4 ms·
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 lib
by not2b 2y ago
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.
- pizlonator 2y agoThat’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.)
- amluto 2y agoIs 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.
- pizlonator 2y agoFil-C doesn’t have those things because it doesn’t need them to achieve safety.
- Dylan16807 2y agoHow 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?
- pizlonator 2y agoData races are memory safe in Fil-C.
- amluto 2y agoI 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).
- pizlonator 2y agoRust 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.
- NobodyNada 2y ago> 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-the-connection-between-memory-management-and-data-race-freedom/ https://smallcultfollowing.com/babysteps/blog/2013/06/11/on-..., https://pcwalton.github.io/_posts/2013-06-02-removing-garbage-collection-from-the-rust-language.html 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.
- jeffrallen 2y agoI 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.
- not2b 2y agoI'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.