Performance/BlockGivenWithExplicitBlock throws an error with Ruby 3.1 anonymous blocks

[nickcampbell18]

Performance/BlockGivenWithExplicitBlock does not seem to be able to handle anonymous blocks well in Ruby 3.1. I had this failure on one of my applications with the following code:

def method(foo, &)
  raise "block needed" unless block_given?
  do_something(&)
end

Expected behaviour

This behaviour may be undefined? I would probably expect this cop to either:

1. Tell me that I should give the block a name (e.g. &block and then change my code to raise "..." unless block)
2. Report no offenses

Actual behaviour

Currently, this code crashes Rubocop:

Failures:

  1) RuboCop::Cop::Performance::BlockGivenWithExplicitBlock in Ruby 3.1 does not break when given an anonymous block
     Failure/Error: block_arg_name = block_arg.loc.name.source.to_sym

     NoMethodError:
       undefined method `source' for nil:NilClass

                 block_arg_name = block_arg.loc.name.source.to_sym
                                                    ^^^^^^^
     # ./lib/rubocop/cop/performance/block_given_with_explicit_block.rb:42:in `on_send'

I've written a test case below to help identify & fix the problem if that helps:

diff --git a/spec/rubocop/cop/performance/block_given_with_explicit_block_spec.rb b/spec/rubocop/cop/performance/block_given_with_explicit_block_spec.rb
index 92e944ce9..afbba760e 100644
--- a/spec/rubocop/cop/performance/block_given_with_explicit_block_spec.rb
+++ b/spec/rubocop/cop/performance/block_given_with_explicit_block_spec.rb
@@ -68,4 +68,17 @@ RSpec.describe RuboCop::Cop::Performance::BlockGivenWithExplicitBlock, :config d
       do_something if block_given?
     RUBY
   end
+
+  describe 'in Ruby 3.1' do
+    let(:ruby_version) { 3.1 }
+
+    it 'does not break when given an anonymous block' do
+      expect_no_offenses(<<~RUBY)
+        def method(x, &)
+          raise ArgumentError, "block required" unless block_given?
+          do_something(&)
+        end
+      RUBY
+    end
+  end
 end

Rubocop version

I can reproduce this on the master branch:

• Ruby 3.1.0
• rubocop (1.24.1 c651461)
• rubocop-ast (1.15.1)
• rubocop-performance (1.13.1)
• rubocop-rspec (2.5.0)