RedundantStringChars auto-correct

[jaycetde]

str = ''

# Existing code:
str.chars.last(2)
  # => []

# Rubocop suggested and auto-corrected to:
str[-2..-1].chars
  # => NoMethodError (undefined method `chars' for nil:NilClass)
# Not equivalent to `str.chars.last(2)` because below returns `nil`
str[-2..-1]
  # => nil

---

Expected behavior

Do not set this cop as auto-correctable as it can cause breakage

Actual behavior

Auto-correct transforms into potentially broken code

Steps to reproduce the problem

NA

RuboCop version

$ [bundle exec] rubocop -V
1.14.0 (using Parser 3.0.1.1, rubocop-ast 1.5.0, running on ruby 2.5.7 x86_64-linux)
  - rubocop-performance 1.6.1
  - rubocop-rails 2.5.2

[jjb]

I experienced string.chars[0] → string[0].chars

rubocop-performance 1.11.5 (held back by standardrb)

[koic]

@jjb Thank you for the feedback. I've opened #273.