Mark RClass instance that may be namespaced with RCLASS_NAMESPACEABLE

class.c

@@ -44,6 +44,8 @@
  *           If unset, the prime classext is writable only from the root namespace.
  * 3:    RCLASS_IS_INITIALIZED
  *           Class has been initialized.
+ * 4:    RCLASS_NAMESPACEABLE
+ *           Is a builtin class that may be namespaced. It larger than a normal class.
  */
 
 /* Flags of T_ICLASS
@@ -51,6 +53,8 @@
  * 2:    RCLASS_PRIME_CLASSEXT_PRIME_WRITABLE
  *           This module's prime classext is the only classext and writable from any namespaces.
  *           If unset, the prime classext is writable only from the root namespace.
+ * 4:    RCLASS_NAMESPACEABLE
+ *           Is a builtin class that may be namespaced. It larger than a normal class.
  */
 
 /* Flags of T_MODULE
@@ -65,6 +69,8 @@
  *           If unset, the prime classext is writable only from the root namespace.
  * 3:    RCLASS_IS_INITIALIZED
  *           Module has been initialized.
+ * 4:    RCLASS_NAMESPACEABLE
+ *           Is a builtin class that may be namespaced. It larger than a normal class.
  */
 
 #define METACLASS_OF(k) RBASIC(k)->klass
@@ -662,6 +668,8 @@ class_alloc(enum ruby_value_type type, VALUE klass)
 
     VALUE flags = type;
     if (RGENGC_WB_PROTECTED_CLASS) flags |= FL_WB_PROTECTED;
+    if (!ruby_namespace_init_done) flags |= RCLASS_NAMESPACEABLE;
+
     NEWOBJ_OF(obj, struct RClass, klass, flags, alloc_size, 0);
 
     memset(RCLASS_EXT_PRIME(obj), 0, sizeof(rb_classext_t));
@@ -676,7 +684,7 @@ class_alloc(enum ruby_value_type type, VALUE klass)
     RCLASS_PRIME_NS((VALUE)obj) = ns;
     // Classes/Modules defined in user namespaces are
     // writable directly because it exists only in a namespace.
-    RCLASS_SET_PRIME_CLASSEXT_WRITABLE((VALUE)obj, !rb_namespace_available() || NAMESPACE_USER_P(ns) ? true : false);
+    RCLASS_SET_PRIME_CLASSEXT_WRITABLE((VALUE)obj, ruby_namespace_init_done || NAMESPACE_USER_P(ns));
 
     RCLASS_SET_ORIGIN((VALUE)obj, (VALUE)obj);
     RCLASS_SET_REFINED_CLASS((VALUE)obj, Qnil);

internal/class.h

@@ -293,6 +293,7 @@ static inline void RCLASS_WRITE_CLASSPATH(VALUE klass, VALUE classpath, bool per
 #define RCLASS_PRIME_CLASSEXT_WRITABLE FL_USER2
 #define RCLASS_IS_INITIALIZED FL_USER3
 // 3 is RMODULE_IS_REFINEMENT for RMODULE
+#define RCLASS_NAMESPACEABLE FL_USER4
 
 /* class.c */
 rb_classext_t * rb_class_duplicate_classext(rb_classext_t *orig, VALUE obj, const rb_namespace_t *ns);

internal/namespace.h

@@ -52,6 +52,7 @@ typedef struct rb_namespace_struct rb_namespace_t;
 #define NAMESPACE_CC_ENTRIES(ccs) (ccs ? NAMESPACE_METHOD_ENTRY(ccs->cme) : NULL)
 
 RUBY_EXTERN bool ruby_namespace_enabled;
+RUBY_EXTERN bool ruby_namespace_init_done;
 
 static inline bool
 rb_namespace_available(void)
@@ -81,5 +82,5 @@ VALUE rb_namespace_exec(const rb_namespace_t *ns, namespace_exec_func *func, VAL
 VALUE rb_namespace_local_extension(VALUE namespace, VALUE fname, VALUE path);
 
 void rb_initialize_main_namespace(void);
-
+void rb_namespace_init_done(void);
 #endif /* INTERNAL_NAMESPACE_H */

lib/rubygems.rb

@@ -1,5 +1,4 @@
 # frozen_string_literal: true
-
 #--
 # Copyright 2006 by Chad Fowler, Rich Kilmer, Jim Weirich and others.
 # All rights reserved.

namespace.c

@@ -48,12 +48,19 @@ static bool tmp_dir_has_dirsep;
 #endif
 
 bool ruby_namespace_enabled = false; // extern
+bool ruby_namespace_init_done = false; // extern
 
 VALUE rb_resolve_feature_path(VALUE klass, VALUE fname);
 static VALUE rb_namespace_inspect(VALUE obj);
 static void namespace_push(rb_thread_t *th, VALUE namespace);
 static VALUE namespace_pop(VALUE th_value);
 
+void
+rb_namespace_init_done(void)
+{
+    ruby_namespace_init_done = true;
+}
+
 void
 rb_namespace_enable_builtin(void)
 {
@@ -1019,6 +1026,9 @@ Init_enable_namespace(void)
     if (env && strlen(env) == 1 && env[0] == '1') {
         ruby_namespace_enabled = true;
     }
+    else {
+        ruby_namespace_init_done = true;
+    }
 }
 
 void

ruby.c

@@ -1822,8 +1822,6 @@ 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();
-
     if (rb_namespace_available())
         rb_initialize_main_namespace();
 
@@ -1844,6 +1842,8 @@ ruby_opt_init(ruby_cmdline_options_t *opt)
     Init_builtin_yjit_hook();
 #endif
 
+    rb_namespace_init_done();
+    ruby_init_prelude();
     ruby_set_script_name(opt->script_name);
     require_libraries(&opt->req_list);
 }