Fix crash in Truffleruby

[jaimerave]

This adds a workaround for oracle/truffleruby#1484. With this change Rubocop no longer crashes when parsing files, an all the tests pass under Truffleruby 20.2.0 and head. Unfortunately, the tests are still too slow to add it to CI.

This change also improves performance a little bit:

require "benchmark/ips"

Benchmark.ips do |x|
  x.config(time: 5, warmup: 2)

  source = "this is my source"

  x.report("to_enum") { source.to_enum(:scan, /this is/).map { Regexp.last_match } }
  x.report("workaround") do
    match_datas = []
    source.scan(/this is/) { match_datas << Regexp.last_match }
    match_datas
  end

  x.compare!
end

With ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-darwin19]

Warming up --------------------------------------
             to_enum    53.745k i/100ms
          workaround    73.683k i/100ms
Calculating -------------------------------------
             to_enum    542.663k (± 1.4%) i/s -      2.741M in   5.052060s
          workaround    727.989k (± 1.7%) i/s -      3.684M in   5.062231s

Comparison:
          workaround:   727989.4 i/s
             to_enum:   542663.3 i/s - 1.34x  (± 0.00) slower

---

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.

[marcandre]

Cool. You'll need to tweak the changelog (see errors)

Ideally, this would add a github action to test on truffleruby too. I think this might help. Same on rubocop-ast... Are you up to do that?

[jaimerave]

@marcandre Github action added, but wow! It is slow. I've have reported the issue to the people in charge of Truffleruby.

[jaimerave]

The slow tests issue is now tracked under oracle/truffleruby#2081

[eregon]

This looks good to me. I would squash the commits when merging though.

[eregon]

@marcandre Who can we ask to review & merge this?

It would be useful to have a release of RuboCop including this fix as it blocks adding TruffleRuby in CI of other gems:
ruby-grape/grape#2099 (comment)

[marcandre]

@jaimerave Thanks for the PR! @eregon Thanks for the ping.

Merged