[woodruffw]

How would Rust perform static checks on dynamic bounds? That seems like an impossible order, i.e. not a reasonable evaluation criteria for any (general-purpose) language.

(I'm separately skeptical that it's the most important memory safety property; I suspect that a review of Chrome and Firefox 0days would show that UAFs and type confusion are, at least to attackers, equally if not more important.)

[LoganDark]

Presumably, they're talking about the possibility of explicitly replacing dynamic bounds checks in cases where it's statically provable that the access will not overrun. That doesn't necessarily mean you lose cases where you genuinely can't prove such a thing, but rather that it would be possible to opt into compiler assistance with proving it, rather than hoping LLVM will have your back. At least, that's how I'd envision such a thing for Rust. This would be similar to how you can already choose whether to invite more of the borrow checker by using references directly, or to do the checks at runtime with Rc/RefCell, etc.

[woodruffw]

Yeah, that seems like a nice thing that Rust could offer. It strikes me as a weird thing to get hung up on, though, given that the norm in compiled languages - including C - is to express your bounds such that an optimizing compiler can (but won't necessarily) eliminate them.

(You mentioned WUFFS below, which is why I qualified with general-purpose! One thing that WUFFS does that I think Rust could add pretty easily is provable indexing, e.g. allow me to use a `u8` to index a `[u8; 256]` without having to widen to a `usize` first and hope that LLVM optimizes it back out.)

[haberman]

> rather than hoping LLVM will have your back

My thought here is to proactively verify that LLVM elided the automatic bounds checks in places where you believe that your explicit checks should be sufficient.

That was a key part of my article on "No-Panic Rust": https://blog.reverberate.org/2025/02/03/no-panic-rust.html ("A Dance With The Optimizer")

[uecker]

I merely pointed out that the statement "Rust prefers to prevent all undefined behavior statically" is misleading in the sense that Rust does not do this for all undefined behavior.

[LoganDark]

Maybe if you think of [] as offsetting a pointer rather than calling into an Index (or IndexMut) implementation. Since the return type isn't optional, a panic is the only way to avoid performing an invalid access or manufacturing an unfaithful return value. There are also optional accessors which do not panic, and unsafe/unchecked accessors.

[uecker]

Maybe? I think my statement is clearly correct in the way I formulated it.

[zem]

ATS could probably do some of this by constructing proofs about the bounds and indices - not in the completely general case of course, but for at least some fraction of what a language without dependent types would have to defer to runtime

[ratmice]

those are not dynamic bounds.

[LoganDark]

All "dynamic" means is that you don't know or don't prove the precise value statically. However you may know the range of possible values, or you may know properties of your algorithm that mean it can never attempt an out-of-bounds access. Sometimes you don't know any of these things, but sometimes you do.

[ratmice]

What I was (badly) trying to express was more that given static bounds rust could also eliminate dynamic checks. So saying e.g. ATS can eliminate static checks, is kind of switching the target.

[zem]

https://ats-lang.sourceforge.net/DOCUMENT/INT2PROGINATS/HTML... is the kind of thing I mean - ATS programs can include proofs that an array access will be safely within bounds, even if we don't have an exact literal subscript in the code.

[uecker]

Rust can not do this. But the original claim is that it is all statically checked in Rust, and this is an obvious counter example. Some dependently types languages can prove this statically, also model checking can do this, etc. So it can be done statically as well, but not in Rust.

[woodruffw]

> But the original claim is that it is all statically checked in Rust

Where? The GP's comment says "prefers," which I didn't read to mean a blanket statement. I don't think anybody with more than passing experience in Rust (or C) would make such a claim.

> Some dependently types languages can prove this statically, also model checking can do this, etc. So it can be done statically as well, but not in Rust.

You're couching the part where it can't be done with full generality or can be done with full generality, but with punishing semantics. The appropriate comparison here is with other normal general purpose compiled languages.

[uecker]

I read "Rust prefers to prevent all undefined behavior statically," to also imply that it actually does this, because otherwise the word "all" would not make sense to me in this sentence.

Dependently types languages and model checking do exist. They come with tradeoffs, but this is also true for Rust.