Add new RSpec/EmptyOutput cop

Instead of calling `output` with an empty string, you should use the
inverse runner and call `output` without an argument. E.g. instead of

    expect { foo }.to output('').to_stdout

you should call

    expect { foo }.not_to output.to_stdout

.rubocop.yml

@@ -164,6 +164,8 @@ RSpec/DuplicatedMetadata:
   Enabled: true
 RSpec/EmptyMetadata:
   Enabled: true
+RSpec/EmptyOutput:
+  Enabled: true
 RSpec/Eq:
   Enabled: true
 RSpec/ExcessiveDocstringSpacing:

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - Fix an autocorrect error for `RSpec/ExpectActual`. ([@bquorning])
 - Add new `RSpec/UndescriptiveLiteralsDescription` cop. ([@ydah])
+- Add new `RSpec/EmptyOutput` cop. ([@bquorning])
 
 ## 2.28.0 (2024-03-30)
 

config/default.yml

@@ -366,6 +366,12 @@ RSpec/EmptyMetadata:
   VersionAdded: '2.24'
   Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/EmptyMetadata
 
+RSpec/EmptyOutput:
+  Description: Check that the `output` matcher is not called with an empty string.
+  Enabled: pending
+  VersionAdded: "<<next>>"
+  Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/EmptyOutput
+
 RSpec/Eq:
   Description: Use `eq` instead of `be ==` to compare objects.
   Enabled: pending

docs/modules/ROOT/pages/cops.adoc

@@ -32,6 +32,7 @@
 * xref:cops_rspec.adoc#rspecemptylineafterhook[RSpec/EmptyLineAfterHook]
 * xref:cops_rspec.adoc#rspecemptylineaftersubject[RSpec/EmptyLineAfterSubject]
 * xref:cops_rspec.adoc#rspecemptymetadata[RSpec/EmptyMetadata]
+* xref:cops_rspec.adoc#rspecemptyoutput[RSpec/EmptyOutput]
 * xref:cops_rspec.adoc#rspeceq[RSpec/Eq]
 * xref:cops_rspec.adoc#rspecexamplelength[RSpec/ExampleLength]
 * xref:cops_rspec.adoc#rspecexamplewithoutdescription[RSpec/ExampleWithoutDescription]

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -1521,6 +1521,37 @@ describe 'Something'
 
 * https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/EmptyMetadata
 
+== RSpec/EmptyOutput
+
+|===
+| Enabled by default | Safe | Supports autocorrection | Version Added | Version Changed
+
+| Pending
+| Yes
+| Always
+| <<next>>
+| -
+|===
+
+Check that the `output` matcher is not called with an empty string.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+expect { foo }.to output('').to_stdout
+expect { bar }.not_to output('').to_stderr
+
+# good
+expect { foo }.not_to output.to_stdout
+expect { bar }.to output.to_stderr
+----
+
+=== References
+
+* https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/EmptyOutput
+
 == RSpec/Eq
 
 |===

lib/rubocop/cop/rspec/empty_output.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Check that the `output` matcher is not called with an empty string.
+      #
+      # @example
+      #   # bad
+      #   expect { foo }.to output('').to_stdout
+      #   expect { bar }.not_to output('').to_stderr
+      #
+      #   # good
+      #   expect { foo }.not_to output.to_stdout
+      #   expect { bar }.to output.to_stderr
+      #
+      class EmptyOutput < Base
+        extend AutoCorrector
+
+        MSG = 'Use `%<runner>s` instead of matching on an empty output.'
+        RESTRICT_ON_SEND = Runners.all
+
+        # @!method matching_empty_output(node)
+        def_node_matcher :matching_empty_output, <<~PATTERN
+          (send
+            (block
+              (send nil? :expect) ...
+            )
+            #Runners.all
+            (send $(send nil? :output (str empty?)) ...)
+          )
+        PATTERN
+
+        def on_send(send_node)
+          matching_empty_output(send_node) do |node|
+            runner = send_node.method?(:to) ? 'not_to' : 'to'
+            message = format(MSG, runner: runner)
+            add_offense(node, message: message) do |corrector|
+              corrector.replace(send_node.loc.selector, runner)
+              corrector.replace(node, 'output')
+            end
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -54,6 +54,7 @@
 require_relative 'rspec/empty_line_after_hook'
 require_relative 'rspec/empty_line_after_subject'
 require_relative 'rspec/empty_metadata'
+require_relative 'rspec/empty_output'
 require_relative 'rspec/eq'
 require_relative 'rspec/example_length'
 require_relative 'rspec/example_without_description'

spec/rubocop/cop/rspec/empty_output_spec.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::EmptyOutput, :config do
+  it 'registers an offense when using `#output` with an empty string' do
+    expect_offense(<<~RUBY)
+      expect { foo }.to output('').to_stderr
+                        ^^^^^^^^^^ Use `not_to` instead of matching on an empty output.
+      expect { foo }.to output('').to_stdout
+                        ^^^^^^^^^^ Use `not_to` instead of matching on an empty output.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      expect { foo }.not_to output.to_stderr
+      expect { foo }.not_to output.to_stdout
+    RUBY
+  end
+
+  it 'registers an offense when negatively matching `#output` with ' \
+     'an empty string' do
+    expect_offense(<<~RUBY)
+      expect { foo }.not_to output('').to_stderr
+                            ^^^^^^^^^^ Use `to` instead of matching on an empty output.
+      expect { foo }.to_not output('').to_stdout
+                            ^^^^^^^^^^ Use `to` instead of matching on an empty output.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      expect { foo }.to output.to_stderr
+      expect { foo }.to output.to_stdout
+    RUBY
+  end
+
+  describe 'compound expectations' do
+    it 'does not register an offense when matching empty strings' do
+      expect_no_offenses(<<~RUBY)
+        expect {
+          :noop
+        }.to output('').to_stdout.and output('').to_stderr
+      RUBY
+    end
+
+    it 'does not register an offense when matching non-empty strings' do
+      expect_no_offenses(<<~RUBY)
+        expect {
+          warn "foo"
+          puts "bar"
+        }.to output("bar\n").to_stdout.and output(/foo/).to_stderr
+      RUBY
+    end
+  end
+
+  it 'does not register an offense when using `#output` with ' \
+     'a non-empty string' do
+    expect_no_offenses(<<~RUBY)
+      expect { foo }.to output('foo').to_stderr
+      expect { foo }.not_to output('foo').to_stderr
+      expect { foo }.to_not output('foo').to_stderr
+    RUBY
+  end
+
+  it 'does not register an offense when using `not_to output`' do
+    expect_no_offenses(<<~RUBY)
+      expect { foo }.not_to output.to_stderr
+      expect { foo }.to_not output.to_stderr
+    RUBY
+  end
+end