Hacker News Reader

Comment by pcwalton | Hacker News Reader

But it's not a fundamental design flaw in the language, nor is it a "hole in the type theory". It's a compiler bug. The compiler isn't checking function contravariance properly. Miri catches the problem, while rustc doesn't.

View on HN

You mean that Miri catches it at runtime, right? If so, that hardly demonstrates anything about the difficulty or lack thereof of fixing rustc’s type checker, since Miri is not a type checker and doesn’t know anything about “lifetimes” in the usual sense.

I agree that this isn’t a “fundamental design flaw in the language”, but Miri is irrelevant to proving that.

Yes. I'm not saying that Miri demonstrates anything other than that it's not a language issue.

I’m not sure it even demonstrates that. For comparison, C’s inability to prevent undefined behavior at compile time definitely is a fundamental weakness of the language. Yet C has its own tools that, like Miri, can detect most undefined behavior at runtime in exchange for a drastic performance cost. (tis-interpreter is probably the closest analogy, though of course you also have ASan and Valgrind.)

Or to put it another way, the reason that Rust’s implied bounds issue is not a fundamental language issue is that it almost certainly can be fixed without massive backwards compatibility breakage or language alterations, whereas making C safe would require such breakage and alterations. But Miri tells us nothing about that.

I believe it really is a flaw in the language, it's impossible for any compiler to check contravariance properly in this edge case. I don't think anything in this link is incorrect: https://counterexamples.org/nearly-universal.html?highlight=... (and it seems the rustc types team endorses this analysis)

I am not at all familiar with Miri. Does Miri consider a slightly different dialect of Rust where implicit constraints like this become explicit but inferred? Sort of like this proposal from the GH issue: https://github.com/rust-lang/rust/issues/25860#issuecomment-... but the "where" clause is inferred at compile time. If so I wouldn't call that a "fix" so much as a partial mitigation, useful for static analysis but not actually a solution to the problem in rustc. I believe that inference problem is undecidable in general and that rustc would need to do something else.

I think you're misinterpreting what "the implicit constraint 'b: 'a has been lost" in that link means. What it means is that the compiler loses track of the constraint. It doesn't mean that the language semantics as everyone understands them allows this constraint to be dropped. That sentence is describing a Rust compiler bug, not a language problem.

I think you're misinterpreting what "implicit" means here. It doesn't mean "a constraint rustc carries around implicitly but sometimes loses due to a programming bug." It means "a constraint Rust programmers use to reason about their code but which rustc itself does not actually keep track of." "The implicit constraint 'b: 'a has been lost" should more precisely read "the implicit constraint 'b: 'a no longer holds."

Look closely at what happens:

  fn foo<'a, 'b, T> (_: &'a &'b (), v: &'b T) -> &'a T

There is an implicit "where 'b : 'a" clause at the end of this declaration - this clause would be explicit if Rust was more like OCaml. The reason this clause is there implicitly is that in correct Rust code you can't get &'a &'b if 'b : 'a doesn't hold. So when a human programmer reasons about this code they implicitly assume 'b: 'a even though there's nothing telling rustc that this must be the case.

This runs into a problem with the requirements of contravariance, which let us replace any instance of 'b in foo with any type 'd such that 'd : 'b, in particular 'static:

  fn foo<'a, 'b, T> (_: &'a &'static (), v: &'b T) -> &'a T

Since contravariance allows us to replace the &'a &'b with &'a &'static without changing the second &'b, the completely implicit constraint 'b : 'a is no longer actually being enforced, and there's no way for it to be enforced. This is not a compiler bug! It's a flaw in the design.

In particular I don't think there's anything especially magical about 'static here except that it works for any type 'a[1]. If you had a specific type 'd such that 'd : 'a then I think you could trigger a similar bug, converting references to 'b into references to 'd.

[1] Actually maybe "static UNIT: &'static &'static () = &&();" is more critical here since I don't think &'d &'d will work. Perhaps there's a more convoluted way to trigger it. This stuff hurts my head :)

Because Rust doesn't have a specification, we can argue back and forth forever about whether something is part of the "language design". The point is that the design problem has been fixed by desugaring to bounds on "for" since 2016 [1]. The only thing that remains is to implement the solution in the compiler, which is an engineering problem. It's not going to break a significant amount of code: even the bigger sledgehammer of banning contravariance on function types was measured to not have much of an impact.

As far as I can tell, the main reason to argue "language design" vs. "compiler bug" is to imply that language design issues threaten to bring down the entire foundation of the language. It doesn't work like that. Rust's type system isn't like a mathematical proof, where either the theorem is true or it's false and one mistake in the proof could invalidate the whole thing. It's a tool to help programmers avoid memory safety issues, and it's proven to be extremely good at that in practice. If there are problems with Rust's type system, they're identified and patched.

[1]: https://github.com/rust-lang/rust/issues/25860#issuecomment-...

> Rust's type system isn't like a mathematical proof, where either the theorem is true or it's false and one mistake in the proof could invalidate the whole thing

Interestingly enough, in practice even mathematical proofs aren't like that either: flaws are routinely found when papers are submitted but most of the time the proof as a whole can be fixed.

Wiles first submission for his proof of Fermat's last theorem in 1993 is the best known example, but it's in fact pretty frequent.

His point seems to be that there are reasons in principle you cannot do this (holding the design fixed).

It's a little like making a type system turing-complete then saying, "we can fix the halting problem in a patch".

If you change the design, then you can fix it. This is what I interpret "design flaw" to mean.

The design problem has been fixed, as I noted above.

In the language,

    fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v }

should be equivalent to

    fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T where 'b: 'a { v }

because the type &'a &'b is only well-formed if 'b: 'a. However, in the implementation, only the first form where the constraint is left implicit is subject to the bug: the implicit constraint is incorrectly lost when 'static is substituted for 'b. This is clearly an implementation bug, not a language bug (insofar as there is a distinction at all—ideally there would be a written formal specification that we could point to, but I don’t think there’s any disagreement in principle about what it should say about this issue).

See my other comment above: https://news.ycombinator.com/item?id=39448909

The point is that rustc does not even implicitly have the "where 'b: 'a { v }" clause. The programmer knows "where 'b: 'a { v }" is true because otherwise &'a &'b would be self-contradictory, and in most cases that's more than enough. But in certain edge cases this runs into problems with the contravariance requirement since we can substitute either one of the 'bs with a type that outlives 'b (i.e. 'static), and the design of contravariance lets us do this pretty arbitrarily.

We could choose to design the language as I wrote where the constraint is implicitly present and variance consequently ought to preserve it, or as you wrote where it is not present at all and variance is unsound. But we want Rust to be sound, so we should choose the former design. We would need a very compelling reason to prefer the latter—for example, some evidence that “it would break too much code” or “it’s impossible to implement”—but I don’t see anyone claiming to actually have such evidence. The only reason the implementation has not been updated to align with the sound design is that it takes work and that work is still ongoing.

The issue isn't contravariance but that higher ranked lifetimes can't have subtyping relations between them in rustc's implementation, so the fn pointer type is treated as a more general type than it is. This user is still wrong, the problem with rustc just isn't how it handles contravariance.

Reply on news.ycombinator.com