RubyVersionFile cop (read version from .ruby-version)

config/default.yml

@@ -8,3 +8,8 @@ EightyFourCodes/CommandLiteralInjection:
   Description: "Check for Command Injection in `` and %x"
   Enabled: true
   VersionAdded: "0.0.1"
+
+EightyFourCodes/RubyVersionFile:
+  Description: "Ensure .ruby-version file use in Gemfile"
+  Enabled: true
+  VersionAdded: "0.0.1"

lib/rubocop/cop/eighty_four_codes/ruby_version_file.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module RuboCop
+    module Cop
+      module EightyFourCodes
+        # Read Ruby version from a .ruby-version file
+        #
+        # Instead of staticly defining the Ruby runtime version in Gemfile, load it from
+        # a .ruby-version file definition. As this Ruby version file is read by rbenv, chruby etc
+        # it's much easier for the developer to work with multiple projects with different versions.
+        #
+        # @example
+        #   # bad
+        #   ruby 2.6.6
+        #
+        #   # good
+        #   ruby File.read('.ruby-version')
+        class RubyVersionFile < Base
+          extend AutoCorrector
+
+          MSG = "Control Ruby version via .ruby-version, fix by replacing with File.read('.ruby-version')"
+
+          RESTRICT_ON_SEND = %i[ruby].freeze
+
+          def_node_matcher :static_version_found?, <<~PATTERN
+            (send nil? :ruby
+              $(str _))
+          PATTERN
+
+          def on_send(node)
+            static_version_found?(node) do |source_node, source|
+              message = format(MSG, source: source)
+
+              add_offense(
+                source_node,
+                message: message
+              ) do |corrector|
+                corrector.replace(
+                  source_node, "File.read('.ruby-version')"
+                )
+              end
+            end
+          end
+        end
+      end
+    end
+  end

spec/rubocop/cop/eighty_four_codes/ruby_version_file_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::EightyFourCodes::RubyVersionFile do
+  subject(:cop) { described_class.new(config) }
+
+  let(:config) { RuboCop::Config.new }
+
+  it 'registers an offense defined with double quotes' do
+    expect_offense(<<~'RUBY')
+      ruby "2.6.6"
+           ^^^^^^^ Control Ruby version via .ruby-version, fix by replacing with File.read('.ruby-version')
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ruby File.read('.ruby-version')
+    RUBY
+  end
+
+  it 'registers an offense defined with single quotes' do
+    expect_offense(<<~'RUBY')
+      ruby '2.1.0'
+           ^^^^^^^ Control Ruby version via .ruby-version, fix by replacing with File.read('.ruby-version')
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ruby File.read('.ruby-version')
+    RUBY
+  end
+
+  it 'does not register an offense when reading .ruby-version' do
+    expect_no_offenses(<<~RUBY)
+      ruby File.read('.ruby-version')
+    RUBY
+  end
+end