the core problem is the Proc is not shareable

bootstraptest/test_ractor.rb

@@ -1130,7 +1130,7 @@ assert_equal 'can not set constants with non-shareable objects by non-main Racto
 }
 
 # define_method is not allowed
-assert_equal "defined in a different Ractor", %q{
+assert_equal "defined with an un-shareable Proc in a different Ractor", %q{
   str = "foo"
   define_method(:buggy){|i| str << "#{i}"}
   begin

vm_insnhelper.c

@@ -3082,7 +3082,7 @@ vm_call_bmethod_body(rb_execution_context_t *ec, struct rb_calling_info *calling
 
     if (!RB_OBJ_SHAREABLE_P(procv) &&
         cme->def->body.bmethod.defined_ractor != rb_ractor_self(rb_ec_ractor_ptr(ec))) {
-        rb_raise(rb_eRuntimeError, "defined in a different Ractor");
+        rb_raise(rb_eRuntimeError, "defined with an un-shareable Proc in a different Ractor");
     }
 
     /* control block frame */