Commits on Dec 7, 2021

1. Add new Performance/StringIdentifierArgument cop …

   This PR adds new `Performance/StringIdentifierArgument` cop.
   
   This cop identifies places where string identifier argument can be replaced
   by symbol identifier argument.
   It prevents the redundancy of the internal string-to-symbol conversion.
   
   This cop targets methods that take identifier (e.g. method name) argument
   and the following examples are parts of it.
   
   ```ruby
   # bad
   send('do_something')
   attr_accessor 'do_something'
   instance_variable_get('@ivar')
   
   # good
   send(:do_something)
   attr_accessor :do_something
   instance_variable_get(:@ivar)
   ```
   
   The `send` method is a representative and other similar methods are
   similar benchmarks. The following is an example benchmark.
   
   ```ruby
   % cat symbol.rb
   puts `ruby -v`
   
   require 'benchmark/ips'
   
   def foo
   end
   
   Benchmark.ips do |x|
     x.report('symbol arg') { send(:foo) }
     x.report('string arg') { send('foo') }
   
     x.compare!
   end
   ```
   
   ```console
   % ruby symbol.rb
   ruby 3.1.0dev (2021-12-05T10:23:42Z master 19f037e452) [x86_64-darwin19]
   Warming up --------------------------------------
             symbol arg     1.025M i/100ms
             string arg   590.038k i/100ms
   Calculating -------------------------------------
             symbol arg     10.665M (± 1.5%) i/s -     53.318M in 5.000551s
   
             string arg      5.895M (± 1.0%) i/s -     29.502M in 5.004999s
   
   Comparison:
             symbol arg: 10665035.8 i/s
             string arg:  5895132.3 i/s - 1.81x  (± 0.00) slower
   ```
   
   I learned about this performance difference from the book "Polished Ruby Programming".

   

   koic committed Dec 7, 2021
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