This cop detects and corrects building up a Hash using reduce and merge, which creates a new copy of the Hash so far on each iteration.

It is preferable to mutate a single Hash, successively adding entries, ideally avoiding small temporary hashes as well.

Real world example/motivation

Reading several hundred thousand key-value pairs from a CSV file to build up a Hash in memory using the following approach takes on the order of 15 minutes:

CSV.foreach("path/to/file.csv").reduce({}) { |hash, row| hash.merge(row[0] => row[1]) }

Meanwhile, building the same Hash by mutation takes under a second

CSV.foreach("path/to/file.csv").each_with_object({}) { |row, hash| hash[row[0]] = row[1] }

require 'tempfile'
require 'benchmark'
require 'csv'

num_rows = 100_000 # This takes a while after 100k records...

file = Tempfile.new('reduce_merge.csv')
file.write(Array.new(num_rows) { "#{_1},#{_1}\n" }.join)
file.flush

Benchmark.bm do |x|
  x.report('reduce + merge') do
    CSV.foreach(file.path).reduce({}) { |hash, row| hash.merge(row[0] => row[1]) }
  end

  x.report('each_with_object + assignment') do
    CSV.foreach(file.path).each_with_object({}) { |row, hash| hash[row[0]] = row[1] }
  end
end

#                                    user     system      total        real
# reduce + merge                15.068653  15.759753  30.828406 ( 31.584389)
# each_with_object + assignment  0.276015   0.013532   0.289547 (  0.298275)

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.