Add new Performance/StringBytesize cop

config/default.yml

@@ -326,6 +326,12 @@ Performance/StartWith:
   VersionAdded: '0.36'
   VersionChanged: '1.10'
 
+Performance/StringBytesize:
+  Description: "Use `String#bytesize` instead of calculating the size of the bytes array."
+  SafeAutoCorrect: false
+  Enabled: 'pending'
+  VersionAdded: '<<next>>'
+
 Performance/StringIdentifierArgument:
   Description: 'Use symbol identifier argument instead of string identifier argument.'
   Enabled: pending

lib/rubocop/cop/performance/string_bytesize.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      # Checks for calls to `#bytes` counting method and suggests using `bytesize` instead.
+      # The `bytesize` method is more efficient and directly returns the size in bytes,
+      # avoiding the intermediate array allocation that `bytes.size` incurs.
+      #
+      # @safety
+      #   This cop is unsafe because it assumes that the receiver
+      #   responds to `#bytesize` method.
+      #
+      # @example
+      #   # bad
+      #   string_var.bytes.count
+      #   "foobar".bytes.size
+      #
+      #   # good
+      #   string_var.bytesize
+      #   "foobar".bytesize
+      class StringBytesize < Base
+        extend AutoCorrector
+
+        MSG = 'Use `String#bytesize` instead of calculating the size of the bytes array.'
+        RESTRICT_ON_SEND = %i[size length count].freeze
+
+        def_node_matcher :string_bytes_method?, <<~MATCHER
+          (send (send !nil? :bytes) {:size :length :count})
+        MATCHER
+
+        def on_send(node)
+          string_bytes_method?(node) do
+            add_offense(node) do |corrector|
+              corrector.replace(node, replacement(node))
+            end
+          end
+        end
+
+        def replacement(node)
+          receiver = node.receiver.receiver.source
+
+          "#{receiver}.bytesize"
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/performance_cops.rb

@@ -44,6 +44,7 @@
 require_relative 'performance/size'
 require_relative 'performance/sort_reverse'
 require_relative 'performance/squeeze'
+require_relative 'performance/string_bytesize'
 require_relative 'performance/start_with'
 require_relative 'performance/string_identifier_argument'
 require_relative 'performance/string_include'

spec/rubocop/cop/performance/string_bytesize_spec.rb

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Performance::StringBytesize, :config do
+  let(:msg) { 'Use `String#bytesize` instead of calculating the size of the bytes array.' }
+
+  it 'registers an offense with `size` method' do
+    expect_offense(<<~RUBY)
+      string.bytes.size
+      ^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+
+    expect_correction(<<~RUBY)
+      string.bytesize
+    RUBY
+  end
+
+  it 'registers an offense with `length` method' do
+    expect_offense(<<~RUBY)
+      string.bytes.length
+      ^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+
+    expect_correction(<<~RUBY)
+      string.bytesize
+    RUBY
+  end
+
+  it 'registers an offense with `count` method' do
+    expect_offense(<<~RUBY)
+      string.bytes.count
+      ^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+
+    expect_correction(<<~RUBY)
+      string.bytesize
+    RUBY
+  end
+
+  it 'registers an offense with string literal' do
+    expect_offense(<<~RUBY)
+      "foobar".bytes.count
+      ^^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+
+    expect_correction(<<~RUBY)
+      "foobar".bytesize
+    RUBY
+  end
+
+  it 'does not register an offenses without array size method' do
+    expect_no_offenses(<<~RUBY)
+      string.bytes
+    RUBY
+  end
+
+  it 'does not register an offenses with `bytes` without explicit receiver' do
+    expect_no_offenses(<<~RUBY)
+      bytes.size
+    RUBY
+  end
+end