Add Rails/AddColumnIndex cop to prevent passing an unused but confusing index: key to add_column migrations

[dvandersluis]

We ran into this at work. add_column in a migration seems to accept an index key like change_column or t.column syntax, but it does not actually do anything. This cop is to prevent appearing like an index will be added without actually adding one, ie.

add_column :table, :my_column, :string, index: true

If this is detected, the cop will register an offense, and correct by adding a add_index line following.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

It would be better if it could this (and other) illegal option be detected at the migration runtime as well. Please ask the Rails team if you like :-) Anyway, this PR looks good to me. Can you rebase with the latest mater branch?

[koic]

@dvandersluis ping :-)

[dvandersluis]

Sorry for the delay @koic, it's been rebased now!

There's rails/rails#39230 but it hasn't had any progress.

[koic]

I get it! I didn't know the Rails's issue. Thank you!

[owst]

Just to note that it looks like this PR to Rails is included in Rails 7.1 rails/rails#46178 which I think will reject the invalid index option