Add option OnlyFor to the Bundler/GemComment cop.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### New features
 
+* [#7978](https://github.com/rubocop-hq/rubocop/pull/7978): Add new option `OnlyIfVersionRestricted` to the `Bumdler/GemComment` cop. ([@ric2b][])
 * [#7735](https://github.com/rubocop-hq/rubocop/issues/7735): `NodePattern` and `AST` classes have been moved to the [`rubocop-ast` gem](https://github.com/rubocop-hq/rubocop-ast). ([@marcandre][])
 * [#7950](https://github.com/rubocop-hq/rubocop/pull/7950): Add new `Lint/DeprecatedOpenSSLConstant` cop. ([@bdewater][])
 
@@ -4524,3 +4525,4 @@
 [@jeffcarbs]: https://github.com/jeffcarbs
 [@laurmurclar]: https://github.com/laurmurclar
 [@jethrodaniel]: https://github.com/jethrodaniel
+[@ric2b]: https://github.com/ric2b

lib/rubocop/cop/bundler/gem_comment.rb

@@ -5,7 +5,7 @@ module Cop
     module Bundler
       # Add a comment describing each gem in your Gemfile.
       #
-      # @example
+      # @example OnlyIfVersionRestricted: false (default)
       #   # bad
       #
       #   gem 'foo'
@@ -15,6 +15,16 @@ module Bundler
       #   # Helpers for the foo things.
       #   gem 'foo'
       #
+      # @example OnlyIfVersionRestricted: true
+      #   # bad
+      #
+      #   gem 'foo', '>= 2.1'
+      #
+      #   # good
+      #
+      #   # Version 2.1 introduces breaking change bar
+      #   gem 'foo', '< 2.1'
+      #
       class GemComment < Cop
         include DefNode
 
@@ -26,6 +36,7 @@ def on_send(node)
           return unless gem_declaration?(node)
           return if ignored_gem?(node)
           return if commented?(node)
+          return if cop_config['OnlyIfVersionRestricted'] && !version_restricted_gem?(node)
 
           add_offense(node)
         end
@@ -58,6 +69,13 @@ def ignored_gem?(node)
           ignored_gems = Array(cop_config['IgnoredGems'])
           ignored_gems.include?(node.first_argument.value)
         end
+
+        # Besides the gem name, all other *positional* arguments to `gem` are version restrictions,
+        # as long as it has one we know there's a version restriction.
+        def version_restricted_gem?(send_node)
+          # arguments[0] is the gem name
+          send_node.arguments[1]&.str_type? == true
+        end
       end
     end
   end

manual/cops_bundler.md

@@ -49,6 +49,8 @@ Add a comment describing each gem in your Gemfile.
 
 ### Examples
 
+#### OnlyIfVersionRestricted: false (default)
+
 ```ruby
 # bad
 
@@ -59,6 +61,18 @@ gem 'foo'
 # Helpers for the foo things.
 gem 'foo'
 ```
+#### OnlyIfVersionRestricted: true
+
+```ruby
+# bad
+
+gem 'foo', '>= 2.1'
+
+# good
+
+# Version 2.1 introduces breaking change bar
+gem 'foo', '< 2.1'
+```
 
 ### Configurable attributes
 

spec/rubocop/cop/bundler/gem_comment_spec.rb

@@ -63,5 +63,61 @@
         GEM
       end
     end
+
+    context 'and the OnlyIfVersionRestricted option is set to true' do
+      before { cop_config['OnlyIfVersionRestricted'] = true }
+
+      context 'and a gem is commented' do
+        it 'does not register an offense' do
+          expect_no_offenses(<<~RUBY, 'Gemfile')
+            # Style-guide enforcer.
+            gem 'rubocop'
+          RUBY
+        end
+      end
+
+      context 'and a gem is uncommented but not version restricted' do
+        it 'does not register an offense in a simple example' do
+          expect_no_offenses(<<-GEM, 'Gemfile')
+            gem 'rubocop'
+          GEM
+        end
+
+        it 'does not register an offense in a more complex example' do
+          expect_no_offenses(<<-GEM, 'Gemfile')
+            gem 'rubocop', group: development
+          GEM
+        end
+      end
+
+      context 'and a gem is uncommented and version restricted' do
+        context 'and has no other paramaters' do
+          it 'registers an offense' do
+            expect_offense(<<-GEM, 'Gemfile')
+              gem 'rubocop', '~> 12.0'
+              ^^^^^^^^^^^^^^^^^^^^^^^^ Missing gem description comment.
+            GEM
+          end
+        end
+
+        context 'and has multiple version restrictions' do
+          it 'registers an offense' do
+            expect_offense(<<-GEM, 'Gemfile')
+              gem 'rubocop', '~> 12.0', '>= 11.0'
+              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Missing gem description comment.
+            GEM
+          end
+        end
+
+        context 'and has extra unrelated keyword arguments' do
+          it 'registers an offense' do
+            expect_offense(<<-GEM, 'Gemfile')
+            gem 'rubocop', '~> 12.0', required: true
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Missing gem description comment.
+            GEM
+          end
+        end
+      end
+    end
   end
 end