Fix subject nesting detection in RSpec/LeadingSubject

For the following code:

    RSpec.describe User do
      let(:foo) { 'bar' }
      it_behaves_like 'a good citizen' do
        subject { described_class.new }
      end
    end

when `on_block` is triggered for `subject`,
`node.parent.each_child_node.to_a` is:

    [
      s(:send, nil, :it_behaves_like, s(:str, "a good citizen")),
      s(:args),
      s(:block, s(:send, nil, :subject), s(:args), s(:send, s(:send, nil, :described_class), :new))
    ]

and autocorrect was putting the subject block above all "offenders"
coming before it, including the `it_behaves_like` send node.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * Add `RSpec/RepeatedIncludeExample` cop. ([@biinari][])
 * Fix `RSpec/FilePath` when checking a file with a shared example. ([@pirj][])
+* Fix subject nesting detection in `RSpec/LeadingSubject`. ([@pirj][])
 
 ## 1.43.1 (2020-08-17)
 

lib/rubocop/cop/rspec/leading_subject.rb

@@ -54,13 +54,17 @@ def check_previous_nodes(node)
         private
 
         def offending_node(node)
-          node.parent.each_child_node.find do |sibling|
+          parent(node).each_child_node.find do |sibling|
             break if sibling.equal?(node)
 
             yield sibling if offending?(sibling)
           end
         end
 
+        def parent(node)
+          node.each_ancestor(:block).first.body
+        end
+
         def autocorrect(corrector, node, sibling)
           RuboCop::RSpec::Corrector::MoveNode.new(
             node, corrector, processed_source

spec/rubocop/cop/rspec/leading_subject_spec.rb

@@ -269,4 +269,16 @@ def helper_method
       end
     RUBY
   end
+
+  it 'ignores subject nested inside a block' do
+    expect_no_offenses(<<-RUBY)
+      RSpec.describe User do
+        let(:foo) { 'bar' }
+
+        it_behaves_like 'a good citizen' do
+          subject { described_class.new }
+        end
+      end
+    RUBY
+  end
 end