List manifest locations when asset is not found

lib/sprockets/rails/helper.rb

@@ -80,7 +80,11 @@ def compute_asset_path(path, options = {})
         if asset_path = resolve_asset_path(path, debug)
           File.join(assets_prefix || "/", legacy_debug_path(asset_path, debug))
         else
-          message =  "The asset #{ path.inspect } is not present in the asset pipeline.\n"
+          message = String.new("The asset #{ path.inspect } is not present in the asset pipeline.\n")
+          asset_resolver_strategies.each do |strat|
+            message << "Checked in manifest file: #{strat.filename}\n" if strat.filename
+          end
+
           raise AssetNotFound, message unless unknown_asset_fallback
 
           if respond_to?(:public_compute_asset_path)
@@ -284,6 +288,10 @@ def initialize(view)
           raise ArgumentError, 'config.assets.resolve_with includes :manifest, but app.assets_manifest is nil' unless @manifest
         end
 
+        def filename
+          @manifest.try(:filename)
+        end
+
         def asset_path(path, digest, allow_non_precompiled = false)
           if digest
             digest_path path, allow_non_precompiled
@@ -320,6 +328,10 @@ def initialize(view)
           @check_precompiled_asset = view.check_precompiled_asset
         end
 
+        def filename
+          nil
+        end
+
         def asset_path(path, digest, allow_non_precompiled = false)
           # Digests enabled? Do the work to calculate the full asset path.
           if digest

sprockets-rails.gemspec

@@ -20,6 +20,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rake"
   s.add_development_dependency "sass"
   s.add_development_dependency "uglifier"
+  s.add_development_dependency "m"
 
   s.author = "Joshua Peek"
   s.email  = "[email protected]"

test/test_railtie.rb

@@ -290,6 +290,25 @@ def test_action_view_helper_when_no_compile
     assert_equal app.assets_manifest, @view.assets_manifest
   end
 
+  def test_lists_manifest_files_on_failed_asset_lookup_in_prod
+    app.configure do
+      config.assets.compile = false
+      config.assets.unknown_asset_fallback = false
+    end
+
+    assert_equal false, app.config.assets.compile
+
+    app.initialize!
+
+    @view = ActionView::Base.new
+    error = assert_raises do
+      @view.asset_path("does_not_exist.js")
+    end
+
+    assert_match(/Checked in manifest/, error.message)
+    assert_match(/\.sprockets-manifest-/, error.message)
+  end
+
   def test_sprockets_context_helper
     app.initialize!