[Fix #8761] Read required_ruby_version from gemspec file if it exis…

…ts (#8885)

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### New features
+
+* [#8761](https://github.com/rubocop-hq/rubocop/issues/8761): Read `required_ruby_version` from gemspec file if it exists #8761. ([@HeroProtagonist][])
+
 ### Bug fixes
 
 * [#8499](https://github.com/rubocop-hq/rubocop/issues/8499): Fix `Style/IfUnlessModifier` and `Style/WhileUntilModifier` to prevent an offense if there are both first-line comment and code after `end` block. ([@dsavochkin][])
@@ -5084,3 +5088,4 @@
 [@miry]: https://github.com/miry
 [@lautis]: https://github.com/lautis
 [@pdobb]: https://github.com/pdobb
+[@HeroProtagonist]: https://github.com/HeroProtagonist

docs/modules/ROOT/pages/configuration.adoc

@@ -559,8 +559,11 @@ AllCops:
   TargetRubyVersion: 2.5
 ----
 
-Otherwise, RuboCop will then check your project for `.ruby-version` and
-use the version specified by it.
+Otherwise, RuboCop will then check your project for a series of files where
+the version may be specified already. The files that will be looked for are
+`.ruby-version`, `Gemfile.lock`, and `*.gemspec`. If Gemspec file has an
+array for `required_ruby_version`, the lowest version will be used.
+If none of the files are found a default version value will be used.
 
 == Automatically Generated Configuration
 

lib/rubocop/target_ruby.rb

@@ -112,6 +112,62 @@ def bundler_lock_file_path
       end
     end
 
+    # The target ruby version may be found in a .gemspec file.
+    # @api private
+    class GemspecFile < Source
+      extend NodePattern::Macros
+
+      GEMSPEC_EXTENSION = '.gemspec'
+
+      def_node_search :required_ruby_version, <<~PATTERN
+        (send _ :required_ruby_version= $_)
+      PATTERN
+
+      def name
+        "`required_ruby_version` parameter (in #{gemspec_filename})"
+      end
+
+      private
+
+      def find_version
+        file = gemspec_filepath
+        return unless file && File.file?(file)
+
+        version = version_from_gemspec_file(file)
+        return if version.nil?
+
+        if version.array_type?
+          versions = version.children.map { |v| version_from_str(v.str_content) }
+          return versions.compact.min
+        end
+
+        version_from_str(version.str_content)
+      end
+
+      def gemspec_filename
+        @gemspec_filename ||= begin
+          basename = Pathname.new(@config.base_dir_for_path_parameters).basename.to_s
+          "#{basename}#{GEMSPEC_EXTENSION}"
+        end
+      end
+
+      def gemspec_filepath
+        @gemspec_filepath ||=
+          @config.find_file_upwards(gemspec_filename, @config.base_dir_for_path_parameters)
+      end
+
+      def version_from_gemspec_file(file)
+        processed_source = ProcessedSource.from_file(file, DEFAULT_VERSION)
+        required_ruby_version(processed_source.ast).first
+      end
+
+      def version_from_str(str)
+        str.match(/^(?:>=|<=)?\s*(?<version>\d+(?:\.\d+)*)/) do |md|
+          md[:version].to_f
+        end
+      end
+    end
+
     # If all else fails, a default version will be picked.
     # @api private
     class Default < Source
@@ -130,7 +186,7 @@ def self.supported_versions
       KNOWN_RUBIES
     end
 
-    SOURCES = [RuboCopConfig, RubyVersionFile, BundlerLockFile, Default].freeze
+    SOURCES = [RuboCopConfig, RubyVersionFile, BundlerLockFile, GemspecFile, Default].freeze
     private_constant :SOURCES
 
     def initialize(config)

spec/rubocop/target_ruby_spec.rb

@@ -256,6 +256,122 @@
           expect(target_ruby.version).to eq default_version
         end
       end
+
+      context 'gemspec file' do
+        context 'when file contains `required_ruby_version` as a string' do
+          let(:base_path) { configuration.base_dir_for_path_parameters }
+          let(:gemspec_file_path) { File.join(base_path, 'example.gemspec') }
+
+          it 'sets target_ruby from required_ruby_version from exact version' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = '2.7.4'
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq 2.7
+          end
+
+          it 'sets target_ruby from required_ruby_version from inclusive range' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = '>= 3.0.0'
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq 3.0
+          end
+
+          it 'sets default target_ruby from exclusive range' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = '< 3.0.0'
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq default_version
+          end
+        end
+
+        context 'when file contains `required_ruby_version` as an array' do
+          let(:base_path) { configuration.base_dir_for_path_parameters }
+          let(:gemspec_file_path) { File.join(base_path, 'example.gemspec') }
+
+          it 'sets target_ruby from the lowest value' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = ['<=2.7.4', '>=2.6.5']
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq 2.6
+          end
+
+          it 'sets target_ruby from required_ruby_version with inclusive range' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = ['<=2.7.4', '>2.6.5']
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq 2.7
+          end
+
+          it 'sets default target_ruby with all exclusive ranges' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = ['<2.7.4', '>2.6.5']
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq default_version
+          end
+        end
+
+        context 'when file does not contain `required_ruby_version`' do
+          let(:base_path) { configuration.base_dir_for_path_parameters }
+          let(:gemspec_file_path) { File.join(base_path, 'example.gemspec') }
+
+          it 'sets default target_ruby' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.platform = Gem::Platform::RUBY
+                  s.licenses = ['MIT']
+                  s.summary = 'test tool.'
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq default_version
+          end
+        end
+      end
     end
 
     context 'when .ruby-version is in a parent directory' do