New cop: Style/RedundantUnless

[fatkodima]

It is a known fact, that unless adds cognitive overhead. Personally, I always try to avoid it if if looks better/simpler.
This cop will detect places where unless is completely unneeded and can be replaced by if.

I was thinking, that there is a similar cop already, but was not able to find it.

For example,

# bad
foo unless !bar
foo unless x != y
foo unless x >= 10
foo unless x.positive?

# good
foo if bar
foo if x == y
foo if x < 10
foo if x.negative?

I saw at least many offenses in the rubocop itself. Here are only a few examples:

rubocop/lib/rubocop/cop/layout/trailing_empty_lines.rb

Line 59 in 1e5f19a

return unless blank_lines != wanted_blank_lines

rubocop/lib/rubocop/cop/layout/multiline_assignment_layout.rb

Line 99 in 1e5f19a

return unless node.loc.operator.line != rhs.first_line

rubocop/lib/rubocop/cop/metrics/parameter_lists.rb

Line 82 in 1e5f19a

return unless count > max_params

Related: #11399

There is a kinda related cop already (Style/NegatedUnless), but it only checks for simple negations with !.

[vlad-pisanov]

This is great, although I'm sure about the name. It's not that the unless is redundant (as in "duplicated"), it's more that it's confusing / unnatural. Style/UnnaturalUnless? Style/ClearCondition? idk, can't think of a better name 🤷‍♂️

[si-lens]

I like the idea. unless is a pain in the ass frequently, especially for non-native English speakers.
However, I agree with @vlad-pisanov that the cop name could be better.