Mark Style/SlicingWithRange as safe

[koic]

Follow #7921.

This PR marks Style/SlicingWithRange as safe.

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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]

Let's also add a note in the cop description explaning why the auto-correct is unsafe, so it's clearer to the end users.

[koic]

Ah, I misunderstood it as (begin_pos..-1) 💦 I think this cop is safe now, and I updated it that way.

@zverok Please let me know if there is any reason why this cop is unsafe.

[bbatsov]

This should be VersionChanged.

[zverok]

I don't believe this cop can be safe. Equivalence of x..-1 and x.. is guaranteed only for Array#[], but as any class might have #[] method (with semantics we do not know from code analysis), generally those two are NOT equivalent. Examples with Ruby core classes:

h = {-3..-1 => 'foo', 0..5 => 'bar'}
h[-3..-1] # => "foo" 
h[-3..] # => nil 

sum = proc { |ary| ary.sum }
sum[-3..-1] # => -6 
sum[-3..] # Hangs forever

While both examples are artificial, they demonstrate the principle: in general, #[] is not expected to treat endless range as range till -1

[koic]

Ah, I was missing on that point of view. I think it's possible to add an unsafe reason to the documentation. I'll close this PR for now. @zverok Thank you for your explanation!