Add RSpec/MemoizedHelperBlockDelimiter cop

[r7kamura]

In response to the review comment at #1414 (comment), I made a cop to be consistent in the way memoized helpers such as let and subject are written.

My personal opinion is that since these helpers are method definitions, they should be written in a style similar to that of writing def. I don't see many cases where def is written in one-liner style. For that reason, I thought that I should prefer to use a consistent style regardless of whether the content of the block would be short or long, and that do_end should be the default.

There is another reason for wanting the default to be this way: the braces style may conflict with Style/BlockDelimiters cop in some cases, so it may not be possible to autocorrect to always use braces. I would also like to avoid the put another style to use different delimiters for different situations as possible because this increases the cognitive load.

I think this cop is somewhat opinionated, so I would like to hear your opinions first.

---

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded in default/config.yml to the next minor version.

If you have modified an existing cop's configuration options:

• Set VersionChanged in config/default.yml to the next major version.

[pirj]

To be useful, I believe the cop should work together with Layout/LineLength somehow. If the style is braces, the helper should be inlined unless this would break Layout/LineLength.

The default style should be braces, and it shouldn't even raise an offence for do/end usages if this would break the line length constraint set by Layout/LineLength.

I believe it is possible to offer auto-correction for braces, too, as:

1. It's not necessary that BlockDelimiters cop is enabled in the user configuration
2. It is possible to inline (except when it breaks line length)

PS The cop should ignore multi-statement definitions for the braces style

[pirj]

This will also match blockpass, should it?

[pirj]

I suggest turning this into a node pattern:

        {
          #{block_pattern('#Helpers.all')}
          #{block_pattern('#Subjects.all')}
        }

[pirj]

This line doesn't semantically belong to "bad", it detects if it's a match.
I suggest moving it to on_block:

memoized_helper_method(node) do |memoized_helper|
  next if preferred_block_delimiter?(memoized_helper)
  add_offense(...)

[pirj]

I disagree with the choice of the default style.

do_end doesn't seem to be a frequent choice in the wild, according to a run against real-world-rspec:

57497 files inspected, 104170 offenses detected, 104170 offenses autocorrectable

as opposed to braces:

57497 files inspected, 17121 offenses detected

And, I suspect some part of do/end usages (offences to braces style) are due to the line length limitation:

    let!(:conference) do
      WimbaConference.create!(title: "my conference", user: @user, duration: 60, context: course_factory)
    end

or because of being multi-line e.g. /Users/pirj/source/real-world-rspec/spree/emails/spec/mailers/spree/order_mailer_spec.rb:25:22::

  let(:second_order) do
    order = create(:completed_order_with_totals, email: 'test2@example.com')
    product = create(:product, name: %{The "BESTEST" product})
    variant = create(:variant, product: product)
    price = create(:price, variant: variant, amount: 15.00)
    store = second_store
    line_item = create(:line_item, variant: variant, order: order, quantity: 1, price: 4.99)
    allow(product).to receive_messages(default_variant: variant)
    allow(variant).to receive_messages(default_price: price)
    allow(order).to receive_messages(line_items: [line_item])
    allow(order).to receive_messages(store: store)
    order
  end

[r7kamura]

Sorry for the late reply.
It is true that the default should be braces, considering the usage ratio of existing code, and Layout/LineLength should be taken into account. I don't know if I can implement this well, but I will give it a try 👌

[r7kamura]

While I was able to implement the do-end style well, I could not come up with a good implementation or appropriate rules for the braces style. I'll close this Pull Request for now 🙇