remove QF generics from all Query/State methods and types (#5170)

# Objective

remove `QF` generics from a bunch of types and methods on query related items. this has a few benefits:
- simplifies type signatures `fn iter(&self) -> QueryIter<'_, 's, Q::ReadOnly, F::ReadOnly>` is (imo) conceptually simpler than `fn iter(&self) -> QueryIter<'_, 's, Q, ROQueryFetch<'_, Q>, F>`
- `Fetch` is mostly an implementation detail but previously we had to expose it on every `iter` `get` etc method
- Allows us to potentially in the future simplify the `WorldQuery` trait hierarchy by removing the `Fetch` trait

## Solution

remove the `QF` generic and add a way to (unsafely) turn `&QueryState<Q1, F1>` into `&QueryState<Q2, F2>`

---

## Changelog/Migration Guide

The `QF` generic was removed from various `Query` iterator types and some methods, you should update your code to use the type of the corresponding worldquery of the fetch type that was being used, or call `as_readonly`/`as_nop` to convert a querystate to the appropriate type. For example:
`.get_single_unchecked_manual::<ROQueryFetch<Q>>(..)` -> `.as_readonly().get_single_unchecked_manual(..)`
`my_field: QueryIter<'w, 's, Q, ROQueryFetch<'w, Q>, F>` -> `my_field: QueryIter<'w, 's, Q::ReadOnly, F::ReadOnly>`

crates/bevy_ecs/src/query/fetch.rs

@@ -318,7 +318,8 @@ use std::{cell::UnsafeCell, marker::PhantomData};
 /// ```
 /// # Safety
 ///
-/// component access of `ROQueryFetch<Self>` should be a subset of `QueryFetch<Self>`
+/// component access of `ROQueryFetch<Self>` must be a subset of `QueryFetch<Self>`
+/// and `ROQueryFetch<Self>` must match exactly the same archetypes/tables as `QueryFetch<Self>`
 pub unsafe trait WorldQuery: for<'w> WorldQueryGats<'w, _State = Self::State> {
     type ReadOnly: ReadOnlyWorldQuery<State = Self::State>;
     type State: FetchState;
@@ -1608,6 +1609,25 @@ macro_rules! impl_anytuple_fetch {
 all_tuples!(impl_tuple_fetch, 0, 15, F, S);
 all_tuples!(impl_anytuple_fetch, 0, 15, F, S);
 
+/// [`WorldQuery`] used to nullify queries by turning `Query<Q>` into `Query<NopWorldQuery<Q>>`
+///
+/// This will rarely be useful to consumers of `bevy_ecs`.
+pub struct NopWorldQuery<Q: WorldQuery>(PhantomData<Q>);
+
+/// SAFETY: `Self::ReadOnly` is `Self`
+unsafe impl<Q: WorldQuery> WorldQuery for NopWorldQuery<Q> {
+    type ReadOnly = Self;
+    type State = Q::State;
+
+    fn shrink<'wlong: 'wshort, 'wshort>(_: ()) {}
+}
+impl<'a, Q: WorldQuery> WorldQueryGats<'a> for NopWorldQuery<Q> {
+    type Fetch = NopFetch<QueryFetch<'a, Q>>;
+    type _State = <Q as WorldQueryGats<'a>>::_State;
+}
+/// SAFETY: `NopFetch` never accesses any data
+unsafe impl<Q: WorldQuery> ReadOnlyWorldQuery for NopWorldQuery<Q> {}
+
 /// [`Fetch`] that does not actually fetch anything
 ///
 /// Mostly useful when something is generic over the Fetch and you don't want to fetch as you will discard the result
@@ -1616,18 +1636,18 @@ pub struct NopFetch<State> {
 }
 
 // SAFETY: NopFetch doesnt access anything
-unsafe impl<'w, State: FetchState> Fetch<'w> for NopFetch<State> {
+unsafe impl<'w, F: Fetch<'w>> Fetch<'w> for NopFetch<F> {
     type Item = ();
-    type State = State;
+    type State = F::State;
 
-    const IS_DENSE: bool = true;
+    const IS_DENSE: bool = F::IS_DENSE;
 
     const IS_ARCHETYPAL: bool = true;
 
     #[inline(always)]
     unsafe fn init(
         _world: &'w World,
-        _state: &State,
+        _state: &F::State,
         _last_change_tick: u32,
         _change_tick: u32,
     ) -> Self {

crates/bevy_ecs/src/query/iter.rs

@@ -13,17 +13,14 @@ use super::{QueryFetch, QueryItem, ReadOnlyWorldQuery};
 ///
 /// This struct is created by the [`Query::iter`](crate::system::Query::iter) and
 /// [`Query::iter_mut`](crate::system::Query::iter_mut) methods.
-pub struct QueryIter<'w, 's, Q: WorldQuery, QF: Fetch<'w, State = Q::State>, F: WorldQuery> {
+pub struct QueryIter<'w, 's, Q: WorldQuery, F: WorldQuery> {
     tables: &'w Tables,
     archetypes: &'w Archetypes,
     query_state: &'s QueryState<Q, F>,
-    cursor: QueryIterationCursor<'w, 's, Q, QF, F>,
+    cursor: QueryIterationCursor<'w, 's, Q, F>,
 }
 
-impl<'w, 's, Q: WorldQuery, QF, F: WorldQuery> QueryIter<'w, 's, Q, QF, F>
-where
-    QF: Fetch<'w, State = Q::State>,
-{
+impl<'w, 's, Q: WorldQuery, F: WorldQuery> QueryIter<'w, 's, Q, F> {
     /// # Safety
     /// This does not check for mutable query correctness. To be safe, make sure mutable queries
     /// have unique access to the components they query.
@@ -44,11 +41,8 @@ where
     }
 }
 
-impl<'w, 's, Q: WorldQuery, QF, F: WorldQuery> Iterator for QueryIter<'w, 's, Q, QF, F>
-where
-    QF: Fetch<'w, State = Q::State>,
-{
-    type Item = QF::Item;
+impl<'w, 's, Q: WorldQuery, F: WorldQuery> Iterator for QueryIter<'w, 's, Q, F> {
+    type Item = QueryItem<'w, Q>;
 
     #[inline(always)]
     fn next(&mut self) -> Option<Self::Item> {
@@ -69,42 +63,32 @@ where
             .map(|id| self.archetypes[*id].len())
             .sum();
 
-        let archetype_query = F::Fetch::IS_ARCHETYPAL && QF::IS_ARCHETYPAL;
+        let archetype_query = Q::Fetch::IS_ARCHETYPAL && F::Fetch::IS_ARCHETYPAL;
         let min_size = if archetype_query { max_size } else { 0 };
         (min_size, Some(max_size))
     }
 }
 
 // This is correct as [`QueryIter`] always returns `None` once exhausted.
-impl<'w, 's, Q: WorldQuery, QF, F: WorldQuery> FusedIterator for QueryIter<'w, 's, Q, QF, F> where
-    QF: Fetch<'w, State = Q::State>
-{
-}
+impl<'w, 's, Q: WorldQuery, F: WorldQuery> FusedIterator for QueryIter<'w, 's, Q, F> {}
 
 /// An [`Iterator`] over query results of a [`Query`](crate::system::Query).
 ///
 /// This struct is created by the [`Query::iter_many`](crate::system::Query::iter_many) method.
-pub struct QueryManyIter<
-    'w,
-    's,
-    Q: WorldQuery,
-    QF: Fetch<'w, State = Q::State>,
-    F: WorldQuery,
-    I: Iterator,
-> where
+pub struct QueryManyIter<'w, 's, Q: WorldQuery, F: WorldQuery, I: Iterator>
+where
     I::Item: Borrow<Entity>,
 {
     entity_iter: I,
     entities: &'w Entities,
     tables: &'w Tables,
     archetypes: &'w Archetypes,
-    fetch: QF,
+    fetch: QueryFetch<'w, Q>,
     filter: QueryFetch<'w, F>,
     query_state: &'s QueryState<Q, F>,
 }
 
-impl<'w, 's, Q: WorldQuery, QF: Fetch<'w, State = Q::State>, F: WorldQuery, I: Iterator>
-    QueryManyIter<'w, 's, Q, QF, F, I>
+impl<'w, 's, Q: WorldQuery, F: WorldQuery, I: Iterator> QueryManyIter<'w, 's, Q, F, I>
 where
     I::Item: Borrow<Entity>,
 {
@@ -119,14 +103,14 @@ where
         entity_list: EntityList,
         last_change_tick: u32,
         change_tick: u32,
-    ) -> QueryManyIter<'w, 's, Q, QF, F, I> {
-        let fetch = QF::init(
+    ) -> QueryManyIter<'w, 's, Q, F, I> {
+        let fetch = Q::Fetch::init(
             world,
             &query_state.fetch_state,
             last_change_tick,
             change_tick,
         );
-        let filter = QueryFetch::<F>::init(
+        let filter = F::Fetch::init(
             world,
             &query_state.filter_state,
             last_change_tick,
@@ -144,12 +128,11 @@ where
     }
 }
 
-impl<'w, 's, Q: WorldQuery, QF: Fetch<'w, State = Q::State>, F: WorldQuery, I: Iterator> Iterator
-    for QueryManyIter<'w, 's, Q, QF, F, I>
+impl<'w, 's, Q: WorldQuery, F: WorldQuery, I: Iterator> Iterator for QueryManyIter<'w, 's, Q, F, I>
 where
     I::Item: Borrow<Entity>,
 {
-    type Item = QF::Item;
+    type Item = QueryItem<'w, Q>;
 
     #[inline(always)]
     fn next(&mut self) -> Option<Self::Item> {
@@ -201,7 +184,7 @@ pub struct QueryCombinationIter<'w, 's, Q: WorldQuery, F: WorldQuery, const K: u
     tables: &'w Tables,
     archetypes: &'w Archetypes,
     query_state: &'s QueryState<Q, F>,
-    cursors: [QueryIterationCursor<'w, 's, Q, QueryFetch<'w, Q>, F>; K],
+    cursors: [QueryIterationCursor<'w, 's, Q, F>; K],
 }
 
 impl<'w, 's, Q: WorldQuery, F: WorldQuery, const K: usize> QueryCombinationIter<'w, 's, Q, F, K> {
@@ -219,11 +202,10 @@ impl<'w, 's, Q: WorldQuery, F: WorldQuery, const K: usize> QueryCombinationIter<
         // Initialize array with cursors.
         // There is no FromIterator on arrays, so instead initialize it manually with MaybeUninit
 
-        let mut array: MaybeUninit<[QueryIterationCursor<'w, 's, Q, QueryFetch<'w, Q>, F>; K]> =
-            MaybeUninit::uninit();
+        let mut array: MaybeUninit<[QueryIterationCursor<'w, 's, Q, F>; K]> = MaybeUninit::uninit();
         let ptr = array
             .as_mut_ptr()
-            .cast::<QueryIterationCursor<'w, 's, Q, QueryFetch<'w, Q>, F>>();
+            .cast::<QueryIterationCursor<'w, 's, Q, F>>();
         if K != 0 {
             ptr.write(QueryIterationCursor::init(
                 world,
@@ -367,10 +349,9 @@ where
     }
 }
 
-impl<'w, 's, Q: WorldQuery, QF, F> ExactSizeIterator for QueryIter<'w, 's, Q, QF, F>
+impl<'w, 's, Q: WorldQuery, F: WorldQuery> ExactSizeIterator for QueryIter<'w, 's, Q, F>
 where
-    QF: Fetch<'w, State = Q::State>,
-    F: WorldQuery + ArchetypeFilter,
+    F: ArchetypeFilter,
 {
     fn len(&self) -> usize {
         self.query_state
@@ -405,21 +386,21 @@ where
 {
 }
 
-struct QueryIterationCursor<'w, 's, Q: WorldQuery, QF: Fetch<'w, State = Q::State>, F: WorldQuery> {
+struct QueryIterationCursor<'w, 's, Q: WorldQuery, F: WorldQuery> {
     table_id_iter: std::slice::Iter<'s, TableId>,
     archetype_id_iter: std::slice::Iter<'s, ArchetypeId>,
-    fetch: QF,
+    fetch: QueryFetch<'w, Q>,
     filter: QueryFetch<'w, F>,
     // length of the table table or length of the archetype, depending on whether both `Q`'s and `F`'s fetches are dense
     current_len: usize,
     // either table row or archetype index, depending on whether both `Q`'s and `F`'s fetches are dense
     current_index: usize,
-    phantom: PhantomData<(&'w (), Q)>,
+    phantom: PhantomData<Q>,
 }
 
-impl<'w, 's, Q: WorldQuery, QF, F: WorldQuery> Clone for QueryIterationCursor<'w, 's, Q, QF, F>
+impl<'w, 's, Q: WorldQuery, F: WorldQuery> Clone for QueryIterationCursor<'w, 's, Q, F>
 where
-    QF: Fetch<'w, State = Q::State> + Clone,
+    QueryFetch<'w, Q>: Clone,
     QueryFetch<'w, F>: Clone,
 {
     fn clone(&self) -> Self {
@@ -435,11 +416,8 @@ where
     }
 }
 
-impl<'w, 's, Q: WorldQuery, QF, F: WorldQuery> QueryIterationCursor<'w, 's, Q, QF, F>
-where
-    QF: Fetch<'w, State = Q::State>,
-{
-    const IS_DENSE: bool = QF::IS_DENSE && <QueryFetch<'static, F>>::IS_DENSE;
+impl<'w, 's, Q: WorldQuery, F: WorldQuery> QueryIterationCursor<'w, 's, Q, F> {
+    const IS_DENSE: bool = Q::Fetch::IS_DENSE && F::Fetch::IS_DENSE;
 
     unsafe fn init_empty(
         world: &'w World,
@@ -460,13 +438,13 @@ where
         last_change_tick: u32,
         change_tick: u32,
     ) -> Self {
-        let fetch = QF::init(
+        let fetch = Q::Fetch::init(
             world,
             &query_state.fetch_state,
             last_change_tick,
             change_tick,
         );
-        let filter = QueryFetch::<F>::init(
+        let filter = F::Fetch::init(
             world,
             &query_state.filter_state,
             last_change_tick,
@@ -485,7 +463,7 @@ where
 
     /// retrieve item returned from most recent `next` call again.
     #[inline]
-    unsafe fn peek_last(&mut self) -> Option<QF::Item> {
+    unsafe fn peek_last(&mut self) -> Option<QueryItem<'w, Q>> {
         if self.current_index > 0 {
             if Self::IS_DENSE {
                 Some(self.fetch.table_fetch(self.current_index - 1))
@@ -509,7 +487,7 @@ where
         tables: &'w Tables,
         archetypes: &'w Archetypes,
         query_state: &'s QueryState<Q, F>,
-    ) -> Option<QF::Item> {
+    ) -> Option<QueryItem<'w, Q>> {
         if Self::IS_DENSE {
             loop {
                 // we are on the beginning of the query, or finished processing a table, so skip to the next

crates/bevy_ecs/src/query/mod.rs

@@ -21,8 +21,8 @@ pub(crate) unsafe fn debug_checked_unreachable() -> ! {
 mod tests {
     use super::WorldQuery;
     use crate::prelude::{AnyOf, Entity, Or, QueryState, With, Without};
-    use crate::query::{ArchetypeFilter, QueryCombinationIter, QueryFetch, ReadOnlyWorldQuery};
-    use crate::system::{IntoSystem, Query, System};
+    use crate::query::{ArchetypeFilter, QueryCombinationIter, QueryFetch};
+    use crate::system::{IntoSystem, Query, System, SystemState};
     use crate::{self as bevy_ecs, component::Component, world::World};
     use std::any::type_name;
     use std::collections::HashSet;
@@ -67,10 +67,11 @@ mod tests {
         }
         fn assert_combination<Q, F, const K: usize>(world: &mut World, expected_size: usize)
         where
-            Q: ReadOnlyWorldQuery,
-            F: ReadOnlyWorldQuery + ArchetypeFilter,
-            for<'w> QueryFetch<'w, Q>: Clone,
-            for<'w> QueryFetch<'w, F>: Clone,
+            Q: WorldQuery,
+            F: WorldQuery,
+            F::ReadOnly: ArchetypeFilter,
+            for<'w> QueryFetch<'w, Q::ReadOnly>: Clone,
+            for<'w> QueryFetch<'w, F::ReadOnly>: Clone,
         {
             let mut query = world.query_filtered::<Q, F>();
             let iter = query.iter_combinations::<K>(world);
@@ -79,10 +80,11 @@ mod tests {
         }
         fn assert_all_sizes_equal<Q, F>(world: &mut World, expected_size: usize)
         where
-            Q: ReadOnlyWorldQuery,
-            F: ReadOnlyWorldQuery + ArchetypeFilter,
-            for<'w> QueryFetch<'w, Q>: Clone,
-            for<'w> QueryFetch<'w, F>: Clone,
+            Q: WorldQuery,
+            F: WorldQuery,
+            F::ReadOnly: ArchetypeFilter,
+            for<'w> QueryFetch<'w, Q::ReadOnly>: Clone,
+            for<'w> QueryFetch<'w, F::ReadOnly>: Clone,
         {
             let mut query = world.query_filtered::<Q, F>();
             let iter = query.iter(world);
@@ -653,4 +655,42 @@ count():       {count}"#
             system.run((), &mut world);
         }
     }
+
+    #[test]
+    fn mut_to_immut_query_methods_have_immut_item() {
+        #[derive(Component)]
+        struct Foo;
+
+        let mut world = World::new();
+        let e = world.spawn().insert(Foo).id();
+
+        // state
+        let mut q = world.query::<&mut Foo>();
+        let _: Option<&Foo> = q.iter(&world).next();
+        let _: Option<[&Foo; 2]> = q.iter_combinations::<2>(&world).next();
+        let _: Option<&Foo> = q.iter_manual(&world).next();
+        let _: Option<&Foo> = q.iter_many(&world, [e]).next();
+        q.for_each(&world, |_: &Foo| ());
+
+        let _: Option<&Foo> = q.get(&world, e).ok();
+        let _: Option<&Foo> = q.get_manual(&world, e).ok();
+        let _: Option<[&Foo; 1]> = q.get_many(&world, [e]).ok();
+        let _: Option<&Foo> = q.get_single(&world).ok();
+        let _: &Foo = q.single(&world);
+
+        // system param
+        let mut q = SystemState::<Query<&mut Foo>>::new(&mut world);
+        let q = q.get_mut(&mut world);
+        let _: Option<&Foo> = q.iter().next();
+        let _: Option<[&Foo; 2]> = q.iter_combinations::<2>().next();
+        let _: Option<&Foo> = q.iter_many([e]).next();
+        q.for_each(|_: &Foo| ());
+
+        let _: Option<&Foo> = q.get(e).ok();
+        let _: Option<&Foo> = q.get_component(e).ok();
+        let _: Option<[&Foo; 1]> = q.get_many([e]).ok();
+        let _: Option<&Foo> = q.get_single().ok();
+        let _: [&Foo; 1] = q.many([e]);
+        let _: &Foo = q.single();
+    }
 }