Since Ruby 2.4

The Warning module contains a single method named #warn, and the module extends itself, making Warning.warn available. Warning.warn is called for all warnings issued by Ruby. By default, warnings are printed to $stderr.

By overriding Warning.warn, you can change how warnings are handled by Ruby, either filtering some warnings, and/or outputting warnings somewhere other than $stderr. When Warning.warn is overridden, super can be called to get the default behavior of printing the warning to $stderr.

Warning Reference