[Fix rubocop#278] Fix a false positive for `Performance/StringIdentif…

…ierArgument`

Fixes rubocop#278.

This PR fixes a false positive for `Performance/StringIdentifierArgument` when using `attr`.
`attr` may not be used because `Style/Attr` registers an offense.

changelog/fix_false_positive_for_performance_string_identifier_argument.md

@@ -0,0 +1 @@
+* [#278](https://github.com/rubocop/rubocop-performance/issues/278): Fix a false positive for `Performance/StringIdentifierArgument` when using `attr`. ([@koic][])

lib/rubocop/cop/performance/string_identifier_argument.rb

@@ -27,8 +27,11 @@ class StringIdentifierArgument < Base
 
         MSG = 'Use `%<symbol_arg>s` instead of `%<string_arg>s`.'
 
+        # NOTE: `attr` method is not included in this list as it can cause false positives in Nokogiri API.
+        # And `attr` may not be used because `Style/Attr` registers an offense.
+        # https://github.com/rubocop/rubocop-performance/issues/278
         RESTRICT_ON_SEND = %i[
-          alias_method attr attr_accessor attr_reader attr_writer autoload autoload?
+          alias_method attr_accessor attr_reader attr_writer autoload autoload?
           class_variable_defined? const_defined? const_get const_set const_source_location
           define_method instance_method method_defined? private_class_method? private_method_defined?
           protected_method_defined? public_class_method public_instance_method public_method_defined?

spec/rubocop/cop/performance/string_identifier_argument_spec.rb

@@ -50,4 +50,13 @@
       send(':foo is :bar', foo, bar)
     RUBY
   end
+
+  # NOTE: `attr` method is not included in this list as it can cause false positives in Nokogiri API.
+  # And `attr` may not be used because `Style/Attr` registers an offense.
+  # https://github.com/rubocop/rubocop-performance/issues/278
+  it "does not register an offense when using string argument for `attr` method" do
+    expect_no_offenses(<<~RUBY)
+      attr('foo')
+    RUBY
+  end
 end