> Someone in the audience also pointed out that a tool like this could be used to compile rust programs, not just C programs, in which case it's odd to hear Fil-C repeatedly framed as a language in opposition to rust rather than as a tool which might complement it.
You can combine both approaches for sure, but that doesn't change that they are indeed separate approaches. One (Rust) intends to characterize and prevent undefined behavior at compile time, and another (Fil-C) intends to make undefined behavior impossible at runtime, as evidenced by decisions like (for example) making unsafe usage of setjmp/longjmp safe.
You could sort of achieve "safety-in-depth" by combining both approaches, but Rust prefers to prevent all undefined behavior statically, and Fil-C prefers to focus on dynamic approaches. They have real differences, so that's probably what comes off as "oppositional"
This presentation wasn’t too bad in terms of us vs theming, but in general throughout the lifetime of the project there’s been a strong us vs them rhetoric. I think it’s working as a marketing tactic to some extent but there are better, albeit harder ways to market the thing.
Rust generally invites us vs them everywhere because it's just so ambitious and attractive (to some). Fil-C probably receives questions all the time about why they don't just do X or Y that Rust does. Maybe they end up supporting their position against that type of pressure and it looks like they're opposing Rust when all they're doing is justifying themselves.
Arguably the most important memory safety property, i.e. bounds checking for dynamic arrays, is also not statically checked in Rust.
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.)
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
those are not dynamic bounds.
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.
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.
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.
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.
> 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.
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.
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.
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.)
> 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")
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.
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.
Maybe? I think my statement is clearly correct in the way I formulated it.
This isn't entirely true. Rust's indexing operator performs a dynamic check but the language and stdlib have a number of ways to access array elements without indexing, which is effectively a form of static checking. For example, the following loop is statically guaranteed to not go OOB, and will not emit any unnecessary bounds checks:
I suspect most techniques you could think of to statically avoid bounds checks are possible in rust's type system.These are not terribly interesting cases though, as it would also be impossible to get an oob access in other languages either.
Yes, rust didn't invent the concept of an iterator, and is thus not the only language which offers a solution to statically avoid bounds checks. Feel free to give a more "interesting" case you believe rust wouldn't be able to handle.
Rust fails to prove basic indirection to be statically safe and instead does a run-time check.
It's possible to do this (WUFFS does it) but it's so invasive that, well, you basically end up with WUFFS. You end up having to track every set of possible and impossible values at every location, etc. (during each program location too.) WUFFS certainly has its place and I've kind of been itching to try it out myself, but I think getting to the point where dynamic arrays can be statically bounds-checked would have highly disabled Rust.
The combination of static safety gracefully degrading to runtime safety where not provable is interesting to me
This is sort of already possible in Rust when you use safe primitives (e.g., RefCell), but it doesn't cover unsafe code invoking Undefined Behavior. Placing something like Fil-C around unsafe code could prove to be interesting, especially since current approaches rely on interpretation (Miri, Soteria Rust) rather than translation.