Instead of complaining, add the dependency automatically

We need to use *both* `depend_on` and `depend_on_asset`, because the
latter (bugfully?) considers changes to the referenced asset's
dependencies, but not the file itself (making it entirely useless for an
image).

In the corresponding test, we call a protected Sprockets method... by
design, this isn't something we're supposed to care about / access. But
actually manipulating assets to ensure the dependency has taken hold
seems rather like overkill.

lib/sprockets/rails/helper.rb

@@ -21,14 +21,6 @@ def raise_runtime_errors
         Sprockets::Rails::Helper.raise_runtime_errors
       end
 
-      class DependencyError < StandardError
-        def initialize(path, dep)
-          msg = "Asset depends on '#{dep}' to generate properly but has not declared the dependency\n"
-          msg << "Please add: `//= depend_on_asset \"#{dep}\"` to '#{path}'"
-          super msg
-        end
-      end
-
       class AssetFilteredError < StandardError
         def initialize(source)
           msg = "Asset filtered out and will not be served: " <<
@@ -71,7 +63,7 @@ def self.extended(obj)
 
       def compute_asset_path(path, options = {})
         # Check if we are inside Sprockets context before calling check_dependencies!.
-        check_dependencies!(path) if defined?(_dependency_assets)
+        check_dependencies!(path) if defined?(depend_on)
 
         if digest_path = asset_digest_path(path)
           path = digest_path if digest_assets
@@ -177,11 +169,10 @@ def stylesheet_link_tag(*sources)
       end
 
       protected
-        # Checks if the asset is included in the dependencies list.
+        # Ensures the asset is included in the dependencies list.
         def check_dependencies!(dep)
-          if raise_runtime_errors && !_dependency_assets.detect { |asset| asset.include?(dep) }
-            raise DependencyError.new(self.pathname, dep)
-          end
+          depend_on(dep)
+          depend_on_asset(dep)
         end
 
         # Raise errors when source does not exist or is not in the precompiled list

test/fixtures/url.css.erb

@@ -1,2 +1 @@
-//= depend_on_asset "logo.png"
 p { background: url(<%= image_path "logo.png" %>); }

test/fixtures/url.js.erb

@@ -1,2 +1 @@
-//= depend_on_asset "foo.js"
 var url = '<%= javascript_path :foo %>';

test/test_helper.rb

@@ -435,18 +435,13 @@ def test_asset_digest
   end
 end
 
-class ErrorsInHelpersTest < HelperTest
+class AutomaticDependenciesFromHelpersTest < HelperTest
 
-  def test_dependency_error
-    Sprockets::Rails::Helper.raise_runtime_errors = true
-    Sprockets::Rails::Helper.precompile           = [ lambda {|logical_path| true } ]
-    @view.assets_environment                      = @assets
+  def test_dependency_added
+    @view.assets_environment = @assets
 
-    assert_raises Sprockets::Rails::Helper::DependencyError do
-      @view.asset_path("error/dependency.js")
-    end
+    @view.asset_path("url.css")
 
-    Sprockets::Rails::Helper.raise_runtime_errors = false
-    @view.asset_path("error/dependency.js")
+    assert_equal ["logo.png", "url.css.erb"], @assets['url.css'].send(:dependency_paths).map {|d| File.basename(d.pathname) }.sort
   end
 end