By the way, this is already improved by nobu:

```
$ benchmark-driver benchmark/irb_exec.yml --rbenv '2.6.3;2.7.0-preview1;before;after' -v
2.6.3: ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
2.7.0-preview1: ruby 2.7.0preview1 (2019-05-31 trunk c55db6aa) [x86_64-linux]
before: ruby 2.7.0dev (2019-06-10T21:13:14+09:00 master 973fd18f) [x86_64-linux]
after: ruby 2.7.0dev (2019-06-10T21:18:56+09:00 master 976c689a) [x86_64-linux]
Calculating -------------------------------------
                          2.6.3  2.7.0-preview1      before       after
            irb_exec     11.868           5.872       6.297      10.278 i/s -      30.000 times in 2.527776s 5.108997s 4.764167s 2.918821s

Comparison:
                         irb_exec
               2.6.3:        11.9 i/s
               after:        10.3 i/s - 1.15x  slower
              before:         6.3 i/s - 1.88x  slower
      2.7.0-preview1:         5.9 i/s - 2.02x  slower

```