Add new Performance/BigDecimalString cop

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### New features
 
+* [#129](https://github.com/rubocop-hq/rubocop-performance/pull/129): Add new `Performance/BigDecimalString` cop. ([@fatkodima][])
 * [#81](https://github.com/rubocop-hq/rubocop-performance/issues/81): Add new `Performance/StringInclude` cop. ([@fatkodima][])
 * [#123](https://github.com/rubocop-hq/rubocop-performance/pull/123): Add new `Performance/AncestorsInclude` cop. ([@fatkodima][])
 * [#125](https://github.com/rubocop-hq/rubocop-performance/pull/125): Support `Range#member?` method for `Performance/RangeInclude` cop. ([@fatkodima][])

config/default.yml

@@ -6,6 +6,11 @@ Performance/AncestorsInclude:
   Enabled: true
   VersionAdded: '1.7'
 
+Performance/BigDecimalString:
+  Description: 'Convert numeric argument to string before passing to BigDecimal.'
+  Enabled: true
+  VersionAdded: '1.7'
+
 Performance/BindCall:
   Description: 'Use `bind_call(obj, args, ...)` instead of `bind(obj).call(args, ...)`.'
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -3,6 +3,7 @@
 = Department xref:cops_performance.adoc[Performance]
 
 * xref:cops_performance.adoc#performanceancestorsinclude[Performance/AncestorsInclude]
+* xref:cops_performance.adoc#performancebigdecimalstring[Performance/BigDecimalString]
 * xref:cops_performance.adoc#performancebindcall[Performance/BindCall]
 * xref:cops_performance.adoc#performancecaller[Performance/Caller]
 * xref:cops_performance.adoc#performancecasewhensplat[Performance/CaseWhenSplat]

docs/modules/ROOT/pages/cops_performance.adoc

@@ -30,6 +30,36 @@ A <= B
 
 * https://github.com/JuanitoFatas/fast-ruby#ancestorsinclude-vs--code
 
+== Performance/BigDecimalString
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Enabled
+| Yes
+| Yes
+| 1.7
+| -
+|===
+
+This cop identifies places where numeric argument to BigDecimal should be
+converted to string. Initializing from String is faster
+than from Numeric for BigDecimal.
+
+BigDecimal(1, 2)
+BigDecimal(1.2, 3, exception: true)
+
+  # good
+BigDecimal('1', 2)
+BigDecimal('1.2', 3, exception: true)
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+----
+
 == Performance/BindCall
 
 NOTE: Required Ruby version: 2.7

lib/rubocop/cop/performance/big_decimal_string.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      # This cop identifies places where numeric argument to BigDecimal should be
+      # converted to string. Initializing from String is faster
+      # than from Numeric for BigDecimal.
+      #
+      # @example
+      #
+      #   # bad
+      # BigDecimal(1, 2)
+      # BigDecimal(1.2, 3, exception: true)
+      #
+      #   # good
+      # BigDecimal('1', 2)
+      # BigDecimal('1.2', 3, exception: true)
+      #
+      class BigDecimalString < Cop
+        MSG = 'Convert numeric argument to string before passing to `BigDecimal`.'
+
+        def_node_matcher :big_decimal_string_candidate?, <<~PATTERN
+          (send nil? :BigDecimal !str_type? ...)
+        PATTERN
+
+        def on_send(node)
+          return unless big_decimal_string_candidate?(node)
+
+          add_offense(node)
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            replacement = build_replacement(node)
+            corrector.replace(node, replacement)
+          end
+        end
+
+        private
+
+        def build_replacement(node)
+          number, *rest = node.arguments
+          number = "'#{number.source}'"
+          args = [number].concat(rest.map(&:source)).join(', ')
+          "BigDecimal(#{args})"
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/performance_cops.rb

@@ -3,6 +3,7 @@
 require_relative 'mixin/regexp_metacharacter'
 
 require_relative 'performance/ancestors_include'
+require_relative 'performance/big_decimal_string'
 require_relative 'performance/bind_call'
 require_relative 'performance/caller'
 require_relative 'performance/case_when_splat'

spec/rubocop/cop/performance/big_decimal_string_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Performance::BigDecimalString do
+  subject(:cop) { described_class.new }
+
+  it 'registers an offense and corrects when using `BigDecimal` with integer' do
+    expect_offense(<<~RUBY)
+      BigDecimal(1)
+      ^^^^^^^^^^^^^ Convert numeric argument to string before passing to `BigDecimal`.
+    RUBY
+  end
+
+  it 'registers an offense and corrects when using `BigDecimal` with float' do
+    expect_offense(<<~RUBY)
+      BigDecimal(1.5, 2, exception: true)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Convert numeric argument to string before passing to `BigDecimal`.
+    RUBY
+  end
+
+  it 'does not register an offense when using `BigDecimal` with string' do
+    expect_no_offenses(<<~RUBY)
+      BigDecimal('1')
+    RUBY
+  end
+end