[Fix rubocop#431] prefer << with push

lib/rubocop/cop/performance/array_push_single.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      # Identifies places where pushing a single element to an array
+      # can be replaced by `Array#<<`.
+      #
+      # @safety
+      #   This cop is unsafe because not all objects that respond to `#push` also respond to `#<<`
+      #
+      # @example
+      #   # bad
+      #   array.push(1)
+      #
+      #   # good
+      #   array << 1
+      #
+      #   # good
+      #   array.push(1, 2, 3) # `<<` only works for one element
+      #
+      class ArrayPushSingle < Base
+        extend AutoCorrector
+
+        MSG = 'Use `<<` instead of `%<current>s`.'
+
+        PUSH_METHODS = Set[:push, :append].freeze
+        RESTRICT_ON_SEND = PUSH_METHODS
+
+        def_node_matcher :push_call?, <<~PATTERN
+          (call $_receiver $%PUSH_METHODS $!(splat _))
+        PATTERN
+
+        def on_send(node)
+          raise
+          push_call?(node) do |receiver, method_name, element|
+            message = format(MSG, current: method_name)
+
+            add_offense(node, message: message) do |corrector|
+              corrector.replace(node, "#{receiver.source} << #{element.source}")
+            end
+          end
+        end
+
+        def on_csend(node)
+          push_call?(node) do |receiver, method_name, element|
+            message = format(MSG, current: method_name)
+
+            add_offense(node, message: message) do |corrector|
+              corrector.replace(node, "#{receiver.source}&.<< #{element.source}")
+            end
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/performance_cops.rb

@@ -4,6 +4,7 @@
 require_relative 'mixin/sort_block'
 
 require_relative 'performance/ancestors_include'
+require_relative 'performance/array_push_single'
 require_relative 'performance/array_semi_infinite_range_slice'
 require_relative 'performance/big_decimal_with_numeric_argument'
 require_relative 'performance/bind_call'

spec/rubocop/cop/performance/array_push_single_spec.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Performance::ArrayPushSingle, :config do
+  it 'registers an offense and corrects when using `push` with a single element' do
+    expect_offense(<<~RUBY)
+      array.push(element)
+      ^^^^^^^^^^^^^^^^^^^ Use `<<` instead of `push`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      array << element
+    RUBY
+  end
+
+  it 'registers an offense and corrects when using `push` with a single element and safe navigation operator' do
+    expect_offense(<<~RUBY)
+      array&.push(element)
+      ^^^^^^^^^^^^^^^^^^^^ Use `<<` instead of `push`.
+    RUBY
+
+    # gross. TODO: make a configuration option?
+    expect_correction(<<~RUBY)
+      array&.<< element
+    RUBY
+  end
+
+  it 'does not register an offense when using `push` with multiple elements' do
+    expect_no_offenses(<<~RUBY)
+      array.push(1, 2, 3)
+    RUBY
+  end
+
+  it 'does not register an offense when using `push` with splatted elements' do
+    expect_no_offenses(<<~RUBY)
+      array.push(*elements)
+    RUBY
+  end
+
+  describe "replacing `push` with `<<`" do
+    subject(:array) { [1, 2, 3] }
+
+    it "returns the same result" do
+      expect([1, 2, 3] << 4).to eq([1, 2, 3].push(4))
+    end
+
+    it "it has the same side-effect" do
+      a = [1, 2, 3]
+      a << 4
+
+      b = [1, 2, 3]
+      b.push(4)
+
+      expect(a).to eq(b)
+    end
+  end
+end