Amazing work, I remember reading the Tree Borrows spec? on Nevin's website a couple years ago and being thoroughly impressed by how it solves some pretty gnarly issue quite elegantly. And in my experience [1] [2] it does indeed allow for sensible code that is illegal under Stacked Borrows.
Bonus: recent talk from Ralf Jung on his group's efforts to precisely specify Rust's operational semantics in executable form in a dialect of Rust: https://youtube.com/watch?v=yoeuW_dSe0o
pvg · 2h ago
The Stacked Borrows mentioned had threads in 2020 and 2018
Just realised that one of the author, Neven Villani, is Cédric Villani's (Fields Medal 2010) son. Apples don't fall far from the tree, indeed.
tandr · 26m ago
> Apples don't fall far from the tree, indeed.
And one could say that they borrow from the tree some of their qualities. Sorry, couldn't resist.
fuhsnn · 33m ago
I wonder if Rust or future PL would evolve into allowing multiple borrow checker implementations with varying characteristics (compile speed, runtime speed, algorithm flexibility, etc.) that projects can choose from.
pjmlp · 3m ago
We already have that by having multiple approaches via affine types (what Rust uses), linear types, effects, dependent types, formal proofs.
All have different costs and capabilities across implementation, performance and developer experience.
Then we have what everyone else besides Rust is actually going for, the productivity of automatic resource management (regardless of how), coupled with one of the type systems above, only for performance critical code paths.
umanwizard · 8m ago
What’s wrong with the compile or runtime speed of the current one?
speed_spread · 13m ago
I cannot imagine how that would work. You couldn't combine code that expect different borrowing rules to be applied. You'd effectively be creating as many sub-dialects as there are borrow checker implementations.
wavemode · 1h ago
From the paper:
> The problem with unsafe code is that it can do things like this:
fn main() {
let mut x = 42;
let ptr = &mut x as *mut i32;
let val = unsafe { write_both(&mut *ptr, &mut *ptr) };
println!("{val}");
}
No it can't? Using pointers to coexist multiple mutable references to the same variable is undefined behavior. Unless I'm just misunderstanding the point they're trying to make here.
pavpanchekha · 1h ago
The point of this work is to pin down the exact boundaries of undefined behavior. Certainly the code above is accepted by the Rust compiler, but it also breaks rules. What rules? In essence, we know that:
- Anything accepted by the borrow checker is legal
- Unsafe can express illegal / undefined behavior
- There's some set of rules, broader than what the borrow checker can check, that is still legal / defined behavior
The goal of this line of work is to precisely specify that set of rules. The outlines are clear (basically, no writable pointers should alias) but the details (interior pointers, invalidation of iterators, is it creating or using bad pointers that's bad, etc) are really hard. The previous paper in this series, on Stacked Borrows, was simpler but more restrictive, and real-world unsafe code often failed its rules (while still seeming correct). Tree Borrows is broader and allows more while still being provably safe.
ralfj · 1h ago
> allows more while still being provably safe.
Note that we have not yet proven this. :) I hope to one day prove that every program accepted by the borrow checker is compatible with TB, but right now, that is only a (very well-tested) conjecture.
ralfj · 1h ago
> Using pointers to coexist multiple mutable references to the same variable is undefined behavior.
Yes, but which exact rule does it violate? What is the exact definition that says that it is UB?
Tree Borrows is a proposal for exactly such a definition.
"code can do things like this" here means "you can write this code and compile it and run it and it will do something, and unless we have something like Tree Borrows we have no argument for why there would be anything wrong with this code".
You seem to have already accepted that we need something like Tree Borrows (i.e., we should say code like this is UB). This part of the paper is arguing why we need something like Tree Borrows. :)
GolDDranks · 9m ago
> Unless I'm just misunderstanding the point they're trying to make here.
You misunderstand the word "can". Yes, you can, in unsafe code, do that. And yes, that is undefined behaviour ;)
You're already getting a lot of replies, and I don't want to pile on, but I think the clearest way to see the intent there is at the start of the following paragraph:
> Given that aliasing optimizations are something that the Rust compiler developers clearly want to support, we need some way of “ruling out” counterexamples like the one above from consideration.
ehsanu1 · 1h ago
I believe that's exactly the point: it's too easy to violate constraints like not allowing multiple mutable references. Unsafe is meant for cases where the validity of the code is difficult to prove with rust's lifetime analysis, but can be abused to do much more than that.
seritools · 1h ago
"can do things" in this case doesn't mean "is allowed to do things".
"Unsafe code allows to express the following, which is UB:"
[1] https://github.com/Voultapher/sort-research-rs/blob/main/wri... Miri column
[2] https://github.com/rust-lang/rust/blob/6b3ae3f6e45a33c2d95fa...
Bonus: recent talk from Ralf Jung on his group's efforts to precisely specify Rust's operational semantics in executable form in a dialect of Rust: https://youtube.com/watch?v=yoeuW_dSe0o
https://news.ycombinator.com/item?id=22281205
https://news.ycombinator.com/item?id=17715399
And one could say that they borrow from the tree some of their qualities. Sorry, couldn't resist.
All have different costs and capabilities across implementation, performance and developer experience.
Then we have what everyone else besides Rust is actually going for, the productivity of automatic resource management (regardless of how), coupled with one of the type systems above, only for performance critical code paths.
> The problem with unsafe code is that it can do things like this:
No it can't? Using pointers to coexist multiple mutable references to the same variable is undefined behavior. Unless I'm just misunderstanding the point they're trying to make here.- Anything accepted by the borrow checker is legal
- Unsafe can express illegal / undefined behavior
- There's some set of rules, broader than what the borrow checker can check, that is still legal / defined behavior
The goal of this line of work is to precisely specify that set of rules. The outlines are clear (basically, no writable pointers should alias) but the details (interior pointers, invalidation of iterators, is it creating or using bad pointers that's bad, etc) are really hard. The previous paper in this series, on Stacked Borrows, was simpler but more restrictive, and real-world unsafe code often failed its rules (while still seeming correct). Tree Borrows is broader and allows more while still being provably safe.
Note that we have not yet proven this. :) I hope to one day prove that every program accepted by the borrow checker is compatible with TB, but right now, that is only a (very well-tested) conjecture.
Yes, but which exact rule does it violate? What is the exact definition that says that it is UB? Tree Borrows is a proposal for exactly such a definition.
"code can do things like this" here means "you can write this code and compile it and run it and it will do something, and unless we have something like Tree Borrows we have no argument for why there would be anything wrong with this code".
You seem to have already accepted that we need something like Tree Borrows (i.e., we should say code like this is UB). This part of the paper is arguing why we need something like Tree Borrows. :)
You misunderstand the word "can". Yes, you can, in unsafe code, do that. And yes, that is undefined behaviour ;)
https://play.rust-lang.org/?version=stable&mode=debug&editio...
> Given that aliasing optimizations are something that the Rust compiler developers clearly want to support, we need some way of “ruling out” counterexamples like the one above from consideration.
"Unsafe code allows to express the following, which is UB:"
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