Add new RSpec/DiscardedMatcher cop

.rubocop.yml

@@ -111,6 +111,8 @@ RSpec/DescribeClass:
   Exclude:
     - spec/project/**/*.rb
 
+RSpec/DiscardedMatcher: {Enabled: true}
+
 RSpec/ExampleLength:
   CountAsOne:
     - heredoc

CHANGELOG.md

@@ -13,6 +13,7 @@
 - Fix a false positive for `RSpec/ScatteredSetup` when the hook is defined inside a class method. ([@d4rky-pl])
 - Fix a false positive for `RSpec/DescribedClass` inside dynamically evaluated blocks (`class_eval`, `module_eval`, `instance_eval`, `class_exec`, `module_exec`, `instance_exec`). ([@sucicfilip])
 - Add new cop `RSpec/Output`. ([@kevinrobell-st])
+- Add new cop `RSpec/DiscardedMatcher` to detect matchers in void context (e.g. missing `.and` between compound matchers). ([@ydakuka])
 
 ## 3.8.0 (2025-11-12)
 
@@ -1109,6 +1110,7 @@ Compatibility release so users can upgrade RuboCop to 0.51.0. No new features.
 [@yasu551]: https://github.com/yasu551
 [@ybiquitous]: https://github.com/ybiquitous
 [@ydah]: https://github.com/ydah
+[@ydakuka]: https://github.com/ydakuka
 [@yevhene]: https://github.com/yevhene
 [@ypresto]: https://github.com/ypresto
 [@yujideveloper]: https://github.com/yujideveloper

config/default.yml

@@ -311,6 +311,13 @@ RSpec/Dialect:
   VersionAdded: '1.33'
   Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/Dialect
 
+RSpec/DiscardedMatcher:
+  Description: Checks for matchers that are used in void context.
+  Enabled: pending
+  VersionAdded: "<<next>>"
+  CustomMatcherMethods: []
+  Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DiscardedMatcher
+
 RSpec/DuplicatedMetadata:
   Description: Avoid duplicated metadata.
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -23,6 +23,7 @@
 * xref:cops_rspec.adoc#rspecdescribedclass[RSpec/DescribedClass]
 * xref:cops_rspec.adoc#rspecdescribedclassmodulewrapping[RSpec/DescribedClassModuleWrapping]
 * xref:cops_rspec.adoc#rspecdialect[RSpec/Dialect]
+* xref:cops_rspec.adoc#rspecdiscardedmatcher[RSpec/DiscardedMatcher]
 * xref:cops_rspec.adoc#rspecduplicatedmetadata[RSpec/DuplicatedMetadata]
 * xref:cops_rspec.adoc#rspecemptyexamplegroup[RSpec/EmptyExampleGroup]
 * xref:cops_rspec.adoc#rspecemptyhook[RSpec/EmptyHook]

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -1216,6 +1216,69 @@ end
 
 * https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/Dialect
 
+[#rspecdiscardedmatcher]
+== RSpec/DiscardedMatcher
+
+|===
+| Enabled by default | Safe | Supports autocorrection | Version Added | Version Changed
+
+| Pending
+| Yes
+| No
+| <<next>>
+| -
+|===
+
+Checks for matchers that are used in void context.
+
+Matcher calls like `change`, `receive`, etc. that appear as
+standalone expressions have their result silently discarded.
+This usually means a missing `.and` to chain compound matchers.
+
+The list of matcher methods can be configured
+with `CustomMatcherMethods`.
+
+[#examples-rspecdiscardedmatcher]
+=== Examples
+
+[source,ruby]
+----
+# bad
+specify do
+  expect { result }
+    .to change { obj.foo }.from(1).to(2)
+    change { obj.bar }.from(3).to(4)
+end
+
+# good
+specify do
+  expect { result }
+    .to change { obj.foo }.from(1).to(2)
+    .and change { obj.bar }.from(3).to(4)
+end
+
+# good
+specify do
+  expect { result }.to change { obj.foo }.from(1).to(2)
+end
+----
+
+[#configurable-attributes-rspecdiscardedmatcher]
+=== Configurable attributes
+
+|===
+| Name | Default value | Configurable values
+
+| CustomMatcherMethods
+| `[]`
+| Array
+|===
+
+[#references-rspecdiscardedmatcher]
+=== References
+
+* https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DiscardedMatcher
+
 [#rspecduplicatedmetadata]
 == RSpec/DuplicatedMetadata
 

lib/rubocop-rspec.rb

@@ -14,6 +14,7 @@
 
 require_relative 'rubocop/cop/rspec/mixin/file_help'
 require_relative 'rubocop/cop/rspec/mixin/final_end_location'
+require_relative 'rubocop/cop/rspec/mixin/inside_example'
 require_relative 'rubocop/cop/rspec/mixin/inside_example_group'
 require_relative 'rubocop/cop/rspec/mixin/location_help'
 require_relative 'rubocop/cop/rspec/mixin/metadata'

lib/rubocop/cop/rspec/discarded_matcher.rb

@@ -0,0 +1,113 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Checks for matchers that are used in void context.
+      #
+      # Matcher calls like `change`, `receive`, etc. that appear as
+      # standalone expressions have their result silently discarded.
+      # This usually means a missing `.and` to chain compound matchers.
+      #
+      # The list of matcher methods can be configured
+      # with `CustomMatcherMethods`.
+      #
+      # @example
+      #   # bad
+      #   specify do
+      #     expect { result }
+      #       .to change { obj.foo }.from(1).to(2)
+      #       change { obj.bar }.from(3).to(4)
+      #   end
+      #
+      #   # good
+      #   specify do
+      #     expect { result }
+      #       .to change { obj.foo }.from(1).to(2)
+      #       .and change { obj.bar }.from(3).to(4)
+      #   end
+      #
+      #   # good
+      #   specify do
+      #     expect { result }.to change { obj.foo }.from(1).to(2)
+      #   end
+      #
+      class DiscardedMatcher < Base
+        include InsideExample
+
+        MSG = 'The result of `%<method>s` is not used. ' \
+              'Did you mean to chain it with `.and`?'
+
+        MATCHER_METHODS = %i[
+          change have_received output
+          receive receive_messages receive_message_chain
+        ].to_set.freeze
+
+        def on_send(node)
+          check_discarded_matcher(node, node)
+        end
+
+        def on_block(node) # rubocop:disable InternalAffairs/NumblockHandler, InternalAffairs/ItblockHandler
+          check_discarded_matcher(node.send_node, node)
+        end
+
+        private
+
+        def check_discarded_matcher(send_node, node)
+          return unless matcher_call?(send_node)
+          return unless inside_example?(node)
+          return unless example_with_matcher_expectation?(node)
+
+          target = find_outermost_chain(node)
+          return unless void_value?(target)
+
+          add_offense(target, message: format(MSG, method: node.method_name))
+        end
+
+        def example_with_matcher_expectation?(node)
+          example_node =
+            node.each_ancestor(:block).find { |ancestor| example?(ancestor) }
+
+          example_node.each_descendant(:send).any? do |send_node|
+            expectation_with_matcher?(send_node)
+          end
+        end
+
+        def expectation_with_matcher?(node)
+          %i[to to_not not_to].include?(node.method_name) &&
+            node.arguments.any? do |arg|
+              arg.each_node(:send).any? { |s| matcher_call?(s) }
+            end
+        end
+
+        def void_value?(node)
+          case node.parent.type
+          when :block
+            example?(node.parent)
+          when :begin, :case, :when
+            void_value?(node.parent)
+          end
+        end
+
+        def matcher_call?(node)
+          node.receiver.nil? && all_matcher_methods.include?(node.method_name)
+        end
+
+        def all_matcher_methods
+          @all_matcher_methods ||=
+            (MATCHER_METHODS + custom_matcher_methods).freeze
+        end
+
+        def custom_matcher_methods
+          cop_config.fetch('CustomMatcherMethods', []).map(&:to_sym)
+        end
+
+        def find_outermost_chain(node)
+          current = node
+          current = current.parent while current.parent.receiver == current
+          current
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec/empty_example_group.rb

@@ -38,6 +38,7 @@ module RSpec
       class EmptyExampleGroup < Base
         extend AutoCorrector
 
+        include InsideExample
         include RangeHelp
 
         MSG = 'Empty example group detected.'
@@ -138,7 +139,7 @@ class EmptyExampleGroup < Base
 
         def on_block(node) # rubocop:disable InternalAffairs/NumblockHandler, InternalAffairs/ItblockHandler
           return if node.each_ancestor(:any_def).any?
-          return if node.each_ancestor(:block).any? { |block| example?(block) }
+          return if inside_example?(node)
 
           example_group_body(node) do |body|
             next unless offensive?(body)

lib/rubocop/cop/rspec/mixin/inside_example.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Helps check if a given node is within an example block.
+      module InsideExample
+        private
+
+        def inside_example?(node)
+          node.each_ancestor(:block).any? { |ancestor| example?(ancestor) }
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec/skip_block_inside_example.rb

@@ -24,6 +24,8 @@ module RSpec
       #   end
       #
       class SkipBlockInsideExample < Base
+        include InsideExample
+
         MSG = "Don't pass a block to `skip` inside examples."
 
         def on_block(node)
@@ -35,12 +37,6 @@ def on_block(node)
 
         alias on_numblock on_block
         alias on_itblock on_block
-
-        private
-
-        def inside_example?(node)
-          node.each_ancestor(:block).any? { |ancestor| example?(ancestor) }
-        end
       end
     end
   end

lib/rubocop/cop/rspec/void_expect.rb

@@ -13,6 +13,8 @@ module RSpec
       #   expect(something).to be(1)
       #
       class VoidExpect < Base
+        include InsideExample
+
         MSG = 'Do not use `expect()` without `.to` or `.not_to`. ' \
               'Chain the methods or remove it.'
         RESTRICT_ON_SEND = %i[expect].freeze
@@ -55,10 +57,6 @@ def void?(expect)
 
           parent.block_type? && parent.body == expect
         end
-
-        def inside_example?(node)
-          node.each_ancestor(:block).any? { |ancestor| example?(ancestor) }
-        end
       end
     end
   end

lib/rubocop/cop/rspec_cops.rb

@@ -21,6 +21,7 @@
 require_relative 'rspec/described_class'
 require_relative 'rspec/described_class_module_wrapping'
 require_relative 'rspec/dialect'
+require_relative 'rspec/discarded_matcher'
 require_relative 'rspec/duplicated_metadata'
 require_relative 'rspec/empty_example_group'
 require_relative 'rspec/empty_hook'