Respect DisabledByDefault for custom cops (#7013)

Before this commit, DisabledByDefault has no effect on
custom cops and they are enabled when the config files
contain no mention of them. This is a problematic
behavior as CI servers might load multiple custom cop
and run Rubocop over repos with vastly different
configs. Custom cops end up enabled for repos that
don't care about them, simply because they are loaded.

CHANGELOG.md

@@ -9,6 +9,7 @@
 
 ### Bug fixes
 
+* [#7013](https://github.com/rubocop-hq/rubocop/pull/7013): Respect DisabledByDefault for custom cops. ([@XrXr][])
 * [#7043](https://github.com/rubocop-hq/rubocop/issues/7043): Prevent RDoc error when installing RuboCop 0.69.0 on Ubuntu. ([@koic][])
 
 ## 0.69.0 (2019-05-13)

lib/rubocop/config.rb

@@ -720,7 +720,7 @@ def enable_cop?(qualified_cop_name, cop_options)
         end
       end
 
-      cop_options.fetch('Enabled', true)
+      cop_options.fetch('Enabled') { !for_all_cops['DisabledByDefault'] }
     end
 
     def smart_loaded_path

spec/rubocop/config_spec.rb

@@ -730,6 +730,40 @@ def cop_enabled(cop_class)
         expect(cop_enabled(cop_class)).to be true
       end
     end
+
+    context 'when configuration has no mention of a cop' do
+      let(:hash) do
+        {}
+      end
+
+      it 'enables the cop that is not mentioned' do
+        expect(cop_enabled('VeryCustomDepartment/CustomCop')).to be true
+      end
+
+      context 'when all cops are disabled by default' do
+        let(:hash) do
+          {
+            'AllCops' => { 'DisabledByDefault' => true }
+          }
+        end
+
+        it 'disables the cop that is not mentioned' do
+          expect(cop_enabled('VeryCustomDepartment/CustomCop')).to be false
+        end
+      end
+
+      context 'when all cops are explicitly enabled by default' do
+        let(:hash) do
+          {
+            'AllCops' => { 'EnabledByDefault' => true }
+          }
+        end
+
+        it 'enables the cop that is not mentioned' do
+          expect(cop_enabled('VeryCustomDepartment/CustomCop')).to be true
+        end
+      end
+    end
   end
 
   describe '#target_rails_version' do