ft: add an alternative analyzer to detect unused code

[enerdgumen]

Hello @hauleth, we at Prima.it are using mix_unused in our Elixir services.
This is a simplified PR that excludes all the discovery modules included in the previous one.

We found some issues with the current implementation, which marked as unused lots of code, depending on the application's design.
We explored a different approach which could make the results more reliable when used properly.

Although the PR is a bit fat, most of the changes are additive and the rest is of course backward compatible.

Summary

• add the MixUnused.Analyzers.Unreachable analyzer, alternative to the Unused one;
• add paths configuration option to report only functions defined in such paths (i.e. to hide code made only for tests purposes);
• add limit configuration option to report only a limited number of functions;
• make the checks options overridable, so that the user can enable the "unreachable" analyzer;
• fix usage detection of public functions called with default arguments;
• detect some generated functions and omit them from report;
• log debug information if the MIX_UNUSED_DEBUG env var is set to true.

The "unreachable" analyzer

It just analyses the calls graph finding all the functions that are reachable from a specific set of well-known used functions. This makes the analysis simple and deterministic.

The analyzer requires the user to specify the set of usages, or "usages-discovery" (modules that try to discover dynamically-called functions - for instance by reading the application config or by analyze the source code).

Unreachable has some differences from Unused:

• Unreachable does not assume that all behaviour implementers are used by default.
• Unreachable does not report generated functions.
• Unreachable, by default, does not report unused functions called transitively (to make the report cleaner).

Please look at the README.md and guides/unreachable-analyzer.md for further details.

[enerdgumen]

No feedback received, so long and thanks for all the fish!

[cpiemontese]

Hey @hauleth! 👋

I've taken over maintaining our (Prima.it) fork and I was wondering if this PR could still be useful to merge
I know it's been sometime but hopefully it's still valuable 😄

[hauleth]

I will take a look today and will see.

[cpiemontese]

Hello! Any news?

[hauleth]

I will look at it today. I was highly occupied with my $WORK and I had little time to look at this in the meantime.

[hauleth]

I would like to view it as a 2 separate PRs - one that adds option for more configuration options, and separate with the unreachable analyser.

When I was creating mix_unused the idea behind MixUnused.Analyze was that it would support externally implemented analysers. So this Unreachable analyser could be library like mix_unused_unreachable. I would need to think whether this particular analyser is useful outside of projects like Phoenix to make it part of the core.

Don't understand me wrong, it looks useful, it is just that additional code will push maintenance burden to me, and as you may have noticed, I often have other stuff that I need to maintain and I would want to avoid having such issues stalled again.

[cpiemontese]

I completely understand and thanks for looking into this 😄

I will open separate PRs and once we actually have the one on the unreachable analyzer we can see if it's actually useful to integrate it in the core or may extract it in a library, does that make sense?

[hauleth]

Yes, that would be awesome.