What is the fastest way to add one thing to an array without mutating the array?

benchmark_append.rb

require 'benchmark/ips'

a = (0..10).to_a

Benchmark.ips do |x|
  x.report("push")      { a.dup.push(100) }
  x.report("shovel")    { a.dup << 100 }
  x.report("concat")    { a.dup.concat([100]) }
  x.report("+")         { a + [100] }
  x.report("unshift")   { a.dup.unshift(100) }
  x.report("insert -1") { a.dup.insert(-1, 100) }
  x.report("insert 0")  { a.dup.insert(0, 100) }
  x.report("[]=")       { b = a.dup ; b[0,0] = 100 }
  x.report("|")         { a | [100] }

  x.report("nil") { nil }
end

pete@balloon:~/projects$ ruby -v benchmark_append.rb 
ruby 2.1.3p242 (2014-09-19 revision 47630) [x86_64-linux]
Calculating -------------------------------------
                push     85651 i/100ms
              shovel     90835 i/100ms
              concat     80403 i/100ms
                   +    121933 i/100ms
             unshift     82516 i/100ms
           insert -1     79081 i/100ms
            insert 0     77149 i/100ms
                 []=     75169 i/100ms
                   |     32619 i/100ms
                 nil    171901 i/100ms
-------------------------------------------------
                push  1805366.8 (±0.7%) i/s -    9079006 in   5.029121s
              shovel  1878389.4 (±1.6%) i/s -    9446840 in   5.030615s
              concat  1579816.1 (±1.8%) i/s -    7959897 in   5.040190s
                   +  3635933.0 (±1.3%) i/s -   18289950 in   5.031245s
             unshift  1788963.8 (±1.3%) i/s -    8994244 in   5.028498s
           insert -1  1638090.0 (±1.4%) i/s -    8224424 in   5.021721s
            insert 0  1642330.2 (±1.2%) i/s -    8254943 in   5.027106s
                 []=  1519751.9 (±1.9%) i/s -    7667238 in   5.046921s
                   |   416418.6 (±1.3%) i/s -    2087616 in   5.014100s
                 nil 10686329.6 (±1.3%) i/s -   53461211 in   5.003663s

Add x.compare!