Add RSpec/IndexedLet cop

[DmitryTsepelev]

Adds the cop that tries to find indexed let statements (item_1, item_2, ...):

# bad
let(:item_1) { create(:item) }
let(:item_2) { create(:item) }

let(:item_1) { create(:item, visible: true) }
let(:item_2) { create(:item, visible: false) }

before do
  item_1 = create(:item)
  item_2 = create(:item)
end

# good
let(:items) { create_list(:item, 2) }

let(:visible_item) { create(:item, visible: true) }
let(:invisible_item) { create(:item, visible: false) }

before do
  create_list(:item, 2)
end

---

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: "<<next>>" in default/config.yml.

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

• Set VersionChanged: "<<next>>" in config/default.yml.

[DmitryTsepelev]

Not sure why that check fails, works fine locally

[bquorning]

I see create_list and think this probably should be a FactoryBot cop: RSpec/FactoryBot/IndexedLet?

[pirj]

Not necessarily, create can be replaced with fixture(:...), and the cop would still inspect the same way.

[DmitryTsepelev]

@pirj Addressed everything, ready for another round 🙂

[ydah]

It sometimes outputs like following, so it seems better to pass a description to it or specify like other specs.

RuboCop::Cop::RSpec::IndexedLet
  is expected to eq describe SomeService do
  let(:item_1) { create(:item) }
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This `let` ...ame. Please give it a meaningful names, use create_list or move creation to the `before` block.
end

[ydah]

I think it would be good to have the following as test cases as well.

describe SomeService do
  let(:item_1) { create(:item) }
  let(:foo_item_1) { create(:item) }
end

describe SomeService do
  let(:item_1) { create(:item) }
  let("item_2") { create(:item) }
end

[DmitryTsepelev]

Applied all suggesstions, updated specs and docs, thanks 🙂

[ydah]

Thank you for doing a great work ❤️

[pirj]

Thank you!
A few nitpicks below.

[DmitryTsepelev]

Updated 🙂

[pirj]

Thank you, @DmitryTsepelev !

[bquorning]

Oops, it looks like we forgot to squash commits before merging 😅