Use :ambiguous_regexp to detect ambiguous Regexp in Ruby 3

[danieldiekmeier]

At work, we recently started using Rubocop via Standard, and we still have a long .rubocop_todo.yml. Luckily, this helped to uncover a bug in RuboCop! :D

After upgrading a Rails app to Ruby 3.0 and setting TargetRubyVersion: 3.0, I noticed that all todos relating to Lint/AmbiguousRegexpLiteral disappeared. But we hadn't changed the files, and the "bad" code was still in there.

- # Offense count: 7
- # Cop supports --auto-correct.
- Lint/AmbiguousRegexpLiteral:
-   Exclude:
-     - 'spec/features/example/example_spec.rb'
-     - 'spec/models/example.rb'
-     - 'spec/views/admin/example/example.haml_spec.rb'

Digging further, a colleague and I noticed that this only happened with TargetRubyVersion: 3.0, and that the offenses showed up when setting TargetRubyVersion: 2.7.

It took a while, but we finally found this code in the parser: https://github.com/whitequark/parser/blob/e160eeb50bf505453c5a9b1538451d2d62e6144a/lib/parser/lexer.rl#L1571-L1578, added in this commit: https://github.com/whitequark/parser/pull/768/files

It adds a new symbol alongside :ambiguous_literal, namely :ambiguous_regexp.

RuboCop only checks for :ambiguous_literal, so the warning didn't propagate and was ultimately not displayed.

This PR uses the new :ambiguous_regexp if target_ruby_version >= 3.0.

Thanks for your consideration!

---

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• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Added tests.
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[koic]

Good catch! Thank you!