I hope we don't settle for unsafe being okay forever. Right now, sometimes it is the right thing to do and there shouldn't be any regret. But in the future, I hope Rusts compilers become better.
There are two things I consider necessary for that. First, that the Rust compilers become smarter in proving the safety of things by themselves. Second, that the Rust compilers become capable of verifying proofs given to them that show the safety of a given piece of code the compilers can't prove as safe on their own.
I think that it is a worthwhile goal to be able to someday formally prove all the unsafe blocks correct in, say, the standard library and popular crates.
However, I honestly feel that the Rust language itself isn't really the right language to be doing these kinds of proofs in. I think that the right language to do the actual formal verification in is likely to be something closer to Coq. Whoever undertakes this effort would probably use an automated theorem prover to prove the unsafe Rust code correct, like was done for seL4 using Isabelle.
You can think of this setup as offering a sort of layered verification: once the small core of code (the unsafe code in the standard library and popular crates, say) is proven correct, the type system and borrow checker effectively prove the rest to be memory- and thread-safe. In fact, that's what would make this system practical: most programmers wouldn't have to understand anything about the complexities of the theorem prover. They would get the benefit of verified memory- and thread-safety for free just by learning Rust.
I also thought foremost about standard libraries and other often used code, or say, critical code in some OS.
We don't need to verify every single occurrence of unsafe, but whenever unsafe is necessary I feel the lack of some other guarantee holding our back.
Having optional small-scale(!) verification in Rust would be awesome.
Using other theorem provers has the same problems as when one tries to establish tools for checking memory safety in C: it isn't the default, just an addon.
In my eyes, there is a scale of verification-readiness in which Rust can position itself. The least ready would be having it completely separate and done by other tools in other files, the most ready would be having syntax for it in the language, having it in the same files, and checked by the standard compiler.
I think every bit of verification-readiness Rust has by default will have a strong effect that we can't achieve through other means.
Maybe, some things don't impact those not interested in verification. Say, have next to 'safe' and 'unsafe' also the 'verified' environment that also contains proof language as a core part of Rust. That way all ordinary code is still valid and everyody is free to use Rust without verification in mind.
Any ideas what could be done to make Rust verification-readier?
Rust needs unsafe to be Rust. It's reasonable to expect (safe) Rust to become more expressive over time so that more things can be written in safe Rust (or written more conveniently) but expecting the escape hatch to go away entirely is misguided. It is as unlikely as C++ or C trying to do away with inline assembly.
IMO that's less of a Rust problem than a problem for the next generation of verified languages. Probably total functional languages, or maybe just usable versions of Coq.
The Rust compiler will not solve the halting problem. It is pretty trivial to write programs which are safe if and only if they halt. So 100% safe is simply absurd.
You don't need to solve the halting problem just to verify an existing proof of a semantic property, nor to use smarter heuristics to avoid requiring such a proof. 100% safe is totally reachable, though I bet the syntax would be pretty hairy added to today's Rust.
Hmm, good point. You'd have to extend the formal verification into the C code, at least. If you can do that, it might be easier to just write verified C.
Inline ASM is exactly as verifiable as the underlying CPU, given an adequate model in the verifier. That's probably easier than verifying C, which introduces extra ambiguity in its semantics. But yeah, verifiable CPUs would be nice.
There are two things I consider necessary for that. First, that the Rust compilers become smarter in proving the safety of things by themselves. Second, that the Rust compilers become capable of verifying proofs given to them that show the safety of a given piece of code the compilers can't prove as safe on their own.