Add Performance/ReadlinesEach Cop

Detects inefficient use of reading each line in a file
just to discard them again.

.rubocop.yml

@@ -389,3 +389,6 @@ Style/FloatDivision:
 
 Cop/BanCatchThrow:
   Enabled: true
+
+Performance/ReadlinesEach:
+  Enabled: true

rubocop/cop/performance/readlines_each.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      class ReadlinesEach < RuboCop::Cop::Cop
+        MESSAGE = 'Avoid `IO.readlines.each`, since it reads contents into memory in full. ' \
+          'Use `IO.each_line` or `IO.each` instead.'
+
+        def_node_matcher :full_file_read_via_class?, <<~PATTERN
+          (send
+            (send (const nil? {:IO :File}) :readlines _) :each)
+        PATTERN
+
+        def_node_matcher :full_file_read_via_instance?, <<~PATTERN
+          (... (... :readlines) :each)
+        PATTERN
+
+        def on_send(node)
+          full_file_read_via_class?(node) { add_offense(node, location: :selector, message: MESSAGE) }
+          full_file_read_via_instance?(node) { add_offense(node, location: :selector, message: MESSAGE) }
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            corrector.replace(node.loc.expression, node.source.gsub('readlines.each', 'each_line'))
+          end
+        end
+      end
+    end
+  end
+end

spec/models/project_services/irker_service_spec.rb

@@ -64,7 +64,7 @@ describe IrkerService do
       irker.execute(sample_data)
 
       conn = @irker_server.accept
-      conn.readlines.each do |line|
+      conn.each_line do |line|
         msg = JSON.parse(line.chomp("\n"))
         expect(msg.keys).to match_array(%w(to privmsg))
         expect(msg['to']).to match_array(["irc://chat.freenode.net/#commits",

spec/rubocop/cop/performance/readlines_each_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require_relative '../../../support/helpers/expect_offense'
+require_relative '../../../../rubocop/cop/performance/readlines_each'
+
+describe RuboCop::Cop::Performance::ReadlinesEach do
+  include CopHelper
+  include ExpectOffense
+
+  subject(:cop) { described_class.new }
+
+  let(:message) { 'Avoid `IO.readlines.each`, since it reads contents into memory in full. Use `IO.each_line` or `IO.each` instead.' }
+
+  shared_examples_for(:class_read) do |klass|
+    context "and it is called as a class method on #{klass}" do
+      # We can't use `expect_offense` here because indentation changes based on `klass`
+      it 'flags it as an offense' do
+        inspect_source "#{klass}.readlines(file_path).each { |line| puts line }"
+
+        expect(cop.offenses.map(&:cop_name)).to contain_exactly('Performance/ReadlinesEach')
+      end
+    end
+
+    context 'when just using readlines without each' do
+      it 'does not flag it as an offense' do
+        expect_no_offenses "contents = #{klass}.readlines(file_path)"
+      end
+    end
+  end
+
+  context 'when reading all lines using IO.readlines.each' do
+    %w(IO File).each do |klass|
+      it_behaves_like(:class_read, klass)
+    end
+
+    context 'and it is called as an instance method on a return value' do
+      it 'flags it as an offense' do
+        expect_offense <<~SOURCE
+          get_file.readlines.each { |line| puts line }
+                             ^^^^ #{message}
+        SOURCE
+      end
+    end
+
+    context 'and it is called as an instance method on an assigned variable' do
+      it 'flags it as an offense' do
+        expect_offense <<~SOURCE
+          file = File.new(path)
+          file.readlines.each { |line| puts line }
+                         ^^^^ #{message}
+        SOURCE
+      end
+    end
+
+    context 'and it is called as an instance method on a new object' do
+      it 'flags it as an offense' do
+        expect_offense <<~SOURCE
+          File.new(path).readlines.each { |line| puts line }
+                                   ^^^^ #{message}
+        SOURCE
+      end
+    end
+
+    it 'autocorrects `readlines.each` to `each_line`' do
+      expect(autocorrect_source('obj.readlines.each { |line| line }')).to(
+        eq('obj.each_line { |line| line }')
+      )
+    end
+  end
+
+  context 'when just using readlines without each' do
+    it 'does not flag it as an offense' do
+      expect_no_offenses 'contents = my_file.readlines'
+    end
+  end
+end