Copied from http://segment7.net/projects/ruby/gem_dependencies.rb

reverse_gem_deps.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'rubygems/remote_fetcher'

file = "Marshal.#{Gem.marshal_version}.Z"
# uri = URI.parse "http://gems.rubyforge.org/#{file}"

# data = Gem::RemoteFetcher.fetcher.fetch_path uri
data = File.read(file)
data = Gem.inflate data
data = Marshal.load data

gem_name = "faraday_middleware"

dependencies = {}
data.each do |name, spec|
  # BUG: Only check the latest version of the gem
  name = spec.name
  version = spec.version.to_s
  date = spec.date
  # name = faraday_middleware
  # dep = faraday
  # what are the things that depened on Fraday
  spec.dependencies.each do |dependency|
    next unless gem_name == dependency.name
    dependencies[dependency.name] ||= []
    dependencies[dependency.name] << name
  end
end

dependencies.each do |name, deps|
  deps.uniq.each do |dep|
    puts "* #{dep}"
  end
end