Merge pull request #13326 from schneems/schneems/add-asset-flag

Flag `config.assets.raise_runtime_errors` in dev

guides/source/asset_pipeline.md

@@ -577,6 +577,33 @@ would generate this HTML:
 
 The `body` param is required by Sprockets.
 
+### Runtime Error Checking
+
+By default the asset pipeline will check for potential errors in development mode during
+runtime. To disable this behavior you can set:
+
+```ruby
+config.assets.raise_runtime_errors = false
+```
+
+When `raise_runtime_errors` is set to `false` sprockets will not check that dependencies of assets are declared properly. Here is a scenario where you must tell the asset pipeline about a dependency:
+
+If you have `application.css.erb` that references `logo.png` like this:
+
+```css
+#logo { background: url(<%= asset_data_uri 'logo.png' %>) }
+```
+
+Then you must declare that `logo.png` is a dependency of `application.css.erb`, so when the image gets re-compiled, the css file does as well. You can do this using the `//= depend_on_asset` declaration:
+
+```css
+//= depend_on_asset "logo.png"
+#logo { background: url(<%= asset_data_uri 'logo.png' %>) }
+```
+
+Without this declaration you may experience strange behavior when pushing to production that is difficult to debug. When you have `raise_runtime_errors` set to `true`, dependencies will be checked at runtime so you can ensure that all dependencies are met.
+
+
 ### Turning Debugging Off
 
 You can turn off debug mode by updating `config/environments/development.rb` to

railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt

@@ -29,5 +29,10 @@ Rails.application.configure do
   # This option may cause significant delays in view rendering with a large
   # number of complex assets.
   config.assets.debug = true
+
+  # Adds additional error checking when serving assets at runtime.
+  # Checks for improperly declared sprockets dependencies.
+  # Raises helpful error messages.
+  config.assets.raise_runtime_errors = true
   <%- end -%>
 end