Use requires_gem API in TargetRailsVersion

changelog/change_migrate_to_requires_gem_api.md

@@ -0,0 +1 @@
+* [#1137](https://github.com/rubocop/rubocop-rails/pull/1137): Migrate to `TargetRailsVersion` the new [`requires_gem` API](https://github.com/rubocop/rubocop/pull/12186). ([@amomchilov][])

lib/rubocop/cop/mixin/target_rails_version.rb

@@ -4,13 +4,40 @@ module RuboCop
   module Cop
     # Common functionality for checking target rails version.
     module TargetRailsVersion
+      # Informs the base RuboCop gem that it the Rails version is checked via `requires_gem` API,
+      # without needing to call this `#support_target_rails_version` method.
+      USES_REQUIRES_GEM_API = true
+
       def minimum_target_rails_version(version)
-        @minimum_target_rails_version = version
+        if respond_to?(:requires_gem)
+          case version
+          when Integer, Float then requires_gem(TARGET_GEM_NAME, ">= #{version}")
+          when String then requires_gem(TARGET_GEM_NAME, version)
+          end
+        else
+          # Fallback path for previous versions of RuboCop which don't support the `requires_gem` API yet.
+          @minimum_target_rails_version = version
+        end
       end
 
       def support_target_rails_version?(version)
-        @minimum_target_rails_version <= version
+        if respond_to?(:requires_gem)
+          return false unless gem_requirements
+
+          gem_requirement = gem_requirements[TARGET_GEM_NAME]
+          return true unless gem_requirement # If we have no requirement, then we support all versions
+
+          gem_requirement.satisfied_by?(Gem::Version.new(version))
+        else
+          # Fallback path for previous versions of RuboCop which don't support the `requires_gem` API yet.
+          @minimum_target_rails_version <= version
+        end
       end
+
+      # Look for `railties` instead of `rails`, to support apps that only use a subset of `rails`
+      # See https://github.com/rubocop/rubocop/pull/11289
+      TARGET_GEM_NAME = 'railties'
+      private_constant :TARGET_GEM_NAME
     end
   end
 end