If RAILS_PATH isn't set, make a dummy doc task

railties/lib/rails/tasks/documentation.rake

@@ -12,45 +12,53 @@ namespace :doc do
   }
 
   desc 'Generate documentation for the Rails framework. Specify path with RAILS_PATH="/path/to/rails"'
-  Rake::RDocTask.new("rails") { |rdoc|
-    path = ENV['RAILS_PATH'] || 'vendor/gems/gems'
-    raise "Missing Rails directory at #{path}" unless File.directory?(path)
-
-    version = "-#{Rails::VERSION::STRING}" unless ENV['RAILS_PATH']
-    rdoc.rdoc_dir = 'doc/api'
-    rdoc.template = "#{ENV['template']}.rb" if ENV['template']
-    rdoc.title    = "Rails Framework Documentation"
-    rdoc.options << '--line-numbers' << '--inline-source'
-    rdoc.rdoc_files.include('README')
-
-    %w(README CHANGELOG lib/action_mailer/base.rb).each do |file|
-      rdoc.rdoc_files.include("#{path}/actionmailer#{version}/#{file}")
+  path = ENV['RAILS_PATH']
+  unless path && File.directory?(path)
+    task :rails do
+      if path
+        $stderr.puts "Skipping doc:rails, missing Rails directory at #{path}"
+      else
+        $stderr.puts "Skipping doc:rails, RAILS_PATH environment variable is not set"
+      end
     end
+  else
+    Rake::RDocTask.new("rails") { |rdoc|
+      version = "-#{Rails::VERSION::STRING}" unless ENV['RAILS_PATH']
+      rdoc.rdoc_dir = 'doc/api'
+      rdoc.template = "#{ENV['template']}.rb" if ENV['template']
+      rdoc.title    = "Rails Framework Documentation"
+      rdoc.options << '--line-numbers' << '--inline-source'
+      rdoc.rdoc_files.include('README')
+
+      %w(README CHANGELOG lib/action_mailer/base.rb).each do |file|
+        rdoc.rdoc_files.include("#{path}/actionmailer#{version}/#{file}")
+      end
 
-    %w(README CHANGELOG lib/action_controller/**/*.rb lib/action_view/**/*.rb).each do |file|
-      rdoc.rdoc_files.include("#{path}/actionpack#{version}/#{file}")
-    end
+      %w(README CHANGELOG lib/action_controller/**/*.rb lib/action_view/**/*.rb).each do |file|
+        rdoc.rdoc_files.include("#{path}/actionpack#{version}/#{file}")
+      end
 
-    %w(README CHANGELOG lib/active_model/**/*.rb).each do |file|
-      rdoc.rdoc_files.include("#{path}/activemodel#{version}/#{file}")
-    end
+      %w(README CHANGELOG lib/active_model/**/*.rb).each do |file|
+        rdoc.rdoc_files.include("#{path}/activemodel#{version}/#{file}")
+      end
 
-    %w(README CHANGELOG lib/active_record/**/*.rb).each do |file|
-      rdoc.rdoc_files.include("#{path}/activerecord#{version}/#{file}")
-    end
+      %w(README CHANGELOG lib/active_record/**/*.rb).each do |file|
+        rdoc.rdoc_files.include("#{path}/activerecord#{version}/#{file}")
+      end
 
-    %w(README CHANGELOG lib/active_resource.rb lib/active_resource/*).each do |file|
-      rdoc.rdoc_files.include("#{path}/activeresource#{version}/#{file}")
-    end
+      %w(README CHANGELOG lib/active_resource.rb lib/active_resource/*).each do |file|
+        rdoc.rdoc_files.include("#{path}/activeresource#{version}/#{file}")
+      end
 
-    %w(README CHANGELOG lib/active_support/**/*.rb).each do |file|
-      rdoc.rdoc_files.include("#{path}/activesupport#{version}/#{file}")
-    end
+      %w(README CHANGELOG lib/active_support/**/*.rb).each do |file|
+        rdoc.rdoc_files.include("#{path}/activesupport#{version}/#{file}")
+      end
 
-    %w(README CHANGELOG MIT-LICENSE lib/{*.rb,commands/*.rb,generators/*.rb}).each do |file|
-      rdoc.rdoc_files.include("#{path}/railties#{version}/#{file}")
-    end
-  }
+      %w(README CHANGELOG MIT-LICENSE lib/{*.rb,commands/*.rb,generators/*.rb}).each do |file|
+        rdoc.rdoc_files.include("#{path}/railties#{version}/#{file}")
+      end
+    }
+  end
 
   plugins = FileList['vendor/plugins/**'].collect { |plugin| File.basename(plugin) }