How do I find out which gem has a specific dependency?

In the bash shell you can do:

gem dependency name_of_the_gem --reverse-dependencies

For instance:

$ gem dependency activesupport --reverse-dependencies                        
Gem activesupport-2.3.14
Used by
actionpack-2.3.14 (activesupport (= 2.3.14))
activerecord-2.3.14 (activesupport (= 2.3.14))
activeresource-2.3.14 (activesupport (= 2.3.14))