[Fix #6378] Add ToJSON Linter

This fixes issue #6378

classes that override to_json need to ensure an optional argument is
available.

class MyClass
  def to_json
  end
end

While calling MyClass.new.to_json works, calling
JSON.generate(MyClass.new) results in an ArgumentError.

CHANGELOG.md

@@ -62,6 +62,7 @@
 * [#6386](https://github.com/rubocop-hq/rubocop/pull/6386): Add `VersionAdded` meta data to config/default.yml when running `rake new_cop`. ([@koic][])
 * [#6395](https://github.com/rubocop-hq/rubocop/pull/6395): Permit to specify TargetRubyVersion 2.6. ([@koic][])
 * [#6392](https://github.com/rubocop-hq/rubocop/pull/6392): Add `Whitelist` config to `Rails/SkipsModelValidations` rule. ([@DiscoStarslayer][])
+* [#6378](https://github.com/rubocop-hq/rubocop/issues/6378): Added ToJSON cop to enforce an argument when overriding #to_json. ([@allcentury][])
 
 ### Bug fixes
 
@@ -3693,3 +3694,4 @@
 [@tom-lord]: https://github.com/tom-lord
 [@bayandin]: https://github.com/bayandin
 [@nadiyaka]: https://github.com/nadiyaka
+[@allcentury]: https://github.com/allcentury

config/default.yml

@@ -1507,6 +1507,10 @@ Lint/Syntax:
   VersionAdded: '0.9'
 
 
+Lint/ToJSON:
+  Description: 'Ensure #to_json includes an optional argument.'
+  Enabled: true
+
 Lint/UnderscorePrefixedVariableName:
   Description: 'Do not use prefix `_` for a variable that is used.'
   Enabled: true

lib/rubocop.rb

@@ -314,6 +314,7 @@
 require_relative 'rubocop/cop/lint/shadowing_outer_local_variable'
 require_relative 'rubocop/cop/lint/string_conversion_in_interpolation'
 require_relative 'rubocop/cop/lint/syntax'
+require_relative 'rubocop/cop/lint/to_json'
 require_relative 'rubocop/cop/lint/underscore_prefixed_variable_name'
 require_relative 'rubocop/cop/lint/unified_integer'
 require_relative 'rubocop/cop/lint/unneeded_cop_disable_directive'

lib/rubocop/cop/lint/to_json.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # This cop checks to make sure `#to_json` includes an optional argument.
+      # When overriding `#to_json`, callers may invoke JSON
+      # generation via `JSON.generate(your_obj)`.  Since `JSON#generate` allows
+      # for an optional argument, your method should too.
+      #
+      # @example
+      #   # bad
+      #   def to_json
+      #   end
+      #
+      #   # good
+      #   def to_json(_opts)
+      #   end
+      #
+      class ToJSON < Cop
+        MSG = ' `#to_json` requires an optional argument to be parsable ' \
+          'via JSON.generate(obj).'.freeze
+
+        def on_def(node)
+          return unless node.method?(:to_json) && node.arguments.empty?
+
+          add_offense(node)
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            corrector.insert_after(node.loc.name, '(_opts)')
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/style/option_hash.rb

@@ -28,6 +28,7 @@ class OptionHash < Cop
 
         def on_args(node)
           return if super_used?(node)
+          return if whitelist.include?(node.parent.method_name.to_s)
 
           option_hash(node) do |options|
             add_offense(options)
@@ -36,6 +37,10 @@ def on_args(node)
 
         private
 
+        def whitelist
+          cop_config['Whitelist'] || []
+        end
+
         def suspicious_name?(arg_name)
           cop_config.key?('SuspiciousParamNames') &&
             cop_config['SuspiciousParamNames'].include?(arg_name.to_s)

manual/cops.md

@@ -247,6 +247,7 @@ In the following section you find all available cops:
 * [Lint/ShadowingOuterLocalVariable](cops_lint.md#lintshadowingouterlocalvariable)
 * [Lint/StringConversionInInterpolation](cops_lint.md#lintstringconversionininterpolation)
 * [Lint/Syntax](cops_lint.md#lintsyntax)
+* [Lint/ToJSON](cops_lint.md#linttojson)
 * [Lint/UnderscorePrefixedVariableName](cops_lint.md#lintunderscoreprefixedvariablename)
 * [Lint/UnifiedInteger](cops_lint.md#lintunifiedinteger)
 * [Lint/UnneededCopDisableDirective](cops_lint.md#lintunneededcopdisabledirective)

manual/cops_lint.md

@@ -1960,6 +1960,29 @@ This is not actually a cop. It does not inspect anything. It just
 provides methods to repack Parser's diagnostics/errors
 into RuboCop's offenses.
 
+## Lint/ToJSON
+
+Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+--- | --- | --- | --- | ---
+Enabled | Yes | Yes  | - | -
+
+This cop checks to make sure `#to_json` includes an optional argument.
+When overriding `#to_json`, callers may invoke JSON
+generation via `JSON.generate(your_obj)`.  Since `JSON#generate` allows
+for an optional argument, your method should too.
+
+### Examples
+
+```ruby
+# bad
+def to_json
+end
+
+# good
+def to_json(_opts)
+end
+```
+
 ## Lint/UnderscorePrefixedVariableName
 
 Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged

spec/rubocop/cop/lint/to_json_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Lint::ToJSON do
+  subject(:cop) { described_class.new(config) }
+
+  let(:config) { RuboCop::Config.new }
+
+  it 'registers an offense when using `#to_json` without arguments' do
+    expect_offense(<<-RUBY.strip_indent)
+      def to_json
+      ^^^^^^^^^^^  `#to_json` requires an optional argument to be parsable via JSON.generate(obj).
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using `#to_json` with arguments' do
+    expect_no_offenses(<<-RUBY.strip_indent)
+      def to_json(opts)
+      end
+    RUBY
+  end
+
+  it 'autocorrects' do
+    corrected = autocorrect_source(<<-RUBY.strip_indent)
+      def to_json
+      end
+    RUBY
+    expect(corrected).to eq(<<-RUBY.strip_indent)
+      def to_json(_opts)
+      end
+    RUBY
+  end
+end

spec/rubocop/cop/style/option_hash_spec.rb

@@ -99,4 +99,15 @@ def allowed(foo, options = {})
       RUBY
     end
   end
+
+  context 'whitelist' do
+    let(:cop_config) { { 'Whitelist' => %w[to_json] } }
+
+    it 'ignores if the method is whitelisted' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        def to_json(options = {})
+        end
+      RUBY
+    end
+  end
 end