Define Kernel#instance_variables_to_inspect

[Bug #21718]

Otherwise objects that don't define it, but define a fairly liberal
`method_missing` method will run into errors that are hard to understand:

```ruby
class Foo
  def method_missing(name, ...)
    name
  end
end

p Foo.new.inspect
```

```
'Kernel#inspect': wrong argument type Symbol (expected Array) (TypeError)
	from ../test.rb:7:in '<main>'
```

1 files changed, 18 insertions, 3 deletions

diff --git a/object.c b/object.c
index 097199479e..4b73fcefa8 100644
--- a/object.c
+++ b/object.c
@@ -854,14 +854,21 @@ rb_obj_inspect(VALUE obj)
 {
     VALUE ivars = rb_check_funcall(obj, id_instance_variables_to_inspect, 0, 0);
     st_index_t n = 0;
-    if (UNDEF_P(ivars)) {
+    if (UNDEF_P(ivars) || NIL_P(ivars)) {
         n = rb_ivar_count(obj);
         ivars = Qnil;
     }
-    else if (!NIL_P(ivars)) {
-        Check_Type(ivars, T_ARRAY);
+    else if (RB_TYPE_P(ivars, T_ARRAY)) {
         n = RARRAY_LEN(ivars);
     }
+    else {
+        rb_raise(
+            rb_eTypeError,
+            "Expected #instance_variables_to_inspect to return an Array or nil, but it returned %"PRIsVALUE,
+            rb_obj_class(ivars)
+        );
+    }
+
     if (n > 0) {
         VALUE c = rb_class_name(CLASS_OF(obj));
         VALUE args[2] = {
@@ -875,6 +882,13 @@ rb_obj_inspect(VALUE obj)
     }
 }
 
+/* :nodoc: */
+static VALUE
+rb_obj_instance_variables_to_inspect(VALUE obj)
+{
+    return Qnil;
+}
+
 static VALUE
 class_or_module_required(VALUE c)
 {
@@ -4535,6 +4549,7 @@ InitVM_Object(void)
 
     rb_define_method(rb_mKernel, "to_s", rb_any_to_s, 0);
     rb_define_method(rb_mKernel, "inspect", rb_obj_inspect, 0);
+    rb_define_private_method(rb_mKernel, "instance_variables_to_inspect", rb_obj_instance_variables_to_inspect, 0);
     rb_define_method(rb_mKernel, "methods", rb_obj_methods, -1); /* in class.c */
     rb_define_method(rb_mKernel, "singleton_methods", rb_obj_singleton_methods, -1); /* in class.c */
     rb_define_method(rb_mKernel, "protected_methods", rb_obj_protected_methods, -1); /* in class.c */