Original PoC for filtering by component value

poc.rs

// Query Filter
pub struct Equals<C: Component, Pred: PartialEq<C> + Default> {
    _c: PhantomData<*const C>,
    _pred: PhantomData<*const Pred>,
}

unsafe impl<C: Component, Pred: PartialEq<C> + Default> WorldQuery for Equals<C, Pred> {
    // All the icky internals that I don't know how to write :(
}

// Const-generics predicate for the query filter
#[derive(Default)]
pub struct Is<const VALUE: bool>;

impl<C, const VALUE: bool> PartialEq<C> for Is<VALUE>
where
    C: Component + Deref<Target = bool>,
{
    fn eq(&self, other: &C) -> bool {
        VALUE == **other
    }
}

// User Component
#[derive(Component, Deref)]
pub struct Poisoned(bool);

// We could reduce the type system word salad with some aliases:
pub type IsTrue<C: Component> = Equals<C, Is<true>>;
pub type IsFalse<C: Component> = Equals<C, Is<false>>;

// Example in a query
pub fn all_poisoned(entities: Query<&Name, Equals<Poisoned, Is<true>>>) {
    // ...
}

// Example with type alias
pub fn all_poisoned2(entities: Query<&Name, IsTrue<Poisoned>>) {
    // ...
}