Generate RuboCop TODO config even without any offenses

Sometimes we enable new cop rules without any offenses and put this rule
in "grace period". This is done in the TODO config in
`.rubocop_todo/**/*.yml` because it's a temporary configuration which
should be removed short after.

However, when regenerating TODOs for cops without any offenses we
discarded to generate them.

This commit fixes this issue and generates a cop TODO config if:
* Cop was previously disabled due to too many offenses
* Cop is in grace period
* Cop excludes files

rubocop/cop_todo.rb

@@ -26,6 +26,10 @@ def autocorrectable?
       @cop_class&.support_autocorrect?
     end
 
+    def generate?
+      previously_disabled || grace_period || files.any?
+    end
+
     def to_yaml
       yaml = []
       yaml << '---'
@@ -39,8 +43,12 @@ def to_yaml
       end
 
       yaml << "  #{RuboCop::Formatter::GracefulFormatter.grace_period_key_value}" if grace_period
-      yaml << '  Exclude:'
-      yaml.concat files.sort.map { |file| "    - '#{file}'" }
+
+      if files.any?
+        yaml << '  Exclude:'
+        yaml.concat files.sort.map { |file| "    - '#{file}'" }
+      end
+
       yaml << ''
 
       yaml.join("\n")

rubocop/formatter/todo_formatter.rb

@@ -31,6 +31,7 @@ def initialize(output, _options = {})
         @config_inspect_todo_dir = load_config_inspect_todo_dir
         @config_old_todo_yml = load_config_old_todo_yml
         check_multiple_configurations!
+        create_empty_todos(@config_inspect_todo_dir)
 
         super
       end
@@ -47,11 +48,9 @@ def file_finished(file, offenses)
 
       def finished(_inspected_files)
         @todos.values.sort_by(&:cop_name).each do |todo|
-          todo.previously_disabled = previously_disabled?(todo)
-          todo.grace_period = grace_period?(todo)
-          validate_todo!(todo)
-          path = @todo_dir.write(todo.cop_name, todo.to_yaml)
+          next unless configure_and_validate_todo(todo)
 
+          path = @todo_dir.write(todo.cop_name, todo.to_yaml)
           output.puts "Written to #{relative_path(path)}\n"
         end
       end
@@ -82,6 +81,14 @@ def check_multiple_configurations!
         raise "Multiple configurations found for cops:\n#{list}\n"
       end
 
+      # For each inspected cop TODO config create a TODO object to make sure
+      # the cop TODO config will be written even without any offenses.
+      def create_empty_todos(inspected_cop_config)
+        inspected_cop_config.each_key do |cop_name|
+          @todos[cop_name]
+        end
+      end
+
       def config_for(todo)
         cop_name = todo.cop_name
 
@@ -101,10 +108,15 @@ def grace_period?(todo)
         GracefulFormatter.grace_period?(todo.cop_name, config)
       end
 
-      def validate_todo!(todo)
-        return unless todo.previously_disabled && todo.grace_period
+      def configure_and_validate_todo(todo)
+        todo.previously_disabled = previously_disabled?(todo)
+        todo.grace_period = grace_period?(todo)
+
+        if todo.previously_disabled && todo.grace_period
+          raise "#{todo.cop_name}: Cop must be enabled to use `#{GracefulFormatter.grace_period_key_value}`."
+        end
 
-        raise "#{todo.cop_name}: Cop must be enabled to use `#{GracefulFormatter.grace_period_key_value}`."
+        todo.generate?
       end
 
       def load_config_inspect_todo_dir

spec/rubocop/cop_todo_spec.rb

@@ -66,6 +66,38 @@
     end
   end
 
+  describe '#generate?' do
+    subject { cop_todo.generate? }
+
+    context 'when empty todo' do
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when previously disabled' do
+      before do
+        cop_todo.previously_disabled = true
+      end
+
+      it { is_expected.to eq(true) }
+    end
+
+    context 'when in grace period' do
+      before do
+        cop_todo.grace_period = true
+      end
+
+      it { is_expected.to eq(true) }
+    end
+
+    context 'with offenses recorded' do
+      before do
+        cop_todo.record('a.rb', 1)
+      end
+
+      it { is_expected.to eq(true) }
+    end
+  end
+
   describe '#to_yaml' do
     subject(:yaml) { cop_todo.to_yaml }
 
@@ -79,7 +111,6 @@
           ---
           # Cop supports --auto-correct.
           #{cop_name}:
-            Exclude:
         YAML
       end
     end

spec/rubocop/formatter/todo_formatter_spec.rb

@@ -309,18 +309,78 @@ def run_formatter
 
   context 'without offenses detected' do
     before do
+      todo_dir.write('A/Cop', yaml) if yaml
+      todo_dir.inspect_all
+
       formatter.started(%w[a.rb b.rb])
       formatter.file_finished('a.rb', [])
       formatter.file_finished('b.rb', [])
       formatter.finished(%w[a.rb b.rb])
+
+      todo_dir.delete_inspected
     end
 
-    it 'does not output anything' do
-      expect(stdout.string).to eq('')
+    context 'without existing TODOs' do
+      let(:yaml) { nil }
+
+      it 'does not output anything' do
+        expect(stdout.string).to eq('')
+      end
+
+      it 'does not write any YAML files' do
+        expect(rubocop_todo_dir_listing).to be_empty
+      end
     end
 
-    it 'does not write any YAML files' do
-      expect(rubocop_todo_dir_listing).to be_empty
+    context 'with existing TODOs' do
+      context 'when existing offenses only' do
+        let(:yaml) do
+          <<~YAML
+            ---
+            A/Cop:
+              Exclude:
+                - x.rb
+          YAML
+        end
+
+        it 'does not output anything' do
+          expect(stdout.string).to eq('')
+        end
+
+        it 'does not write any YAML files' do
+          expect(rubocop_todo_dir_listing).to be_empty
+        end
+      end
+
+      context 'when in grace period' do
+        let(:yaml) do
+          <<~YAML
+            ---
+            A/Cop:
+              Details: grace period
+              Exclude:
+                - x.rb
+          YAML
+        end
+
+        it 'outputs its actions' do
+          expect(stdout.string).to eq(<<~OUTPUT)
+            Written to .rubocop_todo/a/cop.yml
+          OUTPUT
+        end
+
+        it 'creates YAML file with Details only', :aggregate_failures do
+          expect(rubocop_todo_dir_listing).to contain_exactly(
+            'a/cop.yml'
+          )
+
+          expect(todo_yml('A/Cop')).to eq(<<~YAML)
+            ---
+            A/Cop:
+              Details: grace period
+          YAML
+        end
+      end
     end
   end