Linter: Implement erb-no-method-definitions rule

[markokajzer]

closes #195

Rule: erb-no-method-definitions

Description

Disallow defining Ruby methods inside ERB files using def ... end or define_method. View templates should only contain logic necessary for rendering the page. Defining methods in ERB mixes concerns and breaks MVC conventions.

Rationale

Defining methods in a view template:

• Pollutes the view context
• Makes templates harder to read and debug
• Introduces global-like behavior into views
• Encourages non-reusable, non-testable logic
• Violates Rails' separation of concerns

Method definitions belong in helpers, presenters, or view components, not in view files.

Examples

✅ Good

# app/helpers/application_helper.rb

module ApplicationHelper
  def format_date(date)
    date.strftime("%B %d")
  end

  def admin?
    current_user.admin?
  end
end

🚫 Bad

<% def format_date(date) %>
  <%= date.strftime("%B %d") %>
<% end %>

<p><%= format_date(Date.today) %></p>

<% def admin? = current_user.admin? %>

<% define_method(:admin?) { current_user.admin? } %>

[markokajzer]

not sure if we want to catch these

issue description was somewhat specific regarding def ... end blocks, but knowing ruby there are a bunch of other ways to define "methods"

[marcoroth]

I think it's fine to leave them for now. If'd want to catch them we could still introduce a new rule specifically for lambdas.

[marcoroth]

Thanks @markokajzer! 🙏🏼

To only thing to note here is that we can't catch issues like #990 because they are just straight up a parser error.

Though, we might be able to set consumesParserErrors = true on the rule itself, and then, since we can look at the extracted Ruby from that file, detect the method definition that way since then its all valid Ruby again 🤔

[markokajzer]

i didnt even know that was possible haha