Add new Style/SoleNestedConditional cop

[fatkodima]

This cop checks for places where nested conditionals can be merged into outer ones, like:

# bad
if condition_a
  if condition_b
    do_something
  end
end

# good
if condition_a && condition_b
  do_something
end

This, imo, makes code more readable and, for sure, reduces nestings.

Inspired by Style/IfInsideElse cop.
That cop also has (disabled, btw, in rubocops .rubocop.yml file) AllowIfModifier setting. I haven't added that as I haven't get the point of its usefulness and how this

if condition_a
  do_something if condition_b
end

can be better than

if condition_a && condition_b
  do_something
end

Probably, the cop name and wordings can be improved.

[marcandre]

I can see how this:

if important_criteria
  do_something(:important, 'with a long argument') unless unlikely_minor_cond?
end

# could be better than
if important_criteria && !unlikely_minor_cond?
  do_something(:important, 'with a long argument')
end

[marcandre]

Also, did you handle the cases if (var = calculate) manually? That's cheating 🤣. I think that's if a criteria is an assignment than no offense should be raised, no?

I don't see any comments in the specs, are these handled correctly? We should always put comments in our specs, as these are typically difficult to handle, especially when there are line changes.

[fatkodima]

Also, did you handle the cases if (var = calculate) manually? That's cheating 🤣. I think that's if a criteria is an assignment than no offense should be raised, no?

Not sure. Personally, I'm happy with offense raised in this case 😄 I extracted that cases as a separate assignment before conditional, but would also just append nested conditions to that assignment. I looks more simpler/clear to me as a separate line.

[fatkodima]

Added AllowModifier option and test with comment.

[bbatsov]

The changes look good, but your branch has to be rebased on top of the current master branch due to merge conflicts.

[fatkodima]

Rebased.

[bbatsov]

Thanks!