Ruby 2.2 introduced symbol GC.

Prior to symbol GC, if a ruby application took in user input and transformed it into a symbol, it could be DOSed through the creation of many unique symbols.

Symbol GC introduced the concept of immortal vs mortal symbols:

• Immortal symbols aren’t garbage collected. They are created through def foo and :bar.
• Mortal symbols are garbage collected. They are symbols are created through baz.to_sym.

Different behavior can be observed in these three LOC between ruby 2.1 & 2.2.

arr = ['foo'.to_sym.hash]
GC.start
arr.include?('foo'.to_sym.hash) # Value is true in 2.1 but false in 2.2

Ruby 2.2 can be made to return true if we make foo an immortal symbol

:foo
arr = ['foo'.to_sym.hash]
GC.start
arr.include?('foo'.to_sym.hash) # Value is true in 2.2 because :foo is immortal