Add Config#for_badge

This optimization saves about 5MB of unneeded intermediate hashes...

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### New features
 
 * [#8322](https://github.com/rubocop-hq/rubocop/pull/8322): Support autocorrect for `Style/CaseEquality` cop. ([@fatkodima][])
+* [#8339](https://github.com/rubocop-hq/rubocop/pull/8339): Add `Config#for_badge` as an efficient way to get a cop's config merged with its department's. ([@marcandre][])
 
 ### Bug fixes
 

lib/rubocop/config.rb

@@ -47,7 +47,7 @@ def check
     end
 
     def_delegators :@hash, :[], :[]=, :delete, :each, :key?, :keys, :each_key,
-                   :map, :merge, :to_h, :to_hash, :transform_values
+                   :fetch, :map, :merge, :to_h, :to_hash, :transform_values
     def_delegators :@validator, :validate, :target_ruby_version
 
     def to_s
@@ -111,6 +111,14 @@ def for_cop(cop)
       @for_cop[cop.respond_to?(:cop_name) ? cop.cop_name : cop]
     end
 
+    # @return [Config] for the given cop merged with that of its department (if any)
+    # Note: the 'Enabled' attribute is same as that returned by `for_cop`
+    def for_badge(badge)
+      cop_config = for_cop(badge.to_s)
+      fetch(badge.department.to_s) { return cop_config }
+        .merge(cop_config)
+    end
+
     # @return [Config] for the given department name.
     # Note: the 'Enabled' attribute will be present only if specified
     # at the department's level

lib/rubocop/cop/base.rb

@@ -189,8 +189,7 @@ def cop_name
       def cop_config
         # Use department configuration as basis, but let individual cop
         # configuration override.
-        @cop_config ||= @config.for_cop(self.class.department.to_s)
-                               .merge(@config.for_cop(self))
+        @cop_config ||= @config.for_badge(self.class.badge)
       end
 
       def config_to_allow_offenses

spec/rubocop/config_spec.rb

@@ -735,6 +735,31 @@
     end
   end
 
+  describe '#for_badge' do
+    let(:hash) do
+      {
+        'Style' => { 'Foo' => 42, 'Bar' => 666 },
+        'Layout/TrailingWhitespace' => { 'Bar' => 43 },
+        'Style/Alias' => { 'Bar' => 44 }
+      }
+    end
+
+    it 'takes into account the department' do
+      expect(configuration.for_badge(RuboCop::Cop::Style::Alias.badge)).to eq(
+        { 'Enabled' => true,
+          'Foo' => 42,
+          'Bar' => 44 }
+      )
+    end
+
+    it 'works if department has no config' do
+      expect(configuration.for_badge(RuboCop::Cop::Layout::TrailingWhitespace.badge)).to eq(
+        { 'Enabled' => true,
+          'Bar' => 43 }
+      )
+    end
+  end
+
   context 'whether the cop is enabled' do
     def cop_enabled(cop_class)
       configuration.for_cop(cop_class).fetch('Enabled')