Objective

As discussed in #15341, ghost nodes are a contentious and experimental feature. In the interest of enabling ecosystem experimentation, we've decided to keep them in Bevy 0.15.

That said, we don't use them internally, and don't expect third-party crates to support them. If the experimentation returns a negative result (they aren't very useful, an alternative design is preferred etc) they will be removed.

We should clearly communicate this status to users, and make sure that users don't use ghost nodes in their projects without a very clear understanding of what they're getting themselves into.

Solution

To make life easy for users (and Bevy), GhostNode and all associated helpers remain public and are always available.

However, actually constructing these requires enabling a feature flag that's clearly marked as experimental. To do so, I've added a meaningless private field.

When the feature flag is enabled, our constructs (new and default) can be used. I've added a new constructor, which should be preferred over Default::default as that can be readily deprecated, allowing us to prompt users to swap over to the much nicer GhostNode syntax once this is a unit struct again.

Full credit: this was mostly @cart's design: I'm just implementing it!

Testing

I've run the ghost_nodes example and it fails to compile without the feature flag. With the feature flag, it works fine :)