rspec/rspec-expectations — comment on commit f4efada6c86b840802a18e3de12ea68279f9030f
File: lib/rspec/matchers/matcher.rb line 38
By myronmarston:

One suggestion: `Array#include?` is O(N), whereas `Set#include?` is O(1). If I need a collection to check membership, I tend to favor `Set` over `Array`.

That said, it's only 7 items in the collection, so I wouldn't expect it to make a big difference.