Disable Performance/OpenStruct for Rubies >= 2.1

[mikegee]

Is your feature request related to a problem? Please describe.

rubocop-performance told me to avoid instantiating OpenStructs because it invalidates the Ruby global method cache. But I recall Ruby changed the way it cached methods a while ago.

I am fairly confident that the claim:

Instantiation of an OpenStruct invalidates Ruby global method cache

has been untrue since Ruby 2.1.

references:

• initial report and attempt to resolve
• superseded by this change to cache methods at the class level

That change was merged in 2013 and almost certainly released with Ruby 2.1 in December 2013.

See also this tweet from Nate Berkopec:

TIL openstruct does not invalidate the global method cache in Ruby 2.1+ #rubyconf

https://twitter.com/nateberkopec/status/796743575392440320

Describe the solution you'd like

We can probably look at the TargetRubyVersion and only check the older Rubies.