Properly nest core classes under a "Core Extensions" label

Since Active Support is monkey patching a lot of core classes, let's
rather document these changes under a new section so they are still
documented but not encumbering the sidebar.

We can safely remove the rescuing of the `LoadError` since as of
cd7cc525, it's not possible to generate the API from an application.

[ci skip]

[Kasper Timm Hansen & Robin Dupret]

2 files changed, 33 insertions, 9 deletions

diff --git a/railties/lib/rails/api/generator.rb b/railties/lib/rails/api/generator.rb
new file mode 100644
index 0000000000..dcc491783c
--- /dev/null
+++ b/railties/lib/rails/api/generator.rb
@@ -0,0 +1,28 @@
+require "sdoc"
+
+class RDoc::Generator::API < RDoc::Generator::SDoc # :nodoc:
+  RDoc::RDoc.add_generator self
+
+  def generate_class_tree_level(classes, visited = {})
+    # Only process core extensions on the first visit.
+    if visited.empty?
+      core_exts, classes = classes.partition { |klass| core_extension?(klass) }
+
+      super.unshift([ "Core extensions", "", "", build_core_ext_subtree(core_exts, visited) ])
+    else
+      super
+    end
+  end
+
+  private
+    def build_core_ext_subtree(classes, visited)
+      classes.map do |klass|
+        [ klass.name, klass.document_self_or_methods ? klass.path : "", "",
+            generate_class_tree_level(klass.classes_and_modules, visited) ]
+      end
+    end
+
+    def core_extension?(klass)
+      klass.name != "ActiveSupport" && klass.in_files.any? { |file| file.absolute_name.include?("core_ext") }
+    end
+end
diff --git a/railties/lib/rails/api/task.rb b/railties/lib/rails/api/task.rb
index 101e8806d0..d1f984be1d 100644
--- a/railties/lib/rails/api/task.rb
+++ b/railties/lib/rails/api/task.rb
@@ -1,4 +1,5 @@
 require "rdoc/task"
+require "rails/api/generator"
 
 module Rails
   module API
@@ -83,7 +84,7 @@ module Rails
         # Be lazy computing stuff to have as light impact as possible to
         # the rest of tasks.
         before_running_rdoc do
-          load_and_configure_sdoc
+          configure_sdoc
           configure_rdoc_files
           setup_horo_variables
         end
@@ -94,20 +95,15 @@ module Rails
         # no-op
       end
 
-      def load_and_configure_sdoc
-        require "sdoc"
-
+      def configure_sdoc
         self.title    = "Ruby on Rails API"
         self.rdoc_dir = api_dir
 
         options << "-m"  << api_main
         options << "-e"  << "UTF-8"
 
-        options << "-f"  << "sdoc"
+        options << "-f"  << "api"
         options << "-T"  << "rails"
-      rescue LoadError
-        $stderr.puts %(Unable to load SDoc, please add\n\n    gem 'sdoc', require: false\n\nto the Gemfile.)
-        exit 1
       end
 
       def configure_rdoc_files
@@ -150,7 +146,7 @@ module Rails
     end
 
     class RepoTask < Task
-      def load_and_configure_sdoc
+      def configure_sdoc
         super
         options << "-g" # link to GitHub, SDoc flag
       end