"this could sometimes panic" lint

[oli-obk]

minimal range analysis (lots of snippets like if x == 0 { return; } x -= 1; around, for example) to eliminate false positives (range analysis by itself can provide useful optimizations)
the hardest thing to reason about is that v1.len() + v2.len() can't ever overflow if v1, v2: Vec<T> and size_of::<T>() > 0

idea by @eddyb https://botbot.me/mozilla/rust-internals/2016-05-27/?msg=66824497&page=3

[eddyb]

The context is #33905, for which I made a simple change to warn on all #[inline] or generic functions with overflow checks and then looked at the hundreds of results, which showed some interesting patterns.

@nagisa's dataflow framework should usable for tracking ranges based on comparisons, for cases where checks prevent overflows (e.g. if i < n { n - i }), without much difficulty.

Some of the "won't overflow only because you can't have more allocated bytes in total than isize::MAX" situations are in unsafe code deep in collection implementations, so most code could be free of false positives even without understanding that, but it's not a given.

[oli-obk]

Don't https://internals.rust-lang.org/t/mir-compiler-plugins-for-custom-mir-passes/3166/11?u=ker and rust-lang/rust#34066 basically state that we should not be using MIR for linting?

[nagisa]

The AST passes never were stabilised explicitly either so what is said
about the MIR passes was supposed to equally apply to the AST passes too.
On Jun 6, 2016 1:48 PM, "Oliver Schneider" notifications@github.com wrote:

Don't
https://internals.rust-lang.org/t/mir-compiler-plugins-for-custom-mir-passes/3166/11?u=ker
and rust-lang/rust#34066 rust-lang/rust#34066
basically state that we should not be using MIR for linting?

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[eddyb]

@oli-obk Well, that doesn't make MIR less stable than all of the currently unstable compiler internals all lints use. There might be a way to expose the CFG and pieces of the instructions in a less unstable manner, but IMO not having that yet shouldn't stop people for building incredibly powerful tools.

[oli-obk]

@Manishearth can we have a T-MIR label?

[Manishearth]

Done. you can add labels in https://github.com/Manishearth/rust-clippy/labels

[oli-obk]

thanks! I thought that was an owner-only-feature.

[mcarton]

FYI, the ability to register MIR passes was removed: rust-lang/rust#40239.

[oli-obk]

That's not an issue. We can get the MIR from the tcx and the body_id.