Slim Line breaks and formatting

From the docs:

Slim::Engine.set_default_options pretty: true

or directly

Slim::Engine.default_options[:pretty] = true

To expand a bit, as @rubiii mentioned in the comments this is a feature of Slim. For the same reasons it's good practice to minify and compress Javascript and CSS in production Slim strips unecessary whitespace from the HTML it produces without this :pretty option set to true.

If you have some config/initializers/slim.rb file you can configure the :pretty option dynamically by checking the environment.

Slim::Engine.set_default_options pretty: Rails.env.development?

Otherwise you should set this option to true only in config/environments/development.rb, leaving it false in production.