[Fix #7628] Include trailing comma for single arg (#7630)

CHANGELOG.md

@@ -18,6 +18,7 @@
 * [#7602](https://github.com/rubocop-hq/rubocop/pull/7602): Ensure proper handling of Ruby 2.7 syntax. ([@drenmi][])
 * [#7620](https://github.com/rubocop-hq/rubocop/issues/7620): Fix a false positive for `Migration/DepartmentName` when a disable comment contains a plain comment. ([@koic][])
 * [#7616](https://github.com/rubocop-hq/rubocop/issues/7616): Fix an incorrect autocorrect for `Style/MultilineWhenThen` for when statement with then is an array or a hash. ([@koic][])
+* [#7628](https://github.com/rubocop-hq/rubocop/issues/7628): Fix an incorrect autocorrect for `Layout/MultilineBlockLayout` removing trailing comma with single argument. ([@pawptart][])
 * [#7627](https://github.com/rubocop-hq/rubocop/issues/7627): Fix a false negative for `Migration/DepartmentName` when there is space around `:` (e.g. `# rubocop : disable`). ([@koic][])
 
 ### Changes
@@ -4316,3 +4317,4 @@
 [@jethroo]: https://github.com/jethroo
 [@mangara]: https://github.com/mangara
 [@pirj]: https://github.com/pirj
+[@pawptart]: https://github.com/pawptart

lib/rubocop/cop/layout/multiline_block_layout.rb

@@ -97,7 +97,8 @@ def args_on_beginning_line?(node)
         def line_break_necessary_in_args?(node)
           needed_length = node.source_range.column +
                           node.source.lines.first.length +
-                          block_arg_string(node.arguments).length + PIPE_SIZE
+                          block_arg_string(node, node.arguments).length +
+                          PIPE_SIZE
           needed_length > max_line_length
         end
 
@@ -115,7 +116,8 @@ def autocorrect_arguments(corrector, node)
             newlines: false
           ).end_pos
           range = range_between(node.loc.begin.end.begin_pos, end_pos)
-          corrector.replace(range, " |#{block_arg_string(node.arguments)}|")
+          corrector.replace(range,
+                            " |#{block_arg_string(node, node.arguments)}|")
         end
 
         def autocorrect_body(corrector, node, block_body)
@@ -131,14 +133,21 @@ def autocorrect_body(corrector, node, block_body)
                                   "\n  #{' ' * block_start_col}")
         end
 
-        def block_arg_string(args)
-          args.children.map do |arg|
+        def block_arg_string(node, args)
+          arg_string = args.children.map do |arg|
             if arg.mlhs_type?
-              "(#{block_arg_string(arg)})"
+              "(#{block_arg_string(node, arg)})"
             else
               arg.source
             end
           end.join(', ')
+          arg_string += ',' if include_trailing_comma?(node.arguments)
+          arg_string
+        end
+
+        def include_trailing_comma?(args)
+          arg_count = args.each_descendant(:arg).to_a.size
+          arg_count == 1 && args.source.include?(',')
         end
       end
     end

spec/rubocop/cop/layout/multiline_block_layout_spec.rb

@@ -259,4 +259,39 @@ def f
       end
     RUBY
   end
+
+  it 'does not auto-correct a trailing comma when only one argument ' \
+     'is present' do
+    new_source = autocorrect_source(<<~RUBY)
+      def f
+        X.map do |
+          a,
+        |
+        end
+      end
+    RUBY
+    expect(new_source).to eq(<<~RUBY)
+      def f
+        X.map do |a,|
+        end
+      end
+    RUBY
+  end
+
+  it 'auto-corrects nested parens correctly' do
+    new_source = autocorrect_source(<<~RUBY)
+      def f
+        X.map do |
+          (((a), b), c)
+        |
+        end
+      end
+    RUBY
+    expect(new_source).to eq(<<~RUBY)
+      def f
+        X.map do |(((a), b), c)|
+        end
+      end
+    RUBY
+  end
 end