Deprecate asset fallback

When an asset isn't found the behavior is to pass the string through. For example a valid asset will return a url from pipeline

```
asset_path("application.js")
# => assets/application-123098udasvi0mnafd.js
```

While if you make a typo, you won't get an error or anything, it just falls through:

```
asset_path("app1icati0n.js")
# => "app1icati0n.js"
```

Hopefully I don't have to elaborate on why this is bad.

This PR is a child PR to one in Rails that will introduce a `public_asset_path` API.

There are valid reasons for not using the asset pipeline, if you have a purely static asset in the public folder or if you want to link to a static URL in your assets somewhere it makes sense to declare that intention. Eventually we will replace the behavior of the deprecation with an exception so people don't lose hours of their to typos.

We only emit the deprecation when a `public_` api is available in Rails. This means that sprockets-rails can still be used in a backwards compatible manner.

Author: schneems

lib/sprockets/rails/helper.rb

@@ -68,15 +68,35 @@ def assets_environment
         end
       end
 
+      # Writes over the built in ActionView::Helpers::AssetUrlHelper#compute_asset_path
+      # to use the asset pipeline.
       def compute_asset_path(path, options = {})
         debug = options[:debug]
 
         if asset_path = resolve_asset_path(path, debug)
           File.join(assets_prefix || "/", legacy_debug_path(asset_path, debug))
         else
-          super
+          result = super
+          if respond_to?(:public_asset_path)
+            throw(:asset_not_found, result)
+          else
+            result
+          end
+        end
+      end
+
+      # Writes over the built in ActionView::Helpers::AssetUrlHelper#asset_path
+      # to use the asset pipeline.
+      def asset_path(*args)
+        catch_asset_not_found = catch(:asset_not_found) do
+          return super(*args)
         end
+
+        result = catch_asset_not_found
+        deprecate_invalid_asset_lookup(result, caller)
+        result
       end
+      alias_method :path_to_asset, :asset_path # aliased to avoid conflicts with an asset_path named route
 
       # Resolve the asset path against the Sprockets manifest or environment.
       # Returns nil if it's an asset we don't know about.
@@ -244,6 +264,49 @@ def legacy_debug_path(path, debug)
             path
           end
         end
+
+      private
+        # Attempts to extract a method name from a given caller line
+        #
+        # Example:
+        #
+        #   extractd_method_from_call_frame('console.rb:65:in `start'') => 'start'
+        def extract_method_from_call_frame(frame)
+          frame.split("in ".freeze).last.gsub(/`|'/, ''.freeze)
+        end
+
+        # Emits a deprecation warning when asset pipeline
+        # is used with an asset that is not part of the pipeline.
+        #
+        # Attempts to determine proper method name based on caller.
+        def deprecate_invalid_asset_lookup(name, call_stack)
+          message =  "The asset #{ name.inspect } you are looking for is not present in the asset pipeline.\n"
+          message << "The public fallback behavior is being deprecated and will be removed.\n"
+
+          append = nil
+          # Search for the most likely top level method where deprecation occured.
+          # Hash contains, in order, the suffix we're looking for, and where it would appear
+          # in the call stack.
+          # This is needed because every deprecated method eventually calls `asset_path`.
+          { "_url" => [0, 1] , "_tag" => [1, 2], "_path" => [0] }.detect do |suffix, positions|
+
+            positions.detect do |position|
+              public_method_name = "public_" + extract_method_from_call_frame(call_stack[position])
+
+              if public_method_name.end_with?(suffix) && respond_to?(public_method_name)
+                position.times { call_stack.shift }
+                append = "please use the `public_*` helper instead. For example `#{ public_method_name }`.\n"
+              else
+                false
+              end
+            end
+          end
+
+          append ||= "please use the `public_*` helper instead for example `public_asset_path`.\n"
+          message << append
+
+          ActiveSupport::Deprecation.warn(message, call_stack)
+        end
     end
 
     # Use a separate module since Helper is mixed in and we needn't pollute

test/test_helper.rb

@@ -827,6 +827,41 @@ def test_index_files
   end
 end
 
+class DeprecationTest < HelperTest
+  def test_deprecations_for_asset_path
+    @view.send(:define_singleton_method, :public_asset_path, -> {})
+    assert_deprecated(/public_asset_path/) do
+      @view.asset_path("does_not_exist.noextension")
+    end
+  ensure
+    @view.instance_eval('undef :public_asset_path')
+  end
+
+  def test_deprecations_for_asset_url
+    @view.send(:define_singleton_method, :public_asset_path, -> {})
+    @view.send(:define_singleton_method, :public_asset_url, -> {})
+
+    assert_deprecated(/public_asset_url/) do
+      @view.asset_url("does_not_exist.noextension")
+    end
+  ensure
+    @view.instance_eval('undef :public_asset_path')
+    @view.instance_eval('undef :public_asset_url')
+  end
+
+  def test_deprecations_for_image_tag
+    @view.send(:define_singleton_method, :public_asset_path, -> {})
+    @view.send(:define_singleton_method, :public_image_tag, -> {})
+
+    assert_deprecated(/public_image_tag/) do
+      @view.image_tag("does_not_exist.noextension")
+    end
+  ensure
+    @view.instance_eval('undef :public_asset_path')
+    @view.instance_eval('undef :public_image_tag')
+  end
+end
+
 class RaiseUnlessPrecompiledAssetDisabledTest < HelperTest
   def test_check_precompiled_asset_enabled
     @view.check_precompiled_asset = true