Disable decayment of freed Objects to null in debug builds

[RandomShaper]

Fixes #41179. (4.0 already features the right behavior.)

[akien-mga]

Thanks!

[jcarlosrc]

Why not the opposite way? I mean, enable decay of freed objects to null for release builds. I think it makes more sense for a dynamic language and makes checking simpler and more intuitive.

[RandomShaper]

You have a point there. However, there are some tricky remarks:

In 3.x, the ability to decay to null is powered by the so-named "dangling Variant fix", which is a debug-only feature. There's a proposal about enabling it also on release builds (#1589), so we would be able to have null decay in debug and release. That said, I think that for 3.x it's still a bad idea, because it's too compat-breaking.

Regarding 4.0, there's still some uncertainty, given that #43422 has not yet been addressed, so we don't currently know if the mechanism to prevent dangling Variants there will be suitable for release builds. (For what it's worth, maybe it's already fixed by now; that bug has been open for quite a long time now.)

All that is from the technical standpoint. Now, making decay-to-null a feature has never been a design goal. To decide whether it's a good idea or not, I believe the best thing is to open a proposal about that. If you do, feel free to link to this comment of mine, where I've done my best to give a breakdown of the current situation.