Ruby prints some helpful deprecation warnings when using some deprecated features of the Ruby language. However, you won't find these errors when they occur within a rake task run inside Rails.

Warnings Often Indicate Bugs

I prefer to treat warnings as errors, as warnings often indicate the presence of a bug. For example, in Ruby 1.8.7, the id method is deprecated and calling id on an Object that doesn't define an id method prints an error:

warning: Object#id will be deprecated; use Object#object_id

A warning like that is probably a good sign I am calling id on the wrong Object. These deprecation warnings are, surprisingly, explicitly turned off in Rails rake tasks in both Rails 2.3 and Rails 3.

Missing Warnings

A Rake task like the following will output nothing when run from lib/tasks:

namespace :deprec do
  task :test do
    nil.type # => no deprecation warning
    nil.id # => no deprecation warning

    A = Class.new
    a = A.new 
    a.id # =>  no deprecation warning
    a.type # =>  no deprecation warning
  end
end

To add back the warnings, simply put $VERBOSE = true at the top of the rake task:

$VERBOSE = true
namespace :deprec do
  task :test do
    nil.type # => deprecation warning
    nil.id # => deprecation warning

    A = Class.new
    a = A.new 
    a.id # => deprecation warning
    a.type # => deprecation warning
  end
end

A Long History

I'm not sure why silencing warnings is the default behavior for Rails rake tasks. The $VERBOSE = nil seems to originate in the very first commit.