. Like you said, "it can be annoying to find gaps like this in the language that takes years to close" - Rust is young, and the STL is the result of decades of finetuning/nitpicking work from thousands of people. I agree that, in the future, something like Rust's bad heap implementation might be a non-problem.
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