New cop UnneededPercentX

Part of rubocop#835.

CHANGELOG.md

@@ -8,6 +8,7 @@
 * Add new cop `EachWithObject` to prefer `each_with_object` over `inject` or `reduce`. ([@geniou][])
 * [#1010](https://github.com/bbatsov/rubocop/issues/1010): New Cop `Next` check for conditions at the end of an interation and propose to use `next` instead. ([@geniou][])
 * The `GuardClause` cop now also looks for unless and it is configurable how many lines the body of an if / unless needs to have to not be ignored. ([@geniou][])
+* [#835](https://github.com/bbatsov/rubocop/issues/835): New cop `UnneededPercentX` checks for `%x` when backquotes would do. ([@jonas054][])
 
 ### Changes
 

config/enabled.yml

@@ -541,6 +541,10 @@ UnneededCapitalW:
   Description: 'Checks for %W when interpolation is not needed.'
   Enabled: true
 
+UnneededPercentX:
+  Description: 'Checks for %x when `` would do.'
+  Enabled: true
+
 VariableInterpolation:
   Description: >-
                  Don't interpolate global, instance and class variables

lib/rubocop.rb

@@ -219,6 +219,7 @@
 require 'rubocop/cop/style/trivial_accessors'
 require 'rubocop/cop/style/unless_else'
 require 'rubocop/cop/style/unneeded_capital_w'
+require 'rubocop/cop/style/unneeded_percent_x'
 require 'rubocop/cop/style/variable_interpolation'
 require 'rubocop/cop/style/variable_name'
 require 'rubocop/cop/style/when_then'

lib/rubocop/cop/style/unneeded_percent_x.rb

@@ -0,0 +1,26 @@
+# encoding: utf-8
+
+module Rubocop
+  module Cop
+    module Style
+      # This cop checks for usage of the %x() syntax when `` would do.
+      class UnneededPercentX < Cop
+        MSG = 'Do not use `%x` unless the command string contains backquotes.'
+
+        def on_xstr(node)
+          add_offense(node, :expression) if node.loc.expression.source !~ /`/
+        end
+
+        private
+
+        def autocorrect(node)
+          string, = *node
+          @corrections << lambda do |corrector|
+            corrector.replace(node.loc.expression,
+                              "`#{string.loc.expression.source}`")
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/style/unneeded_percent_x_spec.rb

@@ -0,0 +1,34 @@
+# encoding: utf-8
+
+require 'spec_helper'
+
+describe Rubocop::Cop::Style::UnneededPercentX do
+  subject(:cop) { described_class.new }
+
+  it 'registers an offense for a %x string without backquotes' do
+    inspect_source(cop, '%x(ls)')
+    expect(cop.messages)
+      .to eq(['Do not use `%x` unless the command string contains ' \
+              'backquotes.'])
+  end
+
+  it 'accepts a %x string with backquotes' do
+    inspect_source(cop, '%x(echo `ls`)')
+    expect(cop.offenses).to be_empty
+  end
+
+  it 'accepts a `` string without inner backquotes' do
+    inspect_source(cop, '`ls`')
+    expect(cop.offenses).to be_empty
+  end
+
+  it 'accepts a `` string with inner backquotes' do
+    inspect_source(cop, '`echo \`ls\``')
+    expect(cop.offenses).to be_empty
+  end
+
+  it 'auto-corrects' do
+    new_source = autocorrect_source(cop, '%x(ls)')
+    expect(new_source).to eq('`ls`')
+  end
+end