Initialize the main namespace after loading builtin libraries

* For having the common set of loaded libraries between root and main namespaces
* To have the consistent $LOADED_FEATURES in the main namespace

namespace.c

@@ -761,6 +761,9 @@ rb_initialize_main_namespace(void)
     rb_const_set(rb_cNamespace, rb_intern("MAIN"), main_ns);
 
     vm->main_namespace = main_namespace = ns;
+
+    // create the writable classext of ::Object explicitly to finalize the set of visible top-level constants
+    RCLASS_EXT_WRITABLE_IN_NS(rb_cObject, ns);
 }
 
 static VALUE

ruby.c

@@ -1826,10 +1826,13 @@ ruby_opt_init(ruby_cmdline_options_t *opt)
     GET_VM()->running = 1;
     memset(ruby_vm_redefined_flag, 0, sizeof(ruby_vm_redefined_flag));
 
+    ruby_init_prelude();
+
+    /* Initialize the main namespace after loading libraries (including rubygems)
+     * to enable those in both root and main */
     if (rb_namespace_available())
         rb_initialize_main_namespace();
     rb_namespace_init_done();
-    ruby_init_prelude();
 
     // Initialize JITs after ruby_init_prelude() because JITing prelude is typically not optimal.
 #if USE_YJIT

test/ruby/test_namespace.rb

@@ -3,6 +3,10 @@
 require 'test/unit'
 
 class TestNamespace < Test::Unit::TestCase
+  EXPERIMENTAL_WARNINGS = [
+    "warning: Namespace is experimental, and the behavior may change in the future!",
+    "See doc/namespace.md for known issues, etc."
+  ].join("\n")
   ENV_ENABLE_NAMESPACE = {'RUBY_NAMESPACE' => '1'}
 
   def setup
@@ -534,6 +538,51 @@ class TestNamespace < Test::Unit::TestCase
     assert !$LOADED_FEATURES.include?(File.join(namespace_dir, 'blank2.rb'))
   end
 
+  def test_prelude_gems_and_loaded_features
+    assert_in_out_err([ENV_ENABLE_NAMESPACE, "--enable=gems"], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
+      begin;
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+
+        require "error_highlight"
+
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+      end;
+
+      # No additional warnings except for experimental warnings
+      assert_includes error.join("\n"), EXPERIMENTAL_WARNINGS
+      assert_equal error.size, 2
+
+      assert_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
+      assert_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
+      assert_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
+      assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
+    end
+  end
+
+  def test_prelude_gems_and_loaded_features_with_disable_gems
+    assert_in_out_err([ENV_ENABLE_NAMESPACE, "--disable=gems"], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
+      begin;
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+
+        require "error_highlight"
+
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+      end;
+
+      assert_includes error.join("\n"), EXPERIMENTAL_WARNINGS
+      assert_equal error.size, 2
+
+      refute_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
+      refute_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
+      refute_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
+      assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
+    end
+  end
+
   def test_eval_basic
     pend unless Namespace.enabled?