Lint suggestion: benchmarks in main crate code

[skade]

Benchmarks in main crate code mean that the whole code has to opt into test.

Also, in any cargo project, benchmark files can be put into the benches folder, which is a much more appropriate place.

[llogiq]

Note that #[cfg(test)] can be used to conditionally compile code.

Still, using the benches folder is a cleaner solution.

[skade]

That keeps me from running tests on stable :). There's no #[cfg(bench)] equivalent unless introduced yourself through custom features.

[llogiq]

Ah, I see.

[llogiq]

So the lint basically only has to look into the attrs of each item and see if it finds a #[bench] (one can look at the inline_always lint for an example on how to do this). Then one can use the CodeMap's .span_to_filename(span) call to get the filename.

[llogiq]

I've looked into it (while working on #465) and found to my surprise that rustc nightly simply ignores all items that are annotated with #[bench] unless compiling with -bench.

I'm unsure if we can a) coax rustc into compiling benchmarks anyway when running with clippy, or b) should ask our users to compile twice (or thrice, given -test)? Anyway, it doesn't seem worth the effort.

[phansch]

This might be possible with PreExansionPass now?

(Removed good first issue because of the aforementioned potential problems)