Tree Borrows

> On the one hand, compilers would like to exploit the strong guarantees of the type system—particularly those pertaining to aliasing of pointers—in order to unlock powerful intraprocedural optimizations.

How true is this really?

Torvalds has argued for a long time that strict aliasing rules in C are more trouble than they're worth, I find his arguments compelling. Here's one of many examples: https://lore.kernel.org/all/CAHk-=wgq1DvgNVoodk7JKc6BuU1m9Un... (the entire thread worth reading if you find this sort of thing interesting)

Is Rust somehow fundamentally different? Based on limited experience, it seems not (at least, when unsafe is involved...).

I would agree that C's strict aliasing rules are terrible. The rules we are proposing for Rust are very different. They are both more useful for compilers and, in my opinion, less onerous for programmers. We also have an actual in-language opt-out: use raw pointers. And finally, we have a tool you can use to check your code.

But in the end, it's a trade-off, like everything in language design. (In life, really. ;) We think that in Rust we may have found a new sweet spot for this kind of optimizations. Time will tell whether we are right.

As someone who has been writing a lot of unsafe Rust (mostly in an embedded context), I'm thrilled about and thankful for the work that you, your students, and the opsem team are doing.

When you're working with anything below the application level, C's confusing and underspecified rules about UB are almost impossible to keep track of, especially when it comes to aliasing and volatile/MMIO. The spec is so difficult to read and full of complicated cross-references that to actually get a practical answer you have to look for a random Stack Overflow post that may or may not have a correct interpretation of the spec, and may or may not address your specific problem.

Rust right now feels a lot harder to work with, because the spec isn't done. When you have a concrete question about a piece of code, like "is this conversion from an &mut to a *mut and back sound", and you try to look for documentation on it, you get either "Nobody knows, Rust aliasing model isn't defined"; a hand-wavy explanation that is not rigorous or specific; or a model like Stack Borrows or Tree Borrows that's defined a little too formally for easy digestion :)

But when I really started digging, I realized just how much cleaner Rust's semantics are. References aren't actually hard, Tree Borrows basically boils down to "while an &mut reference is live, you can only access the value through pointers or references derived from that reference". Pointer operations have straightforward semantics, there's no confusing notions of typed memory, and no UB "just because" for random things like integer overflow. It's just so much less complicated to understand than C's abstract machine.

I'm really looking forward to things like MiniRust, and to an aliasing model making it into the Reference / other documentation, because at that point I feel like unsafe Rust will be way easier to write confidently and correctly than C.

Congrats on the publication, and thanks again for the work you all have put into this.

In C, you can alias pointers if they have compatible types. Not the case in Rust for mutable references. And the rules of Rust have tripped up even senior Rust developers.

https://github.com/rust-lang/rust/commit/71f5cfb21f3fd2f1740...

Without MIRI, a lot of Rust developers would be lost, as they do not even attempt to understand unsafe. And MIRI cannot and does not cover everything, no matter how good and beloved it is.

It should have been possible for senior Rust developers to write UB-free code without having to hope that MIRI saves them.

The situation is not that bad. The rules of unsafe code were pretty badly defined back then, but they are in process of becoming a lot clearer, and like the grandparent argues, with a well-defined aliasing model like Tree Borrows, they are easier to understand than C's.

If you look into the code you linked, the problem was about accessing undefined bytes though an aliased, differently-typed pointer – something you would have hard time doing in C to begin with. MaybeUninit was a new thing back then. I think that nowadays, a senior Rust developer would clear the hurdles better.

I am very sorry, but you do not address that TBAA, like C has by default, generally is easier than just no aliasing, like what Rust has for mutable references. This is a major difference. C code can opt into a similar kind of aliasing, namely by using _restrict_, but that is opt-in, while it is always on for Rust.

And there is newer UB as well in Rust stdlib

https://github.com/rust-lang/rust/pull/139553