Fix RSpec/InstanceVariable inside custom matcher

fixes #852

Author: pirj

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+* Fix `RSpec/InstanceVariable` detection inside custom matchers. ([@pirj][])
+
 ## 1.37.1 (2019-12-16)
 
 * Improve message and description of `FactoryBot/FactoryClassName`. ([@ybiquitous][])

lib/rubocop/cop/rspec/instance_variable.rb

@@ -58,6 +58,13 @@ class InstanceVariable < Cop
           (block (send (const nil? :Class) :new ...) ...)
         PATTERN
 
+        def_node_matcher :custom_matcher?, <<-PATTERN
+          (block {
+            (send nil? :matcher sym)
+            (send (const (const nil? :RSpec) :Matchers) :define sym)
+          } ...)
+        PATTERN
+
         def_node_search :ivar_usage, '$(ivar $_)'
 
         def_node_search :ivar_assigned?, '(ivasgn % ...)'
@@ -66,17 +73,19 @@ def on_block(node)
           return unless spec_group?(node)
 
           ivar_usage(node) do |ivar, name|
-            return if inside_dynamic_class?(ivar)
-            return if assignment_only? && !ivar_assigned?(node, name)
+            next if valid_usage?(ivar)
+            next if assignment_only? && !ivar_assigned?(node, name)
 
             add_offense(ivar)
           end
         end
 
         private
 
-        def inside_dynamic_class?(node)
-          node.each_ancestor(:block).any? { |block| dynamic_class?(block) }
+        def valid_usage?(node)
+          node.each_ancestor(:block).any? do |block|
+            dynamic_class?(block) || custom_matcher?(block)
+          end
         end
 
         def assignment_only?

spec/rubocop/cop/rspec/instance_variable_spec.rb

@@ -31,6 +31,18 @@
     RUBY
   end
 
+  it 'flags several instance variables inside a describe' do
+    expect_offense(<<-RUBY)
+      describe MyClass do
+        before { @foo = [] }
+        it { expect(@foo).to be_empty }
+                    ^^^^ Avoid instance variables – use let, a method call, or a local variable (if possible).
+        it { expect(@bar).to be_empty }
+                    ^^^^ Avoid instance variables – use let, a method call, or a local variable (if possible).
+      end
+    RUBY
+  end
+
   it 'ignores an instance variable without describe' do
     expect_no_offenses(<<-RUBY)
       @foo = []
@@ -67,6 +79,57 @@ def serialize
     RUBY
   end
 
+  context 'when used in a custom matcher' do
+    it 'ignores instance variables inside `matcher`' do
+      expect_no_offenses(<<~RUBY)
+        describe MyClass do
+          matcher :have_color do
+            match do |object|
+              @matcher = have_attributes(color: anything)
+              @matcher.matches?(object)
+            end
+
+            failure_message do
+              @matcher.failure_message
+            end
+          end
+        end
+      RUBY
+    end
+
+    it 'flags instance variables outside `matcher`' do
+      expect_offense(<<~RUBY)
+        describe MyClass do
+          matcher :have_color do
+            match do |object|
+              @matcher = have_attributes(color: anything)
+              @matcher.matches?(object)
+            end
+          end
+
+          it { expect(color: 1).to @matcher }
+                                   ^^^^^^^^ Avoid instance variables – use let, a method call, or a local variable (if possible).
+        end
+      RUBY
+    end
+
+    it 'ignores instance variables inside `RSpec::Matchers.define`' do
+      expect_no_offenses(<<~RUBY)
+        describe MyClass do
+          RSpec::Matchers.define :be_bigger_than do |first|
+            match do |actual|
+              (actual > first) && (actual < @second)
+            end
+
+            chain :and_smaller_than do |second|
+              @second = second
+            end
+          end
+        end
+      RUBY
+    end
+  end
+
   context 'when configured with AssignmentOnly', :config do
     subject(:cop) { described_class.new(config) }