Extract linting into separate CI job

Because we can simply set `TargetRubyVersion` to the version whose
syntax we want to support, we can run the Rubocop in a single job,
rather than repeating it across all jobs (and matrix combinations).

A side benefit is that this separates test failures from linting
failures, whereas previously tests would be skipped if the linter
failed.

.github/workflows/ci.yml

@@ -41,12 +41,24 @@ jobs:
         run: |
           sudo systemctl start mysql.service
           mysql -uroot -h localhost -proot -e "CREATE DATABASE job_iteration_test;"
-      - name: Rubocop
-        run: bundle exec rubocop
       - name: Ruby tests
         run: bundle exec rake test
         env:
           REDIS_HOST: localhost
           REDIS_PORT: ${{ job.services.redis.ports[6379] }}
+
+  lint:
+    runs-on: ubuntu-latest
+    name: Lint
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.2"
+          bundler-cache: true
+      - name: Rubocop
+        run: bundle exec rubocop
       - name: Documentation correctly written
         run: bundle exec yardoc --no-output --no-save --no-stats --fail-on-warning

test/rubocop_config_test.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "yaml"
+
+module JobIteration
+  class RubocopConfigTest < ActiveSupport::TestCase
+    test "TargetRubyVersion in .rubocop.yml matches oldest Ruby in CI matrix" do
+      assert_equal(
+        oldest_ruby_in_matrix,
+        target_ruby_version,
+        "TargetRubyVersion in .rubocop.yml does not match oldest Ruby in CI matrix",
+      )
+    end
+
+    test "Linting runs on the newest Ruby in CI matrix" do
+      assert_equal(
+        newest_ruby_in_matrix,
+        ruby_version_for_linting,
+        "Linting does not run on the newest Ruby in CI matrix",
+      )
+    end
+
+    test "TargetRubyVersion in .rubocop.yml matches required Ruby version in gemspec" do
+      assert_equal(
+        required_ruby_version,
+        target_ruby_version,
+        "TargetRubyVersion in .rubocop.yml does not match required Ruby version in gemspec",
+      )
+    end
+
+    private
+
+    def oldest_ruby_in_matrix
+      ruby_versions_in_matrix.min
+    end
+
+    def newest_ruby_in_matrix
+      ruby_versions_in_matrix.max
+    end
+
+    def ruby_versions_in_matrix
+      YAML
+        .load_file(".github/workflows/ci.yml")
+        .dig("jobs", "build", "strategy", "matrix", "ruby")
+        .tap { |ruby_versions| refute_nil(ruby_versions, "Ruby versions not found in CI matrix") }
+        .map { |ruby_version| Gem::Version.new(ruby_version) }
+    end
+
+    def ruby_version_for_linting
+      YAML
+        .load_file(".github/workflows/ci.yml")
+        .dig("jobs", "lint", "steps")
+        .tap { |steps| refute_nil(steps, "Steps not found in linting CI config") }
+        .find { |step| step.fetch("uses", "") =~ %r{^ruby/setup-ruby} }
+        .tap { |step| refute_nil(step, "Ruby setup step not found in linting CI config") }
+        .then { |step| step.dig("with", "ruby-version") }
+        .tap { |ruby_version| refute_nil(ruby_version, "Ruby version not found in linting CI config") }
+        .then { |ruby_version| Gem::Version.new(ruby_version) }
+    end
+
+    def target_ruby_version
+      YAML
+        .load_file(".rubocop.yml")
+        .dig("AllCops", "TargetRubyVersion")
+        .tap { |ruby_version| refute_nil(ruby_version, "TargetRubyVersion not found in .rubocop.yml") }
+        .then { |ruby_version| Gem::Version.new(ruby_version) }
+    end
+
+    def required_ruby_version
+      Gem
+        .loaded_specs
+        .fetch("job-iteration")
+        .required_ruby_version
+        .to_s[/(?<=>= )\d+\.\d+/]
+        .then { |ruby_version| Gem::Version.new(ruby_version) }
+    end
+  end
+end