Enhance Gemspec/RequiredRubyVersion cop with check that required_ruby_version is specified

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### New features
 
 * [#8322](https://github.com/rubocop-hq/rubocop/pull/8322): Support autocorrect for `Style/CaseEquality` cop. ([@fatkodima][])
+* [#7876](https://github.com/rubocop-hq/rubocop/issues/7876): Enhance `Gemspec/RequiredRubyVersion` cop with check that `required_ruby_version` is specified. ([@fatkodima][])
 * [#8291](https://github.com/rubocop-hq/rubocop/pull/8291): Add new `Lint/SelfAssignment` cop. ([@fatkodima][])
 * [#8389](https://github.com/rubocop-hq/rubocop/pull/8389): Add new `Lint/DuplicateRescueException` cop. ([@fatkodima][])
 * [#8433](https://github.com/rubocop-hq/rubocop/pull/8433): Add new `Lint/BinaryOperatorWithIdenticalOperands` cop. ([@fatkodima][])

config/default.yml

@@ -210,9 +210,10 @@ Gemspec/OrderedDependencies:
     - '**/*.gemspec'
 
 Gemspec/RequiredRubyVersion:
-  Description: 'Checks that `required_ruby_version` of gemspec and `TargetRubyVersion` of .rubocop.yml are equal.'
+  Description: 'Checks that `required_ruby_version` of gemspec is specified and equal to `TargetRubyVersion` of .rubocop.yml.'
   Enabled: true
   VersionAdded: '0.52'
+  VersionChanged: '0.89'
   Include:
     - '**/*.gemspec'
 

docs/modules/ROOT/pages/cops_gemspec.adoc

@@ -149,11 +149,11 @@ spec.add_dependency 'rspec'
 | Yes
 | No
 | 0.52
-| -
+| 0.89
 |===
 
-Checks that `required_ruby_version` of gemspec and `TargetRubyVersion`
-of .rubocop.yml are equal.
+Checks that `required_ruby_version` of gemspec is specified and
+equal to `TargetRubyVersion` of .rubocop.yml.
 Thereby, RuboCop to perform static analysis working on the version
 required by gemspec.
 
@@ -163,6 +163,11 @@ required by gemspec.
 ----
 # When `TargetRubyVersion` of .rubocop.yml is `2.5`.
 
+# bad
+Gem::Specification.new do |spec|
+  # no `required_ruby_version` specified
+end
+
 # bad
 Gem::Specification.new do |spec|
   spec.required_ruby_version = '>= 2.4.0'

lib/rubocop/cop/gemspec/required_ruby_version.rb

@@ -3,8 +3,8 @@
 module RuboCop
   module Cop
     module Gemspec
-      # Checks that `required_ruby_version` of gemspec and `TargetRubyVersion`
-      # of .rubocop.yml are equal.
+      # Checks that `required_ruby_version` of gemspec is specified and
+      # equal to `TargetRubyVersion` of .rubocop.yml.
       # Thereby, RuboCop to perform static analysis working on the version
       # required by gemspec.
       #
@@ -13,6 +13,11 @@ module Gemspec
       #
       #   # bad
       #   Gem::Specification.new do |spec|
+      #     # no `required_ruby_version` specified
+      #   end
+      #
+      #   # bad
+      #   Gem::Specification.new do |spec|
       #     spec.required_ruby_version = '>= 2.4.0'
       #   end
       #
@@ -41,28 +46,44 @@ module Gemspec
       #     spec.required_ruby_version = '~> 2.5'
       #   end
       class RequiredRubyVersion < Cop
-        MSG = '`required_ruby_version` (%<required_ruby_version>s, ' \
-              'declared in %<gemspec_filename>s) and `TargetRubyVersion` ' \
-              '(%<target_ruby_version>s, which may be specified in ' \
-              '.rubocop.yml) should be equal.'
+        include RangeHelp
+
+        NOT_EQUAL_MSG = '`required_ruby_version` (%<required_ruby_version>s, ' \
+                        'declared in %<gemspec_filename>s) and `TargetRubyVersion` ' \
+                        '(%<target_ruby_version>s, which may be specified in ' \
+                        '.rubocop.yml) should be equal.'
+        MISSING_MSG = '`required_ruby_version` should be specified.'
 
         def_node_search :required_ruby_version, <<~PATTERN
-          (send _ :required_ruby_version= ${(str _) (array (str _))})
+          (send _ :required_ruby_version= $_)
         PATTERN
 
+        def_node_matcher :string_version?, <<~PATTERN
+          {(str _) (array (str _))}
+        PATTERN
+
+        # rubocop:disable Metrics/AbcSize
         def investigate(processed_source)
-          required_ruby_version(processed_source.ast) do |version|
+          version = required_ruby_version(processed_source.ast).first
+
+          if version
+            return unless string_version?(version)
+
             ruby_version = extract_ruby_version(version)
 
             return if ruby_version == target_ruby_version.to_s
 
             add_offense(
               processed_source.ast,
               location: version.loc.expression,
-              message: message(ruby_version, target_ruby_version)
+              message: not_equal_message(ruby_version, target_ruby_version)
             )
+          else
+            range = source_range(processed_source.buffer, 1, 0)
+            add_offense(nil, location: range, message: MISSING_MSG)
           end
         end
+        # rubocop:enable Metrics/AbcSize
 
         private
 
@@ -76,9 +97,9 @@ def extract_ruby_version(required_ruby_version)
           required_ruby_version.str_content.scan(/\d/).first(2).join('.')
         end
 
-        def message(required_ruby_version, target_ruby_version)
+        def not_equal_message(required_ruby_version, target_ruby_version)
           format(
-            MSG,
+            NOT_EQUAL_MSG,
             required_ruby_version: required_ruby_version,
             gemspec_filename: File.basename(processed_source.file_path),
             target_ruby_version: target_ruby_version

spec/rubocop/cop/gemspec/required_ruby_version_spec.rb

@@ -128,4 +128,12 @@
       RUBY
     end
   end
+
+  it 'registers an offense when `required_ruby_version` is not specified' do
+    expect_offense(<<~RUBY, '/path/to/foo.gemspec')
+      Gem::Specification.new do |spec|
+      ^ `required_ruby_version` should be specified.
+      end
+    RUBY
+  end
 end