Don't mount Sprockets at config.assets.prefix if config.assets.compile is false.

actionpack/lib/sprockets/railtie.rb

@@ -67,8 +67,10 @@ module Sprockets
         end
       end
 
-      app.routes.prepend do
-        mount app.assets => config.assets.prefix
+      if config.assets.compile
+        app.routes.prepend do
+          mount app.assets => config.assets.prefix
+        end
       end
 
       if config.assets.digest

railties/test/application/assets_test.rb

@@ -178,6 +178,7 @@ module ApplicationTests
 
     test "assets do not require any assets group gem when manifest file is present" do
       app_file "app/assets/javascripts/application.js", "alert();"
+      app_file "config/initializers/serve_static_assets.rb", "Rails.application.config.serve_static_assets = true"
 
       ENV["RAILS_ENV"] = "production"
       capture(:stdout) do
@@ -314,6 +315,17 @@ module ApplicationTests
       assert_equal 0, files.length, "Expected no assets, but found #{files.join(', ')}"
     end
 
+    test "assets routes are not drawn when compilation is disabled" do
+      app_file "app/assets/javascripts/demo.js.erb", "<%= :alert %>();"
+      app_file "config/initializers/compile.rb", "Rails.application.config.assets.compile = false"
+
+      ENV["RAILS_ENV"] = "production"
+      require "#{app_path}/config/environment"
+
+      get "/assets/demo.js"
+      assert_equal 404, last_response.status
+    end
+
     test "does not stream session cookies back" do
       app_file "app/assets/javascripts/demo.js.erb", "<%= :alert %>();"