[Fixes rubocop#113] Add basic compatibility check with main gem

lib/rubocop/ast.rb

@@ -63,6 +63,7 @@
 require_relative 'ast/node/yield_node'
 require_relative 'ast/builder'
 require_relative 'ast/processed_source'
+require_relative 'ast/rubocop_compatibility'
 require_relative 'ast/token'
 require_relative 'ast/traversal'
 require_relative 'ast/version'

lib/rubocop/ast/rubocop_compatibility.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+module RuboCop
+  # ...
+  module AST
+    # Responsible for compatibility with main gem
+    # @api private
+    module RuboCopCompatibility
+      INCOMPATIBLE_COPS = {
+        '0.89.0' => 'Layout/LineLength'
+      }.freeze
+      def rubocop_loaded
+        loaded = Gem::Version.new(RuboCop::Version::STRING)
+        incompatible = INCOMPATIBLE_COPS.select do |k, _v|
+          loaded < Gem::Version.new(k)
+        end.values
+        return if incompatible.empty?
+
+        warn <<~WARNING
+          *** WARNING – Incompatible version of `rubocop` and `rubocop-ast`
+          You may encounter issues with the following \
+          Cop#{'s' if incompatible.size > 1}: #{incompatible.join(', ')}
+          Please upgrade rubocop to at least v#{INCOMPATIBLE_COPS.keys.last}
+        WARNING
+      end
+    end
+
+    extend RuboCopCompatibility
+  end
+end

spec/rubocop/ast/rubocop_compatibility_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::AST::RuboCopCompatibility do
+  subject(:callback) { RuboCop::AST.rubocop_loaded }
+  before do
+    stub_const '::RuboCop::Version::STRING', rubocop_version
+  end
+
+  context 'when ran from an incompatible version of Rubocop' do
+    let(:rubocop_version) { '0.42.0' }
+
+    it 'issues a warning' do
+      expect { callback }.to output(/LineLength/).to_stderr
+    end
+  end
+
+  context 'when ran from a compatible version of Rubocop' do
+    let(:rubocop_version) { '0.89.0' }
+
+    it 'issues a warning' do
+      expect { callback }.not_to output.to_stderr
+    end
+  end
+end