[Fix #5261] Fix false positive for Style/MixinUsage when using insi…

…de module

Fixes #5261.

Correspondence to use case for `include` belonging to module was
insufficient. This PR added a reproduction test and fixed it.

CHANGELOG.md

@@ -10,6 +10,7 @@
 * [#5234](https://github.com/bbatsov/rubocop/issues/5234): Fix a false positive for `Rails/HasManyOrHasOneDependent` when using `class_name` option. ([@koic][])
 * [#5273](https://github.com/bbatsov/rubocop/issues/5273): Fix `Style/EvalWithLocation` reporting bad line offset. ([@pocke][])
 * [#5228](https://github.com/bbatsov/rubocop/issues/5228): Handle overridden `Metrics/LineLength:Max` for `--auto-gen-config`. ([@jonas054][])
+* [#5261](https://github.com/bbatsov/rubocop/issues/5261): Fix a false positive for `Style/MixinUsage` when using inside class or module. ([@koic][])
 
 ### Changes
 

lib/rubocop/cop/style/mixin_usage.rb

@@ -53,8 +53,9 @@ class MixinUsage < Cop
 
         def on_send(node)
           include_statement(node) do |statement|
-            return if node.argument?
-            return if accepted_include?(node)
+            return if node.argument? ||
+                      accepted_include?(node) ||
+                      belongs_to_class_or_module?(node)
 
             add_offense(node, message: format(MSG, statement: statement))
           end
@@ -65,6 +66,16 @@ def on_send(node)
         def accepted_include?(node)
           node.parent && node.macro?
         end
+
+        def belongs_to_class_or_module?(node)
+          if !node.parent
+            false
+          else
+            return true if node.parent.class_type? || node.parent.module_type?
+
+            belongs_to_class_or_module?(node.parent)
+          end
+        end
       end
     end
   end

spec/rubocop/cop/style/mixin_usage_spec.rb

@@ -29,6 +29,16 @@ class C
       RUBY
     end
 
+    it 'registers an offense when using `include` in method definition ' \
+       'outside class or module' do
+      expect_offense(<<-RUBY.strip_indent)
+        def foo
+          include M
+          ^^^^^^^^^ `include` is used at the top level. Use inside `class` or `module`.
+        end
+      RUBY
+    end
+
     it 'does not register an offense when using outside class' do
       expect_no_offenses(<<-RUBY.strip_indent)
         Foo.include M
@@ -58,6 +68,28 @@ class C
       RUBY
     end
 
+    it 'does not register an offense when using `include` in method ' \
+       'definition inside class' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        class X
+          def foo
+            include M
+          end
+        end
+      RUBY
+    end
+
+    it 'does not register an offense when using `include` in method ' \
+       'definition inside module' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        module X
+          def foo
+            include M
+          end
+        end
+      RUBY
+    end
+
     context 'Multiple definition classes in one' do
       it 'does not register an offense when using inside class' do
         expect_no_offenses(<<-RUBY.strip_indent)