Remove #[system_param(ignore)] and #[world_query(ignore)]

[joseph-gio]

Objective

Follow-up to #8030.

Now that SystemParam and WorldQuery are implemented for PhantomData, the ignore attributes are now unnecessary.

---

Changelog

• Removed the attributes #[system_param(ignore)] and #[world_query(ignore)].

Migration Guide

The attributes #[system_param(ignore)] and #[world_query] ignore have been removed. If you were using either of these with PhantomData fields, you can simply remove the attribute:

#[derive(SystemParam)]
struct MyParam<'w, 's, Marker> {
    ...
    // Before:
    #[system_param(ignore)]
    _marker: PhantomData<Marker>,

    // After:
    _marker: PhantomData<Marker>,
}
#[derive(WorldQuery)]
struct MyQuery<Marker> {
    ...
    // Before:
    #[world_query(ignore)]
    _marker: PhantomData<Marker>,

    // After:
    _marker: PhantomData<Marker>,
}

If you were using this for another type that implements Default, consider wrapping that type in Local<> (this only works for SystemParam):

#[derive(SystemParam)]
struct MyParam<'w, 's> {
    // Before:
    #[system_param(ignore)]
    value: MyDefaultType, // This will be initialized using `Default` each time `MyParam` is created.

    // After:
    value: Local<MyDefaultType>, // This will be initialized using `Default` the first time `MyParam` is created.
}

If you are implementing either trait and need to preserve the exact behavior of the old ignore attributes, consider manually implementing SystemParam or WorldQuery for a wrapper struct that uses the Default trait:

// Before:

#[derive(WorldQuery)
struct MyQuery {
   #[world_query(ignore)]
    str: String,
}

// After:

#[derive(WorldQuery)
struct MyQuery {
    str: DefaultQuery<String>,
}

pub struct DefaultQuery<T: Default>(pub T);

unsafe impl<T: Default> WorldQuery for DefaultQuery<T> {
    type Item<'w> = Self;
    ...
    unsafe fn fetch<'w>(...) -> Self::Item<'w> {
        Self(T::default())
    }
}