New Cop: UnusedImplicitSubject

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Master (Unreleased)
 
+* Add `RSpec/UnusedImplicitSubject` cop. ([@marcandre][])
 * Move our documentation from rubocop-rspec.readthedocs.io to docs.rubocop.org/rubocop-rspec. ([@bquorning][])
 * Add `RSpec/RepeatedIncludeExample` cop. ([@biinari][])
 * Add `RSpec/StubbedMock` cop. ([@bquorning][], [@pirj][])
@@ -565,3 +566,4 @@ Compatibility release so users can upgrade RuboCop to 0.51.0. No new features.
 [@biinari]: https://github.com/biinari
 [@koic]: https://github.com/koic
 [@Rafix02]: https://github.com/Rafix02
+[@marcandre]: https://github.com/marcandre

config/default.yml

@@ -577,6 +577,13 @@ RSpec/UnspecifiedException:
   VersionAdded: '1.30'
   StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/UnspecifiedException
 
+RSpec/UnusedImplicitSubject:
+  Description: Checks for usage of explicit subject that could be implicit.
+  Enabled: false
+  VersionAdded: '1.44'
+  StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/UnusedImplicitSubject
+  Safe: false
+
 RSpec/VariableDefinition:
   Description: Checks that memoized helpers names are symbols or strings.
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -89,6 +89,7 @@
 * xref:cops_rspec.adoc#rspecstubbedmock[RSpec/StubbedMock]
 * xref:cops_rspec.adoc#rspecsubjectstub[RSpec/SubjectStub]
 * xref:cops_rspec.adoc#rspecunspecifiedexception[RSpec/UnspecifiedException]
+* xref:cops_rspec.adoc#rspecunusedimplicitsubject[RSpec/UnusedImplicitSubject]
 * xref:cops_rspec.adoc#rspecvariabledefinition[RSpec/VariableDefinition]
 * xref:cops_rspec.adoc#rspecvariablename[RSpec/VariableName]
 * xref:cops_rspec.adoc#rspecverifieddoubles[RSpec/VerifiedDoubles]

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -3949,6 +3949,40 @@ expect { do_something }.not_to raise_error
 
 * https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/UnspecifiedException
 
+== RSpec/UnusedImplicitSubject
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Disabled
+| No
+| Yes (Unsafe)
+| 1.44
+| -
+|===
+
+Checks for usage of explicit subject that could be implicit.
+
+This Cop is not safe as it might be confused by what is the subject.
+
+  # bad
+  subject(:foo) { :bar }
+  it { expect(foo).to eq :bar }
+  it { foo.should eq :bar }
+
+  # good
+  subject(:foo) { :bar }
+  it { is_expected.to eq :bar }
+  it { should eq :bar }
+
+  # also good
+  it { expect { foo }.to raise_error }
+  it { expect(foo.to_s).to eq 'bar' }
+
+=== References
+
+* https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/UnusedImplicitSubject
+
 == RSpec/VariableDefinition
 
 |===

lib/rubocop/cop/rspec/unused_implicit_subject.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Checks for usage of explicit subject that could be implicit.
+      #
+      # This Cop is not safe as it might be confused by what is the subject.
+      #
+      #   # bad
+      #   subject(:foo) { :bar }
+      #   it { expect(foo).to eq :bar }
+      #   it { foo.should eq :bar }
+      #
+      #   # good
+      #   subject(:foo) { :bar }
+      #   it { is_expected.to eq :bar }
+      #   it { should eq :bar }
+      #
+      #   # also good
+      #   it { expect { foo }.to raise_error }
+      #   it { expect(foo.to_s).to eq 'bar' }
+      #
+      class UnusedImplicitSubject < Base
+        extend AutoCorrector
+
+        MSG = 'Use implicit subject.'
+        RESTRICT_ON_SEND = %i[expect should subject subject!].freeze
+
+        def_node_matcher :subject_definition,
+                         '(send #rspec? {:subject :subject!} (sym $_name))'
+        def_node_matcher :subject_should?,
+                         '(send (send nil? %subject) :should ...)'
+        def_node_matcher :expect_subject?,
+                         '(send nil? :expect (send nil? %subject))'
+
+        def on_send(node)
+          send(node.method_name, node)
+        end
+
+        private
+
+        def subject(node)
+          @cur_subject = subject_definition(node)
+        end
+        alias subject! subject
+
+        def should(node)
+          return unless subject_should?(node, subject: @cur_subject)
+
+          range = node.receiver.loc.expression.join(node.loc.selector)
+          add_offense(range) { |c| c.replace(range, 'should') }
+        end
+
+        def expect(node)
+          return unless expect_subject?(node, subject: @cur_subject)
+
+          add_offense(node) { |c| c.replace(node, 'is_expected') }
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -89,6 +89,7 @@
 require_relative 'rspec/stubbed_mock'
 require_relative 'rspec/subject_stub'
 require_relative 'rspec/unspecified_exception'
+require_relative 'rspec/unused_implicit_subject'
 require_relative 'rspec/variable_definition'
 require_relative 'rspec/variable_name'
 require_relative 'rspec/verified_doubles'

spec/rubocop/cop/rspec/unused_implicit_subject_spec.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::UnusedImplicitSubject, :config do
+  context 'with a subject that can be used implicitly' do
+    it 'flags expect(that_subject)' do
+      expect_offense(<<-RUBY)
+        subject(:example) {}
+        it { expect(example).to be_good }
+             ^^^^^^^^^^^^^^^ Use implicit subject.
+      RUBY
+
+      expect_correction(<<-RUBY)
+        subject(:example) {}
+        it { is_expected.to be_good }
+      RUBY
+    end
+
+    it 'flags that_subject.should' do
+      expect_offense(<<-RUBY)
+        subject(:example) {}
+        it { example.should be_good }
+             ^^^^^^^^^^^^^^ Use implicit subject.
+        it { example.should == 42 }
+             ^^^^^^^^^^^^^^ Use implicit subject.
+      RUBY
+
+      expect_correction(<<-RUBY)
+        subject(:example) {}
+        it { should be_good }
+        it { should == 42 }
+      RUBY
+    end
+  end
+
+  context 'with a subject that can not be used implicitly' do
+    it 'does not flag similar cases' do
+      expect_no_offenses(<<-RUBY)
+        let(:example) {}
+        it { expect(example).to be_good }
+        it { example.should be_good }
+      RUBY
+    end
+
+    it 'does not flag non-simplifyable cases' do
+      expect_no_offenses(<<-RUBY)
+        subject(:example) {}
+        it { expect(example.foo).to be_good }
+        it { example.foo.should be_good }
+        it { expect { example }.to be_good }
+      RUBY
+    end
+  end
+end