Commits on Aug 11, 2023

1. Add new Performance/MapMethodChain cop …

   This PR adds new `Performance/MapMethodChain` cop that checks if the map method is used in a chain.
   
   ```ruby
   # bad
   array.map(&:foo).map(&:bar)
   
   # good
   array.map { |item| item.foo.bar }
   ```
   
   ## Safety
   
   This cop is unsafe because false positives occur if the number of times the first method is executed
   affects the return value of subsequent methods.
   
   ```ruby
   class X
     def initialize
       @@num = 0
     end
   
     def foo
       @@num += 1
       self
     end
   
     def bar
       @@num * 2
     end
   end
   
   [X.new, X.new].map(&:foo).map(&:bar) # => [4, 4]
   [X.new, X.new].map { |x| x.foo.bar } # => [2, 4]
   ```
   ## Benchmark
   
   ```console
   $ cat map_chain.rb
   #!/usr/local/bin/ruby
   
   require 'benchmark/ips'
   
   Benchmark.ips do |x|
   ary = %i[hi bye]
   
   x.report('map.map') { ary.map(&:to_s).map(&:length) }
   x.report('map')     { ary.map { |item| item.to_s.length } }
   
   x.compare!
   end
   ```
   
   ```console
   $ ruby -v
   ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-darwin19]
   
   $ ruby map_chain.rb
   Warming up --------------------------------------
   map.map   215.877k i/100ms
   map   319.746k i/100ms
   Calculating -------------------------------------
   map.map      2.116M (± 2.1%) i/s -     10.578M in   5.002379s
   map      3.227M (± 2.8%) i/s -     16.307M in   5.058091s
   
   Comparison:
   map:  3226860.1 i/s
   map.map:  2115551.0 i/s - 1.53x  slower
   ```

   

   koic committed Aug 11, 2023
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