Usage file in generators shouldn't be fetched only based on source_root

In case `source_roout` is not set, `default_source_root` is used,
which includes also `templates` directory. If there is no `templates`
directory, `default_source_root` is not available and USAGE will not
be displayed. USAGE should be also checked based on default
directory excluding `templates`.

railties/lib/rails/generators/base.rb

@@ -31,10 +31,9 @@ def self.source_root(path=nil)
       # root otherwise uses a default description.
       def self.desc(description=nil)
         return super if description
-        usage = source_root && File.expand_path("../USAGE", source_root)
 
-        @desc ||= if usage && File.exist?(usage)
-          ERB.new(File.read(usage)).result(binding)
+        @desc ||= if usage_path
+          ERB.new(File.read(usage_path)).result(binding)
         else
           "Description:\n    Create #{base_name.humanize.downcase} files for #{generator_name} generator."
         end
@@ -207,7 +206,8 @@ def self.class_option(name, options={}) #:nodoc:
       # root, you should use source_root.
       def self.default_source_root
         return unless base_name && generator_name
-        path = File.expand_path(File.join(base_name, generator_name, 'templates'), base_root)
+        return unless default_generator_root
+        path = File.join(default_generator_root, 'templates')
         path if File.exists?(path)
       end
 
@@ -371,6 +371,19 @@ def self.add_shebang_option!
           }
         end
 
+        def self.usage_path
+          paths = [
+            source_root && File.expand_path("../USAGE", source_root),
+            default_generator_root && File.join(default_generator_root, "USAGE")
+          ]
+          paths.compact.detect { |path| File.exists? path }
+        end
+
+        def self.default_generator_root
+          path = File.expand_path(File.join(base_name, generator_name), base_root)
+          path if File.exists?(path)
+        end
+
     end
   end
 end

railties/test/generators/app_generator_test.rb

@@ -316,7 +316,7 @@ def test_usage_read_from_file
   end
 
   def test_default_usage
-    File.expects(:exist?).returns(false)
+    Rails::Generators::AppGenerator.expects(:usage_path).returns(nil)
     assert_match(/Create rails files for app generator/, Rails::Generators::AppGenerator.desc)
   end
 

railties/test/generators/migration_generator_test.rb

@@ -157,4 +157,8 @@ def test_should_create_empty_migrations_if_name_not_start_with_add_or_remove
       end
     end
   end
+
+  def test_properly_identifies_usage_file
+    assert generator_class.send(:usage_path)
+  end
 end