[Fix #10569] Fix a false positive for Style/FetchEnvVar

[koic]

Fixes #10569.

This PR fixes a false positive for Style/FetchEnvVar when using the same value of ENV as if condition in the body.

The ENV var as a if condition is currently allowed.
So the ENV var already used in the condition will be accepted as well.

And the following (maybe) corner case is accepted in this PR.

if ENV['foo']
  ENV.delete('foo')
  puts 'some code'
  puts ENV['foo']
  puts 'other code'
end

#10569 (comment)

---

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!

[inkstak]

Here is another false positive :

if condition || ENV["FOO"]
   puts ENV.fetch("FOO")
end

# if condition || ENV["FOO"]
#                 ^^^^^^^^^

while inversing conditions is accepted by rubocop:

rubocop -V
1.29.1 (using Parser 3.1.2.0, rubocop-ast 1.18.0, running on ruby 2.7.6 x86_64-darwin21)
  - rubocop-performance 1.13.3
  - rubocop-rails 2.14.2
  - rubocop-rspec 2.11.1

[AlexWayfer]

@inkstak I think, the original issue was about fetch suggestions inside if-block. So, if you'd get an offense about puts line — that would be this issue.

In your case, it's OK to use (I guess):

if condition || ENV.fetch('FOO', false)
  puts ENV['FOO']
end

But I'd still recommend to use fetch inside if because you can get into there by condition, without confidence that there is ENV['FOO'] (until it's || and not &&).

[inkstak]

Yes, that may be another issue : the false positive is in the if statement and not the if block.

# if condition || ENV["FOO"]
#                 ^^^^^^^^^

It's definitely because the condition might be truthy I used the fetch method in puts ENV.fetch("FOO")