Add new Gemspec/AddRuntimeDependency cop

Follow up rubygems/rubygems#7799 (comment).

Prefer `add_dependency` over `add_runtime_dependency` because
`add_runtime_dependency` is considered soft-deprecated.

```ruby
# bad
Gem::Specification.new do |spec|
  spec.add_runtime_dependency('rubocop')
end

# good
Gem::Specification.new do |spec|
  spec.add_dependency('rubocop')
end
```

rubocop/ruby-style-guide#943 has been opened for the Style Guide.

changelog/new_add_new_gemspec_add_runtime_dependency_cop.md

@@ -0,0 +1 @@
+* [#13030](https://github.com/rubocop/rubocop/pull/13030): Add new `Gemspec/AddRuntimeDependency` cop. ([@koic][])

config/default.yml

@@ -262,6 +262,14 @@ Bundler/OrderedGems:
 
 #################### Gemspec ###############################
 
+Gemspec/AddRuntimeDependency:
+  Description: 'Prefer `add_dependency` over `add_runtime_dependency`.'
+  Reference: https://github.com/rubygems/rubygems/issues/7799#issuecomment-2192720316
+  Enabled: pending
+  VersionAdded: '<<next>>'
+  Include:
+    - '**/*.gemspec'
+
 Gemspec/DependencyVersion:
   Description: 'Requires or forbids specifying gem dependency versions.'
   Enabled: false

lib/rubocop.rb

@@ -169,6 +169,7 @@
 require_relative 'rubocop/cop/bundler/insecure_protocol_source'
 require_relative 'rubocop/cop/bundler/ordered_gems'
 
+require_relative 'rubocop/cop/gemspec/add_runtime_dependency'
 require_relative 'rubocop/cop/gemspec/dependency_version'
 require_relative 'rubocop/cop/gemspec/deprecated_attribute_assignment'
 require_relative 'rubocop/cop/gemspec/development_dependencies'

lib/rubocop/cop/gemspec/add_runtime_dependency.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Gemspec
+      # Prefer `add_dependency` over `add_runtime_dependency` because
+      # `add_dependency` is considered soft-deprecated.
+      #
+      # @example
+      #
+      #   # bad
+      #   Gem::Specification.new do |spec|
+      #     spec.add_runtime_dependency('rubocop')
+      #   end
+      #
+      #   # good
+      #   Gem::Specification.new do |spec|
+      #     spec.add_dependency('rubocop')
+      #   end
+      #
+      class AddRuntimeDependency < Base
+        extend AutoCorrector
+
+        MSG = 'Use `add_dependency` instead of `add_runtime_dependency`.'
+
+        RESTRICT_ON_SEND = %i[add_runtime_dependency].freeze
+
+        def on_send(node)
+          return if !node.receiver || node.arguments.empty?
+
+          add_offense(node.loc.selector) do |corrector|
+            corrector.replace(node.loc.selector, 'add_dependency')
+          end
+        end
+      end
+    end
+  end
+end

rubocop.gemspec

@@ -31,14 +31,14 @@ Gem::Specification.new do |s|
     'rubygems_mfa_required' => 'true'
   }
 
-  s.add_runtime_dependency('json', '~> 2.3')
-  s.add_runtime_dependency('language_server-protocol', '>= 3.17.0')
-  s.add_runtime_dependency('parallel', '~> 1.10')
-  s.add_runtime_dependency('parser', '>= 3.3.0.2')
-  s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
-  s.add_runtime_dependency('regexp_parser', '>= 2.4', '< 3.0')
-  s.add_runtime_dependency('rexml', '>= 3.2.5', '< 4.0')
-  s.add_runtime_dependency('rubocop-ast', '>= 1.31.1', '< 2.0')
-  s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
-  s.add_runtime_dependency('unicode-display_width', '>= 2.4.0', '< 3.0')
+  s.add_dependency('json', '~> 2.3')
+  s.add_dependency('language_server-protocol', '>= 3.17.0')
+  s.add_dependency('parallel', '~> 1.10')
+  s.add_dependency('parser', '>= 3.3.0.2')
+  s.add_dependency('rainbow', '>= 2.2.2', '< 4.0')
+  s.add_dependency('regexp_parser', '>= 2.4', '< 3.0')
+  s.add_dependency('rexml', '>= 3.2.5', '< 4.0')
+  s.add_dependency('rubocop-ast', '>= 1.31.1', '< 2.0')
+  s.add_dependency('ruby-progressbar', '~> 1.7')
+  s.add_dependency('unicode-display_width', '>= 2.4.0', '< 3.0')
 end

spec/rubocop/cop/gemspec/add_runtime_dependency_spec.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Gemspec::AddRuntimeDependency, :config do
+  it 'registers an offense when using `add_runtime_dependency`' do
+    expect_offense(<<~RUBY)
+      Gem::Specification.new do |spec|
+        spec.add_runtime_dependency('rubocop')
+             ^^^^^^^^^^^^^^^^^^^^^^ Use `add_dependency` instead of `add_runtime_dependency`.
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Gem::Specification.new do |spec|
+        spec.add_dependency('rubocop')
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using `add_dependency`' do
+    expect_no_offenses(<<~RUBY)
+      Gem::Specification.new do |spec|
+        spec.add_dependency('rubocop')
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using `add_development_dependency`' do
+    expect_no_offenses(<<~RUBY)
+      Gem::Specification.new do |spec|
+        spec.add_development_dependency('rubocop')
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using `add_runtime_dependency` without receiver' do
+    expect_no_offenses(<<~RUBY)
+      add_runtime_dependency('rubocop')
+    RUBY
+  end
+
+  it 'does not register an offense when using `add_runtime_dependency` without arguments' do
+    expect_no_offenses(<<~RUBY)
+      spec.add_runtime_dependency
+    RUBY
+  end
+end