# Objective

#476 changed collision layers to reserve the first bit for a default layer. However, for enums implementing `PhysicsLayer`, the default layer is rather implicit and potentially footgunny.

The default layer should ideally be more explicit.

## Solution

Change `PhysicsLayer` to require `Default` to be implemented. The proc macro orders the variants such that the layer set as the default is always the first bit.

Note that `Default` *must* currently be derived instead of implemented manually, as the macro can only access the default variant if the `#[default]` attribute exists. An error is emitted if `#[default]` does not exist. If someone knows a way to make it work with the manual impl, let me know or consider opening a PR!

I also added documentation for the `PhysicsLayer` macro, and improved error handling.

---

## Migration Guide

Enums that derive `PhysicsLayer` must now also derive `Default` and specify a default layer using the `#[default]` attribute.

Before:

```rust
#[derive(PhysicsLayer)]
enum GameLayer {
    Player,
    Enemy,
    Ground,
}
```

After:

```rust
#[derive(PhysicsLayer, Default)]
enum GameLayer {
    #[default]
    Default, // The name doesn't matter, but Default is used here for clarity
    Player,
    Enemy,
    Ground,
}
```

The default layer always has the bit value `0b0001` regardless of its order relative to the other variants.

---------

Co-authored-by: Jan Hohenheim <jan@hohenheim.ch>

# Objective

#476 changed `CollisionLayers` to use one membership (the first layer) and all filters by default. However, `ColliderConstructorHierarchy` is still using the old default!

## Solution

Use `CollisionLayers::default()` (one membership, all filters) by default for `ColliderConstructorHierarchy`.

---

## Migration Guide

`ColliderConstructorHierarchy` now defaults to one membership (the first layer) and all filters for the `CollisionLayers` of generated colliders. This is consistent with how `CollisionLayers` already works normally.