Add bootsnap to optimize boot time.

[hanachin]

before

    _╰╰,   /ﾊﾞｼｮ % ~/src/github.com/rubocop-hq/rubocop
  _/o ŏｧ  / ﾌﾞﾗﾝﾁ% master ﾔﾊﾞｲ
∈ミ;ﾉ,ﾉ   ＼ﾒｲﾚｲ% time bundle exec rubocop -v
0.70.0
bundle exec rubocop -v  0.79s user 0.12s system 95% cpu 0.952 total
    _╰╰,   /ﾊﾞｼｮ % ~/src/github.com/rubocop-hq/rubocop
  _/o ŏｧ  / ﾌﾞﾗﾝﾁ% master ﾔﾊﾞｲ
∈ミ;ﾉ,ﾉ   ＼ﾒｲﾚｲ% time bundle exec rubocop -v
0.70.0
bundle exec rubocop -v  0.75s user 0.06s system 97% cpu 0.832 total
    _╰╰,   /ﾊﾞｼｮ % ~/src/github.com/rubocop-hq/rubocop
  _/o ŏｧ  / ﾌﾞﾗﾝﾁ% master ﾔﾊﾞｲ
∈ミ;ﾉ,ﾉ   ＼ﾒｲﾚｲ% time bundle exec rubocop -v
0.70.0
bundle exec rubocop -v  0.78s user 0.10s system 95% cpu 0.923 tota

after

    _╰╰,   /ﾊﾞｼｮ % ~/src/github.com/rubocop-hq/rubocop
  _/o ŏｧ  / ﾌﾞﾗﾝﾁ% master● ﾔﾊﾞｲ
∈ミ;ﾉ,ﾉ   ＼ﾒｲﾚｲ% time bundle exec rubocop -v
0.70.0
bundle exec rubocop -v  0.52s user 0.08s system 95% cpu 0.634 total
    _╰╰,   /ﾊﾞｼｮ % ~/src/github.com/rubocop-hq/rubocop
  _/o ŏｧ  / ﾌﾞﾗﾝﾁ% master● ﾔﾊﾞｲ
∈ミ;ﾉ,ﾉ   ＼ﾒｲﾚｲ% time bundle exec rubocop -v
0.70.0
bundle exec rubocop -v  0.55s user 0.10s system 93% cpu 0.689 total
    _╰╰,   /ﾊﾞｼｮ % ~/src/github.com/rubocop-hq/rubocop
  _/o ŏｧ  / ﾌﾞﾗﾝﾁ% master● ﾔﾊﾞｲ
∈ミ;ﾉ,ﾉ   ＼ﾒｲﾚｲ% time bundle exec rubocop -v
0.70.0
bundle exec rubocop -v  0.58s user 0.08s system 93% cpu 0

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

I'm not familiar with bootsnap, so I can't really comment on this. Any thoughts @rubocop-hq/rubocop-core?

[bquorning]

I think Bootsnap is an amazing tool, but I believe it should only be used as part of your application (or system).

It should not be a required dependency of RuboCop (or libraries in general).

[hanachin]

It should not be a required dependency of RuboCop (or libraries in general).

I got it, thanks.
I can use my binstub like following instead of adding dependency to rubocop.

#!/usr/bin/env ruby
require 'bootsnap'
cache_dir = File.join(ENV['HOME'], '.cache', 'rubocop_bootsnap_cache')
Bootsnap.setup(cache_dir: cache_dir, load_path_cache: false, autoload_paths_cache: false)
load Gem.bin_path('rubocop', 'rubocop')

So I close this pull request in favor of binstub.
Thank you for the comment!

[bquorning]

I was going to suggest something along the lines of that binstub, but I couldn’t figure out how to make it work 😅So thanks for sharing that binstub template, I’m sure that pattern can be used to speed up various Ruby CLI tools.