Add new Style/RedundantFetchBlock cop

[fatkodima]

Transferred from rubocop/rubocop-performance#126

Related reference - https://github.com/JuanitoFatas/fast-ruby#hashfetch-with-argument-vs-hashfetch--block-code

When block for fetch just returns Numeric, Symbol, Rational, Complex, true, false, nil, String or constant it is shorter (and faster) to pass an argument instead of using a block.

# bad
hash.fetch(:key) { 5 }
hash.fetch(:key) { true }
hash.fetch(:key) { false }
hash.fetch(:key) { nil }
array.fetch(5) { :value }
ENV.fetch(:key) { VALUE }

# good
hash.fetch(:key, 5)
hash.fetch(:key, true)
hash.fetch(:key, false)
hash.fetch(:key, nil)
array.fetch(5, :value)
ENV.fetch(:key, VALUE)

When hash contains a key, performance is the same for argument vs block syntaxes, but when hash is missing a key - argument version is faster.

Benchmark

# frozen_string_literal: true

require 'bundler/inline'

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

hash_with_key = { key: 1 }
hash_without_key = { not_a_key: 1 }

def fast(hash)
  hash.fetch(:key, 1)
end

def slow(hash)
  hash.fetch(:key) { 1 }
end

Benchmark.ips do |x|
  x.report('fast hash_with_key') { fast(hash_with_key) }
  x.report('slow hash_with_key') { slow(hash_with_key) }
  x.compare!
end

Benchmark.ips do |x|
  x.report('fast hash_without_key') { fast(hash_without_key) }
  x.report('slow hash_without_key') { slow(hash_without_key) }
  x.compare!
end

Results

Warming up --------------------------------------
  fast hash_with_key     1.043M i/100ms
  slow hash_with_key   996.735k i/100ms
Calculating -------------------------------------
  fast hash_with_key     10.292M (± 2.9%) i/s -     52.149M in   5.072099s
  slow hash_with_key      9.940M (± 1.5%) i/s -     49.837M in   5.015103s

Comparison:
  fast hash_with_key: 10291722.7 i/s
  slow hash_with_key:  9939687.7 i/s - same-ish: difference falls within error

Warming up --------------------------------------
fast hash_without_key
                         1.017M i/100ms
slow hash_without_key
                       659.264k i/100ms
Calculating -------------------------------------
fast hash_without_key
                         10.344M (± 0.5%) i/s -     51.892M in   5.016881s
slow hash_without_key
                          6.663M (± 1.6%) i/s -     33.622M in   5.047510s

Comparison:
fast hash_without_key: 10343873.0 i/s
slow hash_without_key:  6663049.0 i/s - 1.55x  (± 0.00) slower

[fatkodima]

Updated with suggestions.

[fatkodima]

@koic Updated.

[bbatsov]

Thanks!

[Drenmi]

Since this is only concerned with the performance of one case, and not the style of #fetch in general, it should probably have gone to rubocop-performance. 😅