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Comment by simonask | Hacker News Reader

Wait, what? Unsafe in Rust _is_ the actual benefit. That is to say, what `unsafe` does is that it allows you to implement a fundamentally tricky thing and express its safety invariants in the type system and lifetime system, resulting in a safe API with no tricky parts.

That's the whole point.

There's tons of trivial unsafe in the Rust ecosystem, and a little bit of nontrivial unsafe, because crates.io is full of libraries doing interesting things (high-performance data structures, synchronization primitives, FFI bindings, etc.) while providing a safe API, so you can do all of that without writing any unsafe yourself.

The point of Rust isn't that you can implement low-level data structures in safe code, but that you can use them without fear.

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Saying that unsafe is a benefit is backwards, I think. Unsafe is the compromise Rust made to achieve its other goals, but of course Rust would be better if there was some way of doing things without unsafe.

Consider the split_at_mut function. It takes one mutable slice, and returns two mutable slices by chopping at a caller-specified point. It's in the standard library.

The operation is completely safe, as after the call the original mutable slice is no longer live, but the borrow checker won't let you write such a function yourself unless you tag it as unsafe, so that's what the implementation must do.

The same thing happens in the implementation of Vec: there is low-level code that is unsafe, used to provide safe abstractions.

That’s not a benefit of Rust. In other safe languages you could implement those things without writing a single bit of unsafe code. (This is true in Fil-C for example.)

Is that really fair? Neither Fil-C nor C have anything that particularly resembles & or &mut. If Rust had an &shared_mut style of reference, you could presumably split it without unsafe. For that matter, Rust does have various interior-mutable types, and you could have a shared reference to a slice of AtomicBool or whatever, and you can split that without any particular magic.

Fil-C doesn’t have those things because it doesn’t need them to achieve safety.

How does it make sure data races are safe?

Rust can split up an array without unsafe if you use Rc. That has overhead, but is it more than Fil-C in this situation?

Data races are memory safe in Fil-C.

I think the disconnect here is that some people want their data races (a) not to result in memory safety violations; some people want them to also (b) not result in nonsensical control flow or code producing results that look impossible, and some people want (c) data races to simply not occur.

You seem to like (a), Linus Torvalds seems to like (b), and Rust targets (c).

Rust doesn’t target (c). That’s a hugely disingenuous claim.

The truth is: Rust’s approach to memory safety only works if you also have no data races. This makes it a strictly inferior approach to memory safety, since programs sometimes do have to race (sometimes it’s just the best solution). So, Rust has data race prevention not because it’s a good idea but because the whole language falls apart without it.

> Rust has data race prevention not because it’s a good idea but because the whole language falls apart without it.

In terms of the design goals and evolution of the Rust language, this is exactly backwards. Rust was originally conceived as a garbage-collected, green-threaded language designed for concurrent programming -- think something similar to Go but with a stronger type system and no data races. The ownership model was created, first as foremost, not for memory safety but to prevent data races; and, more broadly, to help programmers reason about the correctness of their code.

Midway through development, as people started using the language & getting a feel for it, the language designers realized that the ownership model was powerful enough to express not just data race freedom but full memory safety without needing a GC (some writing from this stage in Rust's development: https://smallcultfollowing.com/babysteps/blog/2013/06/11/on-..., https://pcwalton.github.io/_posts/2013-06-02-removing-garbag...). So they removed the GC, and that decision positioned Rust where it is today: a nicer, memory-safe "C/C++ competitor", popular for systems code where the performance overhead or runtime complexity of a garbage collector is considered unacceptable.

Yes but how do you accomplish that? If you do nothing at all, pointers can tear. And allocation sizes too if they're stored inline.

The capability is internally a pointer to a monotonic GC-allocated object.

That capability pointer is always stored and loaded using monotonic 64-bit accesses.

Therefore, in the worst case you'll get a pointer that is torn from its capability, and then you'll trap accessing that pointer. But you'll never get a corrupt capability.

If you don't want the pointer to tear from its capability, just use `_Atomic`, `volatile`, or `std::atomic`. Then Fil-C uses lock-free shenanigans to make sure that the capability and pointer travel together and don't tear from one another.

Okay, in that case I do think one of the data-holding types in rust is a fair comparison, adding some runtime checks to let you be looser about ownership. Then you can write a function to split/share an array without a single bit of unsafe code.

I don't see the interest in split_at_mut. I can get the same thing by reslicing in Go. And also the GC will do the job the borrow checker foists off onto the unlucky Rust programmer.

Pfft, whatever. Rusters gonna rust, I guess.

I'm not a Ruster, I've spent my career writing a ton of C++. But I'm interested in Rust as an alternative because it doesn't need GC.

But in this case, it's not just the memory safety I'm interested in, it's the data races. If we have multiple threads but can guarantee that any object either has only read-only references, or one mutable references and no readers, we don't have data race issues.

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