For Methods and Procs Ruby 1.9 has method called source_location:
Returns the Ruby source filename and line number containing this method or nil if this method was not defined in Ruby (i.e. native)
So you can request for the method:
m = Foo::Bar.method(:create)
And then ask for the source_location of that method:
m.source_location
This will return an array with filename and line number.
E.g for ActiveRecord::Base#validates this returns:
ActiveRecord::Base.method(:validates).source_location
# => ["/Users/laas/.rvm/gems/ruby-1.9.2-p0@arveaurik/gems/activemodel-3.2.2/lib/active_model/validations/validates.rb", 81]
For classes and modules, Ruby does not offer built in support, but there is an excellent Gist out there that builds upon source_location to return file for a given method or first file for a class if no method was specified:
- ruby where_is module
EDIT: For Ruby 1.8.7 there is a gem that backports source_location:
- ruby18_source_location