Add --core-ext option

The `--core-ext` option can be used to indicate files that define core
extensions.  It accepts a regular expression pattern.  If a module is
defined exclusively in files that match the pattern, the module will be
designated as a core extension and will be listed separately in the main
page nav.

The default value for `--core-ext` is `/core_ext/` which matches Rails'
core extension files in `activesupport/lib/active_support/core_ext/`.

lib/rdoc/generator/template/rails/_index_nav.rhtml

@@ -2,9 +2,20 @@
   <%= outline(@context) %>
 </div>
 
-<div class="nav__heading">Modules</div>
-<ul class="nav__list">
-  <% top_modules(@context.store).each do |rdoc_module| %>
-    <li><%= link_to rdoc_module %></li>
-  <% end %>
-</ul>
+<% unless (modules = top_modules(@context.store)).empty? %>
+  <div class="nav__heading">Modules</div>
+  <ul class="nav__list">
+    <% modules.each do |rdoc_module| %>
+      <li><%= link_to rdoc_module %></li>
+    <% end %>
+  </ul>
+<% end %>
+
+<% unless (extensions = core_extensions(@context.store)).empty? %>
+  <div class="nav__heading">Core Extensions</div>
+  <ul class="nav__list">
+    <% extensions.each do |rdoc_module| %>
+      <li><%= link_to rdoc_module %></li>
+    <% end %>
+  </ul>
+<% end %>

lib/sdoc/generator.rb

@@ -11,6 +11,12 @@
 require "sdoc/version"
 
 class RDoc::Options
+  attr_writer :core_ext_pattern
+
+  def core_ext_pattern
+    @core_ext_pattern ||= /core_ext/
+  end
+
   attr_accessor :github
   attr_accessor :search_index
 end
@@ -38,6 +44,12 @@ def self.setup_options(options)
     opt.separator nil
     opt.separator "SDoc generator options:"
 
+    opt.separator nil
+    opt.on("--core-ext=PATTERN", Regexp, "Regexp pattern indicating files that define core extensions. " \
+      "Defaults to 'core_ext'.") do |pattern|
+      options.core_ext_pattern = pattern
+    end
+
     opt.separator nil
     opt.on("--github", "-g",
             "Generate links to github.") do |value|

lib/sdoc/helpers.rb

@@ -148,11 +148,30 @@ def more_less_ul(items, limit)
   end
 
   def top_modules(rdoc_store)
+    _top_modules(rdoc_store).reject { |rdoc_module| _core_ext?(rdoc_module) }
+  end
+
+  def core_extensions(rdoc_store)
+    _top_modules(rdoc_store).select { |rdoc_module| _core_ext?(rdoc_module) }
+  end
+
+  def _top_modules(rdoc_store)
     rdoc_store.all_classes_and_modules.select do |rdoc_module|
       !rdoc_module.full_name.include?("::")
     end.sort
   end
 
+  def _core_ext?(rdoc_module)
+    # HACK There is currently a bug in RDoc v6.5.0 that causes the value of
+    # RDoc::ClassModule#in_files for `Object` to become polluted. The cause is
+    # unclear, but it might be related to setting global constants (for example,
+    # setting `APP_PATH = "..."` outside of a class or module). To work around
+    # this bug, we always treat `Object` as a core extension.
+    rdoc_module.full_name == "Object" ||
+
+    rdoc_module.in_files.all? { |rdoc_file| @options.core_ext_pattern.match?(rdoc_file.full_name) }
+  end
+
   def module_breadcrumbs(rdoc_module)
     parent_names = rdoc_module.full_name.split("::")[0...-1]
 

spec/helpers_spec.rb

@@ -640,6 +640,32 @@ module Bar::Fuga; end
       _(@helpers.top_modules(top_level.store)).
         must_equal [top_level.find_module_named("Bar"), top_level.find_module_named("Foo")]
     end
+
+    it "excludes core extensions (based on options.core_ext_pattern)" do
+      top_level = rdoc_top_level_for <<~RUBY
+        module Foo; end
+      RUBY
+
+      @helpers.options.core_ext_pattern = /#{Regexp.escape top_level.name}/
+
+      _(@helpers.top_modules(top_level.store)).must_be_empty
+    end
+  end
+
+  describe "#core_extensions" do
+    it "returns top-level core extensions in sorted order (based on options.core_ext_pattern)" do
+      top_level = rdoc_top_level_for <<~RUBY
+        class Foo; module Hoge; end; end
+        module Bar; class Fuga; end; end
+      RUBY
+
+      _(@helpers.core_extensions(top_level.store)).must_be_empty
+
+      @helpers.options.core_ext_pattern = /#{Regexp.escape top_level.name}/
+
+      _(@helpers.core_extensions(top_level.store)).
+        must_equal [top_level.find_module_named("Bar"), top_level.find_module_named("Foo")]
+    end
   end
 
   describe "#module_breadcrumbs" do

spec/rdoc_generator_spec.rb

@@ -55,6 +55,16 @@ def parse_options(*options)
     end
   end
 
+  describe "options.core_ext_pattern" do
+    it "is /core_ext/ by default" do
+      _(parse_options().core_ext_pattern).must_equal %r"core_ext"
+    end
+
+    it "can be set via --core-ext" do
+      _(parse_options("--core-ext", "foo.*bar").core_ext_pattern).must_equal %r"foo.*bar"
+    end
+  end
+
   describe "options.github" do
     it "is disabled by default" do
       _(parse_options().github).must_be_nil