Stop transitioning to UNDEF when undefining an instance variable

Cases like this:

```ruby
obj = Object.new
loop do
  obj.instance_variable_set(:@foo, 1)
  obj.remove_instance_variable(:@foo)
end
```

can cause us to use many more shapes than we want (and even run out).
This commit changes the code such that when an instance variable is
removed, we'll walk up the shape tree, find the shape, then rebuild any
child nodes that happened to be below the "targetted for removal" IV.

This also requires moving any instance variables so that indexes derived
from the shape tree will work correctly.

Co-Authored-By: Jemma Issroff <jemmaissroff@gmail.com>
Co-authored-by: John Hawthorn <jhawthorn@github.com>

1 files changed, 16 insertions, 6 deletions

diff --git a/marshal.c b/marshal.c
index e1612303fe..7152be2924 100644
--- a/marshal.c
+++ b/marshal.c
@@ -721,13 +721,23 @@ w_ivar_each(VALUE obj, st_index_t num, struct dump_call_arg *arg)
     struct w_ivar_arg ivarg = {arg, num};
     if (!num) return;
     rb_ivar_foreach(obj, w_obj_each, (st_data_t)&ivarg);
-    if (ivarg.num_ivar) {
-        rb_raise(rb_eRuntimeError, "instance variable removed from %"PRIsVALUE" instance",
-                 CLASS_OF(arg->obj));
-    }
+
     if (shape_id != rb_shape_get_shape_id(arg->obj)) {
-        rb_raise(rb_eRuntimeError, "instance variable added to %"PRIsVALUE" instance",
-                 CLASS_OF(arg->obj));
+        rb_shape_t * expected_shape = rb_shape_get_shape_by_id(shape_id);
+        rb_shape_t * actual_shape = rb_shape_get_shape(arg->obj);
+
+        // If the shape tree got _shorter_ then we probably removed an IV
+        // If the shape tree got longer, then we probably added an IV.
+        // The exception message might not be accurate when someone adds and
+        // removes the same number of IVs, but they will still get an exception
+        if (rb_shape_depth(expected_shape) > rb_shape_depth(actual_shape)) {
+            rb_raise(rb_eRuntimeError, "instance variable removed from %"PRIsVALUE" instance",
+                    CLASS_OF(arg->obj));
+        }
+        else {
+            rb_raise(rb_eRuntimeError, "instance variable added to %"PRIsVALUE" instance",
+                    CLASS_OF(arg->obj));
+        }
     }
 }