A hundred lints – The sequel

05 February 2016

My last blog entry got some nice feedback, and here I’m trying to give my thoughts to some of it:

Rust-clippy is great, but I always had something like Ada’s restrictions in mind too (https://en.wikibooks.org/wiki/Ada_Programming/Pragmas/Restrictions …) .. anyone for a rust-dogme?

— Graydon Hoare on twitter

@Graydon: You are hereby cordially invited to submit an issue to discuss this further. :-)

With that said, we should bear in mind that some things are still hard to ensure (or at least check) in Rust:

That an arbitrary function within an external library will not diverge (i.e. check for panic!() )

) That a function is pure (mcarton recently suggested viewing functions with immutable arguments as pure, but this fails to account for interior mutability, alas. Still it may be a useful heuristic nonetheless, depending on the level of “purity” required)

That a function won’t recurse (endlessly or at all)

That a function won’t do IO

That a function won’t allocate

That a function does not include undefined behavior

That a function does not call into FFI code

That a function stays within a particular stack size

The last one is currenly a real problem, as it causes people’s code to segfault due to stack overflow. Some of those people subsequently go on reddit to complain that their “safe” language let them crash their program. :-P I think the next-to-last one is very interesting – we obviously have less UB in Rust than in other languages, but you never know if that function you call introduces some of it.

Why is this hard? I blame the crate barrier. Note that I don’t want to break it down and recompile everything all the time. But we may want to think about upping rustc’s compiled-code introspection capabilities so we can answer those questions with some certainty. Apart from allowing us to reason about the aforementioned questions, it could make some of our current lints better, which now have to take a rather conservative stance.

Now how would this be done? I figure we’d need to make some crate metadata that we should be able to get accessible to lints (and the rest of the compiler). The call graph of a function, for example (a list of functions that each function calls would be sufficient, though this obviously stops at the FFI boundary, so there may be some false negatives). If a function accesses any global state (which is locally discoverable). Note that we have all this information crate-locally while we’re compiling a crate. We just don’t store it in the rlib, as far as I know.

Some have expressed their belief that many lints of clippy belong in rustc. I’m personally at best lukewarm on this for various reasons: We can develop lints better if we don’t have to wait half an hour for the thing to compile :-P. Also we can try out more stuff without slowing the rustc people down with our follies – the resulting communication fallout from a lint-breaking change (which, while not too frequent, still occurs every now and then) would take up time that both teams can put to better purposes. Finally I think that we shouldn’t think about the compiler too much, but focus on the development environment and focus on improving integration there.

Since I wrote the last post, we have gained a good number of new lints (and are at 110 at the time of this writing, a mere 8 days after the post). Some bugs have been fixed, yet others remain. I’m personally very happy with how the project advances, Manish’s formidable leadership and our focus on being inclusive has given us fertile soil for many more helpful improvements, which will hopefully benefit a good part of the Rust ecosystem.

I’ve also just started writing library-specific lints (for Andrew Gallant’s glorious regex crate) – we have not yet decided if those will stay in clippy or become their own regex-lints crate. So library crate authors: If you find that your library API can be misused, but lack a zero-overhead way of preventing that misuse, perhaps a lint could at least flag the problematic combinations? I’d like to hear from security-relevant crates in particular.

Discuss this on r/rust or rust-users!