OneOf WorldQuery via a Xor filter, alternative to Enum filtering

[ItsDoot]

What problem does this solve or what need does it fill?

A somewhat common question in the Bevy Discord involves being able to filter by a component's enum variant:

enum MyComponent {
    Foo,
    Bar,
}

fn my_system(query: Query<Entity, With<MyComponent::Foo>>) {
    // ...
}

This doesn't work because Rust doesn't represent enum variants as full types. A common alternative is to use marker components instead, but currently you lose out on the mutual exclusivity that standard enums provide.

I propose to add a new OneOf<(A, B, C, ...)> WorldQuery similar to AnyOf, but backed by a Xor filter rather than Or.

What solution would you like?

Similar to how AnyOf works:

// equivalent to `Query<(Option<&A>, Option<&B>, Option<&mut C>), Or<(With<A>, With<B>, With<C>)>>
fn my_system(query: Query<AnyOf<(&A, &B, &mut C)>>) {}

This new OneOf should work like:

// equivalent to `Query<(Option<&A>, Option<&B>, Option<&mut C>), Xor<(With<A>, With<B>, With<C>)>>
fn my_system(query: Query<OneOf<(&A, &B, &mut C)>>) {}

Could also look at having an enum return type per arity, like

enum OneOf2<A, B> {
    A(A),
    B(B),
}

enum OneOf3<A, B, C> {
    A(A),
    B(B),
    C(C),
}

// etc ...

This might be best paired with Archetype Invariants.

What alternative(s) have you considered?

Use enums and deal with its inherent issues manually.

[teohhanhui]

This is not ergonomic. Bevy could generate marker types for each variant of the enum and they can then be used in the query like Query<Entity, with!(MyEnum::A)>