Add new cop Lint/UnexpectedBlockArity

[dvandersluis]

Follows #9071 (comment):

Maybe it should be a different cop that checks the block arity for methods that are known to have arity 2. There are less of them now that Hash#each is being changed to arity 1. Maybe just Enumerable#each_with_object, min, max, minmax, slice_when, sort? Maybe we even have such a cop already?

This new cop checks for method blocks where the given block args don't match what the method is known to need. It is configurable using Methods, which is preloaded with all the Enumerable methods that take 2 args, as per the above comment.

---

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• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
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[bbatsov]

Overall the cop looks great. My only suggestion would be to ignore method calls without receivers, as those are going to be more likely to be false positives.

[dvandersluis]

@bbatsov makes sense! I've updated it.

[bbatsov]

Thanks!

[inkstak]

Would it be possible to add a deny list an allow list using regexp ?

We have a lot of sort blocks in jbuilder templates which are json properties.

json.sort do
  json.order     @current_order
  json.direction @current_order_direction
end

[dvandersluis]

@inkstak configuring the cop like this probably would solve this for you:

Lint/UnexpectedBlockArity:
  Exclude:
    - '**/*.jbuilder'

[inkstak]

@dvandersluis Yes this is my current workaround, but it's too bad that this helpful cop doesn't cover regular Enum#sort methods present in our templates.

[dvandersluis]

There isn't really a way for rubocop to know if sort is Enumerable#sort or another sort. Generally when cops exclude methods it's done across the board, not based on receiver (because in general we can't tell what the receiver is).

[marcandre]

(until we plug into rbs...)

[inkstak]

I actually suspect we can't tell what type is the receiver, but isn't it impossible to exclude cases using the name of the receiver or the whole line ? I humbly ask, the subtleties of rubocop parser are still beyonds me.

Lint/UnexpectedBlockArity:
  Allowlist:
    - 'json.sort'
    - /\Ajson\.sort do\Z/

Still, some real uses of Enumerable#sort could go through, but this should be more restricted