Add Stubject Cop

config/default.yml

@@ -83,3 +83,7 @@ RSpec/ExampleLength:
 RSpec/NamedSubject:
   Description: 'Name your RSpec subject if you reference it explicitly'
   Enabled: true
+
+RSpec/Stubject:
+  Description: 'Checks for stubbed test subjects'
+  Enabled: true

lib/rubocop-rspec.rb

@@ -26,4 +26,5 @@
 require 'rubocop/cop/rspec/named_subject'
 require 'rubocop/cop/rspec/nested_groups'
 require 'rubocop/cop/rspec/not_to_not'
+require 'rubocop/cop/rspec/stubject'
 require 'rubocop/cop/rspec/verified_doubles'

lib/rubocop/cop/rspec/stubject.rb

@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Checks for stubs on test subjects
+      #
+      # @example
+      #   # bad
+      #   describe Foo do
+      #     subject(:bar) { baz }
+      #
+      #     before do
+      #       allow(bar).to receive(:qux?).and_return(true)
+      #     end
+      #   end
+      #
+      class Stubject < Cop
+        include RuboCop::RSpec::TopLevelDescribe, RuboCop::RSpec::Language
+
+        MSG = 'Do not stub your test subject.'.freeze
+
+        def_node_matcher :subject, <<-PATTERN
+        (block (send nil :subject (sym $_)) args ...)
+        PATTERN
+
+        def_node_matcher :anonymous_subject?, <<-PATTERN
+        (block (send nil :subject) args ...)
+        PATTERN
+
+        def_node_matcher :defines_subject?, <<-PATTERN
+        (block (send nil :subject (sym [!%])) args ...)
+        PATTERN
+
+        def_node_matcher :subject_stub, '(send nil {:allow :spy} (send nil %))'
+        def_node_matcher :subject_mock, <<-PATTERN
+        (send
+          (send nil :expect
+            (send nil %)) :to
+            {(send (send nil :receive ...) ...) (send nil :receive ...)})
+        PATTERN
+
+        def_node_matcher :example_group?, <<-PATTERN
+          (block
+            (send _ {#{ExampleGroups::ALL.to_node_pattern}} ...)
+            ...)
+        PATTERN
+
+        def on_block(node)
+          describe, = described_constant(node)
+          return unless describe
+
+          find_subject(node) do |subject_name, context|
+            find_subject_stub(context, subject_name) do |stub|
+              add_offense(stub, :expression)
+            end
+          end
+        end
+
+        private
+
+        def find_subject_stub(context, subject_name, &block)
+          return if example_group?(context) && defines_subject?(context)
+
+          if subject_stub(context, subject_name) || subject_mock(context, subject_name)
+            yield(context)
+          end
+
+          context.each_child_node do |node|
+            find_subject_stub(node, subject_name, &block)
+          end
+        end
+
+        def defines_subject?(node)
+          node.each_child_node.any? do |child|
+            subject(child)              ||
+              anonymous_subject?(child) ||
+              defines_subject?(child)
+          end
+        end
+
+        def find_subject(node, parent: nil, &block)
+          subject(node) { |name| yield(name, parent) }
+
+          node.each_child_node do |child|
+            find_subject(child, parent: node, &block)
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/rspec/stubject_spec.rb

@@ -0,0 +1,164 @@
+# frozen_string_literal: true
+
+describe RuboCop::Cop::RSpec::Stubject do
+  subject(:cop) { described_class.new }
+
+  it 'complains when subject is stubbed' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        before do
+          allow(foo).to receive(:bar).and_return(baz)
+          ^^^^^^^^^^ Do not stub your test subject.
+        end
+
+        it 'uses expect twice' do
+          expect(foo.bar).to eq(baz)
+        end
+      end
+    RUBY
+  end
+
+  it 'complains when subject is mocked' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        before do
+          expect(foo).to receive(:bar).and_return(baz)
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Do not stub your test subject.
+          expect(foo).to receive(:bar)
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Do not stub your test subject.
+          expect(foo).to receive(:bar).with(1)
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Do not stub your test subject.
+        end
+
+        it 'uses expect twice' do
+          expect(foo.bar).to eq(baz)
+        end
+      end
+    RUBY
+  end
+
+  it 'ignores stub within context where subject name changed' do
+    expect_no_violations(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        context 'when I shake things up' do
+          subject(:bar) { described_class.new }
+
+          it 'tries to trick rubocop-rspec' do
+            allow(foo).to receive(:baz)
+          end
+        end
+      end
+    RUBY
+  end
+
+  it 'flags nested stubjects when nested subject uses same name' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        context 'when I shake things up' do
+          subject(:foo) { described_class.new }
+
+          before do
+            allow(foo).to receive(:wow)
+            ^^^^^^^^^^ Do not stub your test subject.
+          end
+
+          it 'tries to trick rubocop-rspec' do
+            expect(foo).to eql(:neat)
+          end
+        end
+      end
+    RUBY
+  end
+
+  it 'ignores nested stubs when nested subject is anonymous' do
+    expect_no_violations(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        context 'when I shake things up' do
+          subject { described_class.new }
+
+          before do
+            allow(foo).to receive(:wow)
+          end
+
+          it 'tries to trick rubocop-rspec' do
+            expect(foo).to eql(:neat)
+          end
+        end
+      end
+    RUBY
+  end
+
+  it 'flags nested stubjects when example group does not define subject' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        context 'when I shake things up' do
+          before do
+            allow(foo).to receive(:wow)
+            ^^^^^^^^^^ Do not stub your test subject.
+          end
+
+          it 'tries to trick rubocop-rspec' do
+            expect(foo).to eql(:neat)
+          end
+        end
+      end
+    RUBY
+  end
+
+  it 'flags nested stubjects' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        context 'when I shake things up' do
+          subject(:bar) { described_class.new }
+
+          before do
+            allow(foo).to receive(:wow)
+            allow(bar).to receive(:wow)
+            ^^^^^^^^^^ Do not stub your test subject.
+          end
+
+          it 'tries to trick rubocop-rspec' do
+            expect(bar).to eql(foo)
+          end
+        end
+      end
+    RUBY
+  end
+
+  it 'flags deeply nested stubjects' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        subject(:foo) { described_class.new }
+
+        context 'level 1' do
+          subject(:bar) { described_class.new }
+
+          context 'level 2' do
+            subject(:baz) { described_class.new }
+
+            before do
+              allow(foo).to receive(:wow)
+              allow(bar).to receive(:wow)
+              allow(baz).to receive(:wow)
+              ^^^^^^^^^^ Do not stub your test subject.
+            end
+          end
+        end
+      end
+    RUBY
+  end
+end