Add performance rubocop and fixes

.rubocop.yml

@@ -1,5 +1,10 @@
 inherit_from:
   - .rubocop.shopify.yml
 
+require: rubocop-performance
+
+Performance:
+  Enabled: true
+
 Metrics/LineLength:
   Enabled: false

.travis.yml

@@ -21,10 +21,6 @@ matrix:
     - rvm: ruby-head
     - rvm: jruby-head
 
-before_install:
-  - gem update --system
-  - gem install bundler
-
 install:
   - bundle install
 

Gemfile

@@ -18,6 +18,7 @@ end
 
 group :test do
   gem 'rubocop', '~> 0.74.0', require: false
+  gem 'rubocop-performance', require: false
 
   platform :mri, :truffleruby do
     gem 'liquid-c', github: 'Shopify/liquid-c', ref: '7ba926791ef8411984d0f3e41c6353fd716041c6'

lib/liquid/block_body.rb

@@ -65,7 +65,7 @@ module Liquid
     end
 
     def render(context)
-      render_to_output_buffer(context, ''.dup)
+      render_to_output_buffer(context, +'')
     end
 
     def render_to_output_buffer(context, output)
@@ -81,7 +81,7 @@ module Liquid
         when Variable
           render_node(context, output, node)
         when Block
-          render_node(context, node.blank? ? ''.dup : output, node)
+          render_node(context, node.blank? ? +'' : output, node)
           break if context.interrupt? # might have happened in a for-block
         when Continue, Break
           # If we get an Interrupt that means the block must stop processing. An

lib/liquid/errors.rb

@@ -7,7 +7,7 @@ module Liquid
     attr_accessor :markup_context
 
     def to_s(with_prefix = true)
-      str = ''.dup
+      str = +''
       str << message_prefix if with_prefix
       str << super()
 
@@ -22,7 +22,7 @@ module Liquid
     private
 
     def message_prefix
-      str = ''.dup
+      str = +''
       str << if is_a?(SyntaxError)
         'Liquid syntax error'
       else

lib/liquid/parser.rb

@@ -70,7 +70,7 @@ module Liquid
     end
 
     def argument
-      str = ''.dup
+      str = +''
       # might be a keyword argument (identifier: expression)
       str << consume << consume << ' ' if look(:id) && look(:colon, 1)
 

lib/liquid/tags/ifchanged.rb

@@ -4,7 +4,7 @@ module Liquid
   class Ifchanged < Block
     def render_to_output_buffer(context, output)
       context.stack do
-        block_output = ''.dup
+        block_output = +''
         super(context, block_output)
 
         if block_output != context.registers[:ifchanged]

lib/liquid/tags/raw.rb

@@ -12,7 +12,7 @@ module Liquid
     end
 
     def parse(tokens)
-      @body = ''.dup
+      @body = +''
       while (token = tokens.shift)
         if token =~ FULL_TOKEN_POSSIBLY_INVALID
           @body << Regexp.last_match(1) if Regexp.last_match(1) != ''

lib/liquid/template.rb

@@ -204,7 +204,7 @@ module Liquid
         # render the nodelist.
         # for performance reasons we get an array back here. join will make a string out of it.
         with_profiling(context) do
-          @root.render_to_output_buffer(context, output || ''.dup)
+          @root.render_to_output_buffer(context, output || +'')
         end
       rescue Liquid::MemoryError => e
         context.handle_error(e)

test/integration/drop_test.rb

@@ -33,7 +33,7 @@ class ProductDrop < Liquid::Drop
 
   class CatchallDrop < Liquid::Drop
     def liquid_method_missing(method)
-      'catchall_method: '.dup << method.to_s
+      +'catchall_method: ' << method.to_s
     end
   end
 
@@ -50,7 +50,7 @@ class ProductDrop < Liquid::Drop
   end
 
   def user_input
-    'foo'.dup.taint
+    (+'foo').taint
   end
 
   protected

test/unit/block_unit_test.rb

@@ -63,7 +63,7 @@ class BlockUnitTest < Minitest::Test
 
       assert_equal 'hello', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'hello', output
       assert_equal 'hello', buf
@@ -81,7 +81,7 @@ class BlockUnitTest < Minitest::Test
 
       assert_equal 'foohellobar', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'foohellobar', output
       assert_equal 'foohellobar', buf

test/unit/tag_unit_test.rb

@@ -33,7 +33,7 @@ class TagUnitTest < Minitest::Test
 
       assert_equal 'hello', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'hello', output
       assert_equal 'hello', buf
@@ -51,7 +51,7 @@ class TagUnitTest < Minitest::Test
 
       assert_equal 'foohellobar', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'foohellobar', output
       assert_equal 'foohellobar', buf