Add RSpec/StringAsInstanceDoubleConstant

Addresses rubocop#1136

Adds a cop which can autocorrect from String declarations for instance_double to Class declarations. Symbol declarations are not affected.

.rubocop.yml

@@ -253,4 +253,5 @@ Style/YAMLFileRead: {Enabled: true}
 
 # Enable our own pending cops.
 #
-# No pending cops yet.
+RSpec/StringAsInstanceDoubleConstant:
+  Enabled: true

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+- Add `RSpec/StringAsInstanceDoubleConstant` to check for and correct strings used as instance_doubles. ([@corsonknowles])
+
 ## 3.0.5 (2024-09-07)
 
 - Fix false-negative and error for `RSpec/MetadataStyle` when non-literal args are used in metadata in `EnforceStyle: hash`. ([@cbliard])
@@ -920,6 +922,7 @@ Compatibility release so users can upgrade RuboCop to 0.51.0. No new features.
 [@cfabianski]: https://github.com/cfabianski
 [@clupprich]: https://github.com/clupprich
 [@composerinteralia]: https://github.com/composerinteralia
+[@corsonknowles]: https://github.com/corsonknowles
 [@corydiamand]: https://github.com/corydiamand
 [@darhazer]: https://github.com/Darhazer
 [@daveworth]: https://github.com/daveworth

config/default.yml

@@ -929,6 +929,13 @@ RSpec/SpecFilePathSuffix:
     - "**/spec/**/*"
   Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/SpecFilePathSuffix
 
+RSpec/StringAsInstanceDoubleConstant:
+  Description: Do not use a string as `instance_double` constant.
+  Enabled: pending
+  Safe: false
+  VersionAdded: "<<next>>"
+  Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/StringAsInstanceDoubleConstant
+
 RSpec/StubbedMock:
   Description: Checks that message expectations do not have a configured response.
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -101,6 +101,7 @@
 * xref:cops_rspec.adoc#rspecsortmetadata[RSpec/SortMetadata]
 * xref:cops_rspec.adoc#rspecspecfilepathformat[RSpec/SpecFilePathFormat]
 * xref:cops_rspec.adoc#rspecspecfilepathsuffix[RSpec/SpecFilePathSuffix]
+* xref:cops_rspec.adoc#rspecstringasinstancedoubleconstant[RSpec/StringAsInstanceDoubleConstant]
 * xref:cops_rspec.adoc#rspecstubbedmock[RSpec/StubbedMock]
 * xref:cops_rspec.adoc#rspecsubjectdeclaration[RSpec/SubjectDeclaration]
 * xref:cops_rspec.adoc#rspecsubjectstub[RSpec/SubjectStub]

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -5493,6 +5493,35 @@ spec/models/user.rb       # shared_examples_for 'foo'
 
 * https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/SpecFilePathSuffix
 
+== RSpec/StringAsInstanceDoubleConstant
+
+|===
+| Enabled by default | Safe | Supports autocorrection | Version Added | Version Changed
+
+| Pending
+| No
+| Always (Unsafe)
+| <<next>>
+| -
+|===
+
+Do not use a string as `instance_double` constant.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+instance_double('Foo', failure_message, stubs)
+
+# good
+instance_double(Foo, failure_message, stubs)
+----
+
+=== References
+
+* https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/StringAsInstanceDoubleConstant
+
 == RSpec/StubbedMock
 
 |===

lib/rubocop/cop/rspec/string_as_instance_double_constant.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Do not use a string as `instance_double` constant.
+      #
+      # @example
+      #   # bad
+      #   instance_double('Foo', failure_message, stubs)
+      #
+      #   # good
+      #   instance_double(Foo, failure_message, stubs)
+      #
+      class StringAsInstanceDoubleConstant < Base
+        extend AutoCorrector
+
+        MSG = 'Do not use a string as `instance_double` constant.'
+        RESTRICT_ON_SEND = %i[instance_double].freeze
+
+        # @!method stringified_instance_double_const?(node)
+        def_node_matcher :stringified_instance_double_const?, <<~PATTERN
+          (send nil? :instance_double $str ...)
+        PATTERN
+
+        def on_send(node)
+          stringified_instance_double_const?(node) do |args_node|
+            add_offense(args_node) do |corrector|
+              autocorrect(corrector, args_node)
+            end
+          end
+        end
+
+        def autocorrect(corrector, node)
+          corrector.replace(node, node.value)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -99,6 +99,7 @@
 require_relative 'rspec/sort_metadata'
 require_relative 'rspec/spec_file_path_format'
 require_relative 'rspec/spec_file_path_suffix'
+require_relative 'rspec/string_as_instance_double_constant'
 require_relative 'rspec/stubbed_mock'
 require_relative 'rspec/subject_declaration'
 require_relative 'rspec/subject_stub'

spec/rubocop/cop/rspec/string_as_instance_double_constant_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::StringAsInstanceDoubleConstant,
+               :config do
+  context 'when using a class for instance_double' do
+    it do
+      expect_no_offenses(<<~RUBY)
+        instance_double(Foo, failure_message, stubs)
+      RUBY
+    end
+  end
+
+  context 'when using a symbol for instance_double' do
+    it do
+      expect_no_offenses(<<~RUBY)
+        instance_double(:Foo, failure_message, stubs)
+      RUBY
+    end
+  end
+
+  context 'when using a string for instance_double' do
+    it 'replaces the string with the class' do
+      expect_offense <<~RUBY
+        instance_double('Foo', failure_message, stubs)
+                        ^^^^^ Do not use a string as `instance_double` constant.
+      RUBY
+
+      expect_correction <<~RUBY
+        instance_double(Foo, failure_message, stubs)
+      RUBY
+    end
+  end
+end