Add new Performance/Sum cop

[fatkodima]

Instead of custom implementations of array summation, we should use sum method, which is a lot faster.

# bad
[1, 2, 3].inject(:+)
[1, 2, 3].reduce(10, :+)
[1, 2, 3].reduce { |acc, elem| acc + elem }

# good
[1, 2, 3].sum
[1, 2, 3].sum(10)
[1, 2, 3].sum

Benchmark

# frozen_string_literal: true

require 'bundler/inline'

gemfile(true) do
  gem 'benchmark-ips'
end

RANGE = 1..10

Benchmark.ips do |x|
  x.report("reduce(:+)")   { RANGE.reduce(:+) }
  x.report("reduce { |sum, n| sum + n }")   { RANGE.reduce { |sum, n| sum + n } }
  x.report("sum") { RANGE.sum }

  x.compare!
end

Results

When RANGE = 1..10:

Comparison:
                 sum:  8472475.9 i/s
          reduce(:+):  1421098.7 i/s - 5.96x  (± 0.00) slower
reduce { |sum, n| sum + n }:  1274392.8 i/s - 6.65x  (± 0.00) slower

When RANGE=1..1000:

Comparison:
                 sum:  8309208.6 i/s
          reduce(:+):    19003.9 i/s - 437.24x  (± 0.00) slower
reduce { |sum, n| sum + n }:    15918.6 i/s - 521.98x  (± 0.00) slower

When RANGE=1..1_000_000:

Comparison:
                 sum:  8478595.9 i/s
          reduce(:+):       19.2 i/s - 442082.64x  (± 0.00) slower
reduce { |sum, n| sum + n }:       17.0 i/s - 498855.95x  (± 0.00) slower

We cannot safely autocorrect when initial value is not provided, because, for example, if we convert code

('a'..'z').to_a.reduce { |acc, elem| acc + elem }

into code

('a'..'z').to_a.sum

and this will result in TypeError (String can't be coerced into Integer), because for sum method initial is 0 if not specified explicitly.

When initial value is provided - we can safely autocorrect.

[bbatsov]

Good work overall. As usual - it's nice to attach some references to the cops.

[bbatsov]

Btw, it can also be argued that's a matter of style, not just performance.

[fatkodima]

Updated with:

1. better cop description, as suggested
2. link reference. There is just one non finished PR mentioning Enumerable#sum for the whole fast-ruby, so I added mentioned link (didn't find better on the internets).