Add options hash cop

CHANGELOG.md

@@ -14,6 +14,7 @@
 * [#2060](https://github.com/bbatsov/rubocop/issues/2060): New cop `Style/RescueEnsureAlignment` checks for bad alignment of `rescue` and `ensure` keywords. ([@lumeet][])
 * New cop `Style/OptionalArguments` checks for optional arguments that do not appear at the end of an argument list. ([@rrosenblum][])
 * New cop `Lint/CircularArgumentReference` checks for "circular argument references" in keyword arguments, which Ruby 2.2 warns against. ([@maxjacobson][], [@sliuu][])
+* [#2030](https://github.com/bbatsov/rubocop/issues/2030): New cop `Lint/OptionHash` checks for option hashes and encourages changing them to keyword arguments (disabled by default). ([@maxjacobson][])
 
 ### Changes
 

config/disabled.yml

@@ -44,6 +44,10 @@ Style/MissingElse:
     - case
     - both
 
+Style/OptionHash:
+  Description: "Don't use option hashes when you can use keyword arguments."
+  Enabled: false
+
 Style/SymbolArray:
   Description: 'Use %i or %I for arrays of symbols.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#percent-i'

lib/rubocop.rb

@@ -228,6 +228,7 @@
 require 'rubocop/cop/style/one_line_conditional'
 require 'rubocop/cop/style/op_method'
 require 'rubocop/cop/style/optional_arguments'
+require 'rubocop/cop/style/option_hash'
 require 'rubocop/cop/style/parallel_assignment'
 require 'rubocop/cop/style/parentheses_around_condition'
 require 'rubocop/cop/style/percent_literal_delimiters'

lib/rubocop/cop/style/option_hash.rb

@@ -0,0 +1,56 @@
+# encoding: utf-8
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks for options hashes and discourages them if the
+      # current Ruby version supports keyword arguments.
+      #
+      # @example
+      #   Instead of:
+      #
+      #   def fry(options = {})
+      #     temperature = options.fetch(:temperature, 300)
+      #     ...
+      #   end
+      #
+      #   Prefer:
+      #
+      #   def fry(temperature: 300)
+      #     ...
+      #   end
+      class OptionHash < Cop
+        MSG = 'Prefer keyword arguments to options hashes.'
+
+        def on_args(node)
+          return unless supports_keyword_arguments?
+
+          *_but_last, last_arg = *node
+
+          # asserting that there was an argument at all
+          return unless last_arg
+
+          # asserting last argument is an optional argument
+          return unless last_arg.optarg_type?
+
+          _, default_value = *last_arg
+
+          # asserting default value is a hash
+          return unless default_value.hash_type?
+
+          # asserting default value is empty hash
+          *key_value_pairs = *default_value
+          return unless key_value_pairs.empty?
+
+          add_offense(last_arg, :expression, MSG)
+        end
+
+        private
+
+        def supports_keyword_arguments?
+          RUBY_VERSION >= '2.0.0'
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/style/option_hash_spec.rb

@@ -0,0 +1,80 @@
+# encoding: utf-8
+
+require 'spec_helper'
+
+describe RuboCop::Cop::Style::OptionHash do
+  subject(:cop) { described_class.new }
+  before(:each) do
+    inspect_source(cop, source)
+  end
+
+  let(:source) do
+    [
+      'def some_method(options = {})',
+      '  puts some_arg',
+      'end'
+    ]
+  end
+
+  context 'ruby < 2.0, which has no keyword arguments', ruby_less_than: 2.0 do
+    it 'does not register an offense' do
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'ruby >= 2.0', ruby_greater_than_or_equal: 2.0 do
+    it 'registers an offense' do
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.messages.first)
+        .to eq('Prefer keyword arguments to options hashes.')
+      expect(cop.highlights).to eq ['options = {}']
+    end
+
+    context 'when the last argument is an options hash named something else' do
+      let(:source) do
+        [
+          'def steep(flavor, duration, config={})',
+          '  mug = config.fetch(:mug)',
+          '  prep(flavor, duration, mug)',
+          'end'
+        ]
+      end
+
+      it 'registers an offense' do
+        expect(cop.offenses.size).to eq(1)
+        expect(cop.messages.first)
+          .to eq('Prefer keyword arguments to options hashes.')
+        expect(cop.highlights).to eq ['config={}']
+      end
+    end
+
+    context 'when there are no arguments' do
+      let(:source) do
+        [
+          'def meditate',
+          '  puts true',
+          '  puts true',
+          'end'
+        ]
+      end
+
+      it 'does not register an offense' do
+        expect(cop.offenses).to be_empty
+      end
+    end
+
+    context 'when the last argument is a non-options-hash optional hash' do
+      let(:source) do
+        [
+          'def cook(instructions, ingredients = { hot: [], cold: [] })',
+          '  prep(ingredients)',
+          'end'
+        ]
+      end
+
+      it 'does not register an offense' do
+        expect(cop.offenses).to be_empty
+      end
+    end
+  end
+end