How to detect what causes a deprecation warning in Rake
<p>When I do <code>bundle exec rake -T</code> (or <code>bundle exec rake</code>), I get deprecation warnings:</p>

<pre><code>Andrews-Air:nabu agrimm$ bundle exec rake --trace -T
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
rake about                               # List versions of all Rails frameworks and the environment
[snip]
</code></pre>

<p>Although not really relevant, here's the Gemfile:</p>

<pre><code>source 'https://rubygems.org'

gem 'rails', '~&gt; 3.2.22.2'

gem 'mysql2'

group :assets do
  gem 'coffee-rails', '~&gt; 3.2.1'
  gem 'compass-rails'

  gem 'therubyracer'
  gem 'libv8'

  gem 'uglifier', '&gt;= 1.0.3'
end

gem 'jquery-rails'
gem 'haml-rails'
gem 'to-csv', :require =&gt; 'to_csv'
gem 'kaminari'
gem 'oai'
gem 'analytical'

gem 'country-select'
gem 'activeadmin'
gem 'sass-rails',  '~&gt; 3.2.3'
gem 'meta_search', '&gt;= 1.1.0.pre'

gem 'devise', '2.2.3'
gem 'cancancan', '~&gt; 1.12.0'

gem 'squeel'
gem 'nilify_blanks'

gem 'sunspot_rails'
gem 'sunspot_solr'

gem 'unicorn'

gem 'ruby-filemagic'

gem 'capistrano'
gem 'capistrano-ext'
gem 'capistrano-unicorn'

gem 'rollbar', '~&gt; 2.8.3'
gem 'newrelic_rpm'

gem 'progress_bar'
gem 'paper_trail', '~&gt; 2'
gem 'quiet_assets'
gem 'roo', '~&gt; 2.1.0'
gem 'roo-xls', :github =&gt; 'roo-rb/roo-xls', :ref =&gt; '0a5ef88'
gem 'streamio-ffmpeg'
gem 'rake'

gem 'rmagick'

gem 'whenever', :require =&gt; false

group :development, :test do
  gem 'turn', '~&gt; 0.8.3', :require =&gt; false
  gem 'rspec-rails', '~&gt; 2.0'
  gem 'sextant'
  gem 'thin'

  gem 'spring'
  gem 'spring-commands-rspec'

  gem 'pry'
  gem 'pry-rails'

  gem 'letter_opener'

  gem 'guard-bundler'
  gem 'guard-rails'
  gem 'guard-rspec'
  gem 'guard-sunspot'

  gem 'rb-inotify', :require =&gt; RUBY_PLATFORM.include?('linux') ? 'rb-inotify' : false
  gem 'rb-fsevent', :require =&gt; RUBY_PLATFORM.include?('darwin') ? 'rb-fsevent' : false
end

group :development do
  gem 'annotate'
  gem 'binding_of_caller'
  gem 'better_errors'
  gem 'rubocop'
end

group :test do
  gem 'capybara'
  gem 'poltergeist'
  gem 'factory_girl_rails'
  gem 'database_cleaner'
  gem 'email_spec'
  gem 'launchy'
end
</code></pre>

<p>Doing a <code>git grep -i last_comment</code> indicates it's not in my code - presumably it's in a third-party gem.</p>

<p>How do I work out what causes the deprecation warnings?</p>