[Fix #1164] Fix autocorrection in Rails/Delegate to preserve comments

changelog/fix_autocorrection_for_rails_delegate.md

@@ -0,0 +1 @@
+* [#1164](https://github.com/rubocop/rubocop-rails/issues/1164): Fix autocorrection for `Rails/Delegate` to preserve comments. ([@masato-bkn][])

lib/rubocop/cop/rails/delegate.rb

@@ -82,7 +82,11 @@ def on_def(node)
 
         def register_offense(node)
           add_offense(node.loc.keyword) do |corrector|
-            corrector.replace(node, build_delegation(node))
+            comments = processed_source.ast_with_comments[node.body]
+            preferred = build_comment(comments)
+            preferred += build_delegation(node)
+
+            corrector.replace(node, preferred)
           end
         end
 
@@ -97,6 +101,20 @@ def build_delegation(node)
           delegation.join(', ')
         end
 
+        def build_comment(comments)
+          return '' if comments.empty?
+
+          comments.map.with_index do |comment, i|
+            comment.text + ("\n" * newline_count_between_comments(comments, i))
+          end.join << "\n"
+        end
+
+        def newline_count_between_comments(comments, index)
+          return 0 if index >= comments.size - 1
+
+          comments[index + 1].loc.line - comments[index].loc.line
+        end
+
         def trivial_delegate?(def_node)
           delegate?(def_node) &&
             method_name_matches?(def_node.method_name, def_node.body) &&

spec/rubocop/cop/rails/delegate_spec.rb

@@ -55,6 +55,80 @@ def bar_foo
     RUBY
   end
 
+  it 'finds trivial delegate with single line comment' do
+    expect_offense(<<~RUBY)
+      def foo
+      ^^^ Use `delegate` to define delegations.
+        # comment
+        bar.foo
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      # comment
+      delegate :foo, to: :bar
+    RUBY
+  end
+
+  it 'finds trivial delegate with multiline comments' do
+    expect_offense(<<~RUBY)
+      def foo
+      ^^^ Use `delegate` to define delegations.
+        # comment 1
+        # comment 2
+        bar.foo
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      # comment 1
+      # comment 2
+      delegate :foo, to: :bar
+    RUBY
+  end
+
+  it 'finds trivial delegate with multiline comments including blank lines' do
+    expect_offense(<<~RUBY)
+      def foo
+      ^^^ Use `delegate` to define delegations.
+        # comment 1
+
+        # comment 2
+
+
+        # comment 3
+        bar.foo
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      # comment 1
+
+      # comment 2
+
+
+      # comment 3
+      delegate :foo, to: :bar
+    RUBY
+  end
+
+  it 'finds trivial delegate with preceding comments' do
+    expect_offense(<<~RUBY)
+      # comment 1
+      def foo
+      ^^^ Use `delegate` to define delegations.
+        # comment 2
+        bar.foo
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      # comment 1
+      # comment 2
+      delegate :foo, to: :bar
+    RUBY
+  end
+
   it 'finds trivial delegate to `self` when underscored method' do
     expect_offense(<<~RUBY)
       def bar_foo