Add new Rails/ValidationsGrouping cop

[fatkodima]

Closes #555.

Autocorrection is more involved, so I suggest to leave it as a future improvement.

Tested on gitlab's codebase: a bunch of offenses, no errors.

[Bhacaz]

@fatkodima
A suggestion, to simplify the autocorrection, maybe the config SingleAttributeValidations could be instead an other cop that can be disable if not wanted.

A cop with check and autocorrect

# bad
validates :email, format: { with: /@/ }
validates :email, presence: true
# good
validates :email, presence: true, format: { with: /@/ }

and the other

# bad
validates :name, :bio, presence: true
# good
validates :name, presence: true
validates :bio, presence: true

The combination of both cops will help to follow the style guide.

What do you think?

[fatkodima]

@koic Can you review this?

[Earlopain]

Can you show in tests how this handles things like this:

# class-level conditionals
if something?
  validates :email, presence: true
end

# or https://api.rubyonrails.org/classes/Object.html#method-i-with_options
with_options if: :something? do
  validates :email, presence: true
end

I don't believe there is code to handle that at the moment.

Also, can you add a test where validations have overlapping conditions:

validates :email, format: { with: /foo/ }
validates :email, format: { with: /bar/ }

[fatkodima]

Can you show in tests how this handles things like this

Made that validations inside each branch/etc are considered separately.

Also, can you add a test where validations have overlapping conditions

Added.

[Earlopain]

What would be the expected correction for this case?

validates :bool, inclusion: { in: [false], if: :foo? }
validates :bool, inclusion: { in: [true], if: :bar? }

I can't come up with something that doesn't make this more complicated.

[fatkodima]

validates :bool, inclusion: { in: :some_meaningfull_method_name }

def some_meaningfull_method_name
  if foo?
    [false]
  else
    [true]
  end
end