[Fix rubocop#9734] Add new Style/TopLevelMethodDefinition cop

Closes rubocop#9734

changelog/new_style_top_level_method_definition_cop.md

@@ -0,0 +1 @@
+* [#9734](https://github.com/rubocop/rubocop/pull/9734): Add `Style/TopLevelMethodDefinition` cop. ([@tejasbubane][])

config/default.yml

@@ -4651,6 +4651,11 @@ Style/TernaryParentheses:
     - require_parentheses_when_complex
   AllowSafeAssignment: true
 
+Style/TopLevelMethodDefinition:
+  Description: 'This cop looks for top-level method definitions.'
+  Enabled: false
+  VersionAdded: '<<next>>'
+
 Style/TrailingBodyOnClass:
   Description: 'Class body goes below class statement.'
   Enabled: true

lib/rubocop.rb

@@ -598,6 +598,7 @@
 require_relative 'rubocop/cop/style/symbol_literal'
 require_relative 'rubocop/cop/style/symbol_proc'
 require_relative 'rubocop/cop/style/ternary_parentheses'
+require_relative 'rubocop/cop/style/top_level_method_definition'
 require_relative 'rubocop/cop/style/trailing_body_on_class'
 require_relative 'rubocop/cop/style/trailing_body_on_method_definition'
 require_relative 'rubocop/cop/style/trailing_body_on_module'

lib/rubocop/cop/style/top_level_method_definition.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # Newcomers to ruby applications may write top-level methods,
+      # when ideally they should be organized in appropriate classes or modules.
+      # This cop looks for definitions of top-level methods and warns about them.
+      #
+      # However for ruby scripts it is perfectly fine to use top-level methods.
+      # Hence this cop is disabled by default.
+      #
+      # @example
+      #   # bad
+      #   def some_method
+      #   end
+      #
+      #   # bad
+      #   def self.some_method
+      #   end
+      #
+      #   # bad
+      #   define_method(:foo) { puts 1 }
+      #
+      #   # good
+      #   module Foo
+      #     def some_method
+      #     end
+      #   end
+      #
+      #   # good
+      #   class Foo
+      #     def self.some_method
+      #     end
+      #   end
+      #
+      #   # good
+      #   Struct.new do
+      #     def some_method
+      #     end
+      #   end
+      #
+      #   # good
+      #   class Foo
+      #     define_method(:foo) { puts 1 }
+      #   end
+      class TopLevelMethodDefinition < Base
+        MSG = 'Do not define methods at the top-level.'
+
+        RESTRICT_ON_SEND = %i[define_method].freeze
+
+        def on_def(node)
+          return unless node.root?
+
+          add_offense(node)
+        end
+        alias on_defs on_def
+        alias on_send on_def
+
+        def on_block(node)
+          return unless define_method_block?(node) && node.root?
+
+          add_offense(node)
+        end
+
+        private
+
+        # @!method define_method_block?(node)
+        def_node_matcher :define_method_block?, <<~PATTERN
+          (block (send _ {:define_method} _) ...)
+        PATTERN
+      end
+    end
+  end
+end

spec/rubocop/cop/style/top_level_method_definition_spec.rb

@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Style::TopLevelMethodDefinition, :config do
+  it 'registers an offense top-level methods' do
+    expect_offense(<<~RUBY)
+      def foo; end
+      ^^^^^^^^^^^^ Do not define methods at the top-level.
+    RUBY
+  end
+
+  it 'registers an offense top-level class methods' do
+    expect_offense(<<~RUBY)
+      def self.foo; end
+      ^^^^^^^^^^^^^^^^^ Do not define methods at the top-level.
+    RUBY
+  end
+
+  context 'top-level define_method' do
+    it 'registers offense with inline block' do
+      expect_offense(<<~RUBY)
+        define_method(:foo) { puts 1 }
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Do not define methods at the top-level.
+      RUBY
+    end
+
+    it 'registers offense for multi-line block' do
+      expect_offense(<<~RUBY)
+        define_method(:foo) do |x|
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^ Do not define methods at the top-level.
+          puts 1
+        end
+      RUBY
+    end
+
+    it 'registers offense for proc argument' do
+      expect_offense(<<~RUBY)
+        define_method(:foo, instance_method(:bar))
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Do not define methods at the top-level.
+      RUBY
+    end
+  end
+
+  it 'does not register an offense when using module' do
+    expect_no_offenses(<<~RUBY)
+      module Foo
+        def foo; end
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using class' do
+    expect_no_offenses(<<~RUBY)
+      class Foo
+        def self.foo; end
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using Struct' do
+    expect_no_offenses(<<~RUBY)
+      Foo = Struct.new do
+        def foo; end
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when defined within arbitrary block' do
+    expect_no_offenses(<<~RUBY)
+      Foo = types.each do |type|
+        def foo(type)
+          puts type
+        end
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when define_method is not top-level' do
+    expect_no_offenses(<<~RUBY)
+      class Foo
+        define_method(:a) { puts 1 }
+
+        define_method(:b) do |x|
+          puts x
+        end
+
+        define_method(:c, instance_method(:d))
+      end
+    RUBY
+  end
+end