Suggest compact over reject nil values

[baelter]

Rubocop could suggest replacing:

{}.reject { |_k, v| v.nil? } with {}.compact

[rrosenblum]

It looks like this would be a good candidate for a performance cop. I haven't tested this with a hash yet. I would expect to see similar results from a hash.

ARRAY_100_NIL = Array.new(100)
ARRAY_1000_NIL = Array.new(1000)
ARRAY_100 = (1..100).to_a.map { |v| rand(3) >= 2 ? nil : v }
ARRAY_1000 = (1..1000).to_a.map { |v| rand(3) >= 2 ? nil : v }

Benchmark.ips do |x|
  x.report('100 nil reject') { ARRAY_100_NIL.reject(&:nil?) }
  x.report('1000 nil reject') { ARRAY_1000_NIL.reject(&:nil?) }
  x.report('100 reject') { ARRAY_100.reject(&:nil?) }
  x.report('1000 reject') { ARRAY_1000.reject(&:nil?) }
  x.report('100 nil compact') { ARRAY_100_NIL.compact }
  x.report('1000 nil compact') { ARRAY_1000_NIL.compact }
  x.report('100 compact') { ARRAY_100.compact }
  x.report('1000 compact') { ARRAY_1000.compact }
  x.compare!
end

Warming up --------------------------------------
      100 nil reject    30.078k i/100ms
     1000 nil reject     3.094k i/100ms
          100 reject    18.859k i/100ms
         1000 reject     2.010k i/100ms
     100 nil compact   317.667k i/100ms
    1000 nil compact   123.605k i/100ms
         100 compact   146.194k i/100ms
        1000 compact    29.429k i/100ms
Calculating -------------------------------------
      100 nil reject    297.746k (± 3.7%) i/s -      1.504M in   5.058602s
     1000 nil reject     31.080k (± 2.4%) i/s -    157.794k in   5.080001s
          100 reject    194.301k (± 4.3%) i/s -    980.668k in   5.056359s
         1000 reject     22.523k (± 3.8%) i/s -    112.560k in   5.004890s
     100 nil compact      3.336M (± 3.9%) i/s -     16.836M in   5.054570s
    1000 nil compact      1.260M (± 2.7%) i/s -      6.304M in   5.007786s
         100 compact      1.743M (±14.5%) i/s -      8.625M in   5.057635s
        1000 compact    345.905k (±24.5%) i/s -      1.648M in   5.047389s

Comparison:
     100 nil compact:  3336043.7 i/s
         100 compact:  1743292.6 i/s - 1.91x  (± 0.00) slower
    1000 nil compact:  1259767.4 i/s - 2.65x  (± 0.00) slower
        1000 compact:   345904.8 i/s - 9.64x  (± 0.00) slower
      100 nil reject:   297745.5 i/s - 11.20x  (± 0.00) slower
          100 reject:   194301.3 i/s - 17.17x  (± 0.00) slower
     1000 nil reject:    31079.8 i/s - 107.34x  (± 0.00) slower
         1000 reject:    22523.0 i/s - 148.12x  (± 0.00) slower