bevy_ecs: Special-case Entity::PLACEHOLDER formatting

[MrGVSV]

Objective

Oftentimes, users will store an entity on a component or resource. To make this component/resource Default-able, they might initialize it with Entity::PLACEHOLDER. This is sometimes done to avoid the need for an Option<Entity>, especially if it complicates other logic.

For example, it's used in this Selection resource to denote "no selection":

#[derive(Resource, Debug)]
struct Selection(Entity);

impl Default for Selection {
    fn default() -> Self {
        Self(Entity::PLACEHOLDER)
    }
}

The problem is that if we try to Debug the current Selection, we get back: 4294967295v1#8589934591. It's not immediately obvious whether or not the entity is an actual entity or the placeholder.

Now while it doesn't take long to realize that this is in fact just the value of Entity::PLACEHOLDER, it would be a lot clearer if this was made explicit, especially for these particular use cases.

Solution

This PR makes the Debug and Display impls for Entity return PLACEHOLDER for the Entity::PLACEHOLDER constant.

Feel free to bikeshed the actual value returned here. I think PLACEHOLDER on its own could work too. Swapped to PLACEHOLDER from Entity::PLACEHOLDER.

Testing

You can test locally by running:

cargo test --package bevy_ecs

---

Migration Guide

The Debug and Display impls for Entity now return PLACEHOLDER for the Entity::PLACEHOLDER constant. If you had any code relying on these values, you may need to account for this change.

[alice-i-cecile]

I prefer just PLACEHOLDER.

[MrGVSV]

Two votes for PLACEHOLDER so I swapped to that for now. If anyone has any other opinions, feel free to share!