New cop idea: specifying response content when rendering a 204 No Content response

[samrjenkins]

Is your feature request related to a problem? Please describe.

Occasionally I see Rails engineers specify a 204 No Content response within a controller action along with content for the response's body:

render json: { a_name: 'a value' }, status: :no_content

This is misleading. As specified in the Rails docs (https://guides.rubyonrails.org/layouts_and_rendering.html):

If you try to render content along with a non-content status code (100-199, 204, 205, or 304), it will be dropped from the response.

and can give engineers the impression that their response body will contain the content they specify.

Describe the solution you'd like

A new cop to register an offence whenever body content is specified for a response with a no-content status code.
I would be very happy to work on such a cop.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.

[vlad-pisanov]

Should such a cop recommend head :no_content instead?

[samrjenkins]

I am unsure what is best practice in this regard. I suppose head is slightly more explicit as to the lack of body than render.

Interested to hear other opinions.

[koic]

I have never encountered a case like this (it may be lucky 😅 ). Can you show expected bad and good case example codes in practice?

[samrjenkins]

@koic of course.

# Bad
render status: :continue, plain: 'content for the response body'
render status: :no_content, html: helpers.tag.strong('content for the response body')
render status: :reset_content, json: { content_for: 'the response body' }
render status: :not_modified, xml: { content_for: 'the response body' }.to_xml

# Good
render status: :continue
render status: :no_content
render status: :reset_content
render status: :not_modified

# Also good
head :continue
head :no_content
head :reset_content
head :not_modified

[samrjenkins]

@koic I have spun up a PR here: #970