let rake tasks be robust to a missing RDoc in Rubinius [Fixes #10462]

See the comment in the rescue clause towards the top of the patch for the rationale.

railties/lib/rails/tasks/documentation.rake

@@ -1,61 +1,72 @@
-require 'rdoc/task'
-require 'rails/api/task'
+begin
+  require 'rdoc/task'
+rescue LoadError
+  # Rubinius installs RDoc as a gem, and for this interpreter "rdoc/task" is
+  # available only if the application bundle includes "rdoc" (normally as a
+  # dependency of the "sdoc" gem.)
+  #
+  # If RDoc is not available it is fine that we do not generate the tasks that
+  # depend on it. Just be robust to this gotcha and go on.
+else
+  require 'rails/api/task'
 
-# Monkey-patch to remove redoc'ing and clobber descriptions to cut down on rake -T noise
-class RDocTaskWithoutDescriptions < RDoc::Task
-  include ::Rake::DSL
+  # Monkey-patch to remove redoc'ing and clobber descriptions to cut down on rake -T noise
+  class RDocTaskWithoutDescriptions < RDoc::Task
+    include ::Rake::DSL
 
-  def define
-    task rdoc_task_name
+    def define
+      task rdoc_task_name
 
-    task rerdoc_task_name => [clobber_task_name, rdoc_task_name]
+      task rerdoc_task_name => [clobber_task_name, rdoc_task_name]
 
-    task clobber_task_name do
-      rm_r rdoc_dir rescue nil
-    end
-
-    task :clobber => [clobber_task_name]
-
-    directory @rdoc_dir
-    task rdoc_task_name => [rdoc_target]
-    file rdoc_target => @rdoc_files + [Rake.application.rakefile] do
-      rm_r @rdoc_dir rescue nil
-      @before_running_rdoc.call if @before_running_rdoc
-      args = option_list + @rdoc_files
-      if @external
-        argstring = args.join(' ')
-        sh %{ruby -Ivendor vendor/rd #{argstring}}
-      else
-        require 'rdoc/rdoc'
-        RDoc::RDoc.new.document(args)
+      task clobber_task_name do
+        rm_r rdoc_dir rescue nil
       end
+
+      task :clobber => [clobber_task_name]
+
+      directory @rdoc_dir
+      task rdoc_task_name => [rdoc_target]
+      file rdoc_target => @rdoc_files + [Rake.application.rakefile] do
+        rm_r @rdoc_dir rescue nil
+        @before_running_rdoc.call if @before_running_rdoc
+        args = option_list + @rdoc_files
+        if @external
+          argstring = args.join(' ')
+          sh %{ruby -Ivendor vendor/rd #{argstring}}
+        else
+          require 'rdoc/rdoc'
+          RDoc::RDoc.new.document(args)
+        end
+      end
+      self
     end
-    self
+  end
+
+  namespace :doc do
+    def gem_path(gem_name)
+      path = $LOAD_PATH.grep(/#{gem_name}[\w.-]*\/lib$/).first
+      yield File.dirname(path) if path
+    end
+
+    RDocTaskWithoutDescriptions.new("app") { |rdoc|
+      rdoc.rdoc_dir = 'doc/app'
+      rdoc.template = ENV['template'] if ENV['template']
+      rdoc.title    = ENV['title'] || "Rails Application Documentation"
+      rdoc.options << '--line-numbers'
+      rdoc.options << '--charset' << 'utf-8'
+      rdoc.rdoc_files.include('README.rdoc')
+      rdoc.rdoc_files.include('app/**/*.rb')
+      rdoc.rdoc_files.include('lib/**/*.rb')
+    }
+    Rake::Task['doc:app'].comment = "Generate docs for the app -- also available doc:rails, doc:guides (options: TEMPLATE=/rdoc-template.rb, TITLE=\"Custom Title\")"
+
+    # desc 'Generate documentation for the Rails framework.'
+    Rails::API::AppTask.new('rails')
   end
 end
 
 namespace :doc do
-  def gem_path(gem_name)
-    path = $LOAD_PATH.grep(/#{gem_name}[\w.-]*\/lib$/).first
-    yield File.dirname(path) if path
-  end
-
-  RDocTaskWithoutDescriptions.new("app") { |rdoc|
-    rdoc.rdoc_dir = 'doc/app'
-    rdoc.template = ENV['template'] if ENV['template']
-    rdoc.title    = ENV['title'] || "Rails Application Documentation"
-    rdoc.options << '--line-numbers'
-    rdoc.options << '--charset' << 'utf-8'
-    rdoc.rdoc_files.include('README.rdoc')
-    rdoc.rdoc_files.include('app/**/*.rb')
-    rdoc.rdoc_files.include('lib/**/*.rb')
-  }
-  Rake::Task['doc:app'].comment = "Generate docs for the app -- also available doc:rails, doc:guides (options: TEMPLATE=/rdoc-template.rb, TITLE=\"Custom Title\")"
-
-  # desc 'Generate documentation for the Rails framework.'
-  Rails::API::AppTask.new('rails')
-
-  # desc "Generate Rails Guides"
   task :guides do
     rails_gem_dir = Gem::Specification.find_by_name("rails").gem_dir
     require File.expand_path(File.join(rails_gem_dir, "/guides/rails_guides"))