Linter: Implement erb-no-module-definitions rule

[markokajzer]

closes #197

Rule: erb-no-module-definitions

Description

Disallow Ruby module ... end definitions inside ERB templates.

Rationale

Modules introduce reusable application structure, which belongs in a helper, library, or initializer rather than a view. Defining one while rendering a template can pollute the view context, prevents normal autoloading, and makes the template harder to understand and reuse.

Examples

✅ Good

# app/helpers/display_helpers.rb
module DisplayHelpers
  def highlight(text)
    "<mark>#{text}</mark>".html_safe
  end
end

<%= highlight("Important") %>

🚫 Bad

<%
  module DisplayHelpers
    def highlight(text)
      "<mark>#{text}</mark>".html_safe
    end
  end
%>

<%= highlight("Important") %>

<%
  decorator = Module.new do
    def highlight
      "<mark>#{text}</mark>".html_safe
    end
  end
%>

[marcoroth]

lets use a different example here. Promoting .html_safe is not great, especially since this could become a problem if highlight gets called with user-controlled data.

Maybe an example with like pluralize could work?

[markokajzer]

switched to pluralize for example and specs, and used a more generic error message 🙏

let me know if any other changes are necessary!

i'm currently running against a few repos of https://github.com/steveclarke/real-world-rails, and i'm slowly adding more of them to have a wider base of examples. if you have a good one, feel free to suggest 👀

[marcoroth]

@markokajzer directly related: https://github.com/marcoroth/herb-corpus

We are running against this corpus on every commit, at least for the parse + printer, but not the linter yet.

[marcoroth]

Sweet, thank you @markokajzer! 🙏🏼