1 files changed, 38 insertions, 0 deletions
diff --git a/railties/lib/rails/api/generator.rb b/railties/lib/rails/api/generator.rb
new file mode 100644
index 0000000000..126d4d0438
--- /dev/null
+++ b/railties/lib/rails/api/generator.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "sdoc"
+require "active_support/core_ext/array/extract"
+
+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 and remove
+    # Active Storage duplicated classes that are at the top level
+    # since they aren't nested under a definition of the `ActiveStorage` module.
+    if visited.empty?
+      classes = classes.reject { |klass| active_storage?(klass) }
+      core_exts = classes.extract! { |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
+
+    def active_storage?(klass)
+      klass.name != "ActiveStorage" && klass.in_files.all? { |file| file.absolute_name.include?("active_storage") }
+    end
+end