Implement WorldQuery for EntityRef (#6960)

# Objective
Partially address #5504. Fix #4278. Provide "whole entity" access in
queries. This can be useful when you don't know at compile time what
you're accessing (i.e. reflection via `ReflectComponent`).

## Solution
Implement `WorldQuery` for `EntityRef`. 

- This provides read-only access to the entire entity, and supports
anything that `EntityRef` can normally do.
- It matches all archetypes and tables and will densely iterate when
possible.
- It marks all of the ArchetypeComponentIds of a matched archetype as
read.
- Adding it to a query will cause it to panic if used in conjunction
with any other mutable access.
 - Expanded the docs on Query to advertise this feature.
 - Added tests to ensure the panics were working as intended.
 - Added `EntityRef` to the ECS prelude.

To make this safe, `EntityRef::world` was removed as it gave potential
`UnsafeCell`-like access to other parts of the `World` including aliased
mutable access to the components it would otherwise read safely.

## Performance
Not great beyond the additional parallelization opportunity over
exclusive systems. The `EntityRef` is fetched from `Entities` like any
other call to `World::entity`, which can be very random access heavy.
This could be simplified if `ArchetypeRow` is available in
`WorldQuery::fetch`'s arguments, but that's likely not something we
should optimize for.

## Future work
An equivalent API where it gives mutable access to all components on a
entity can be done with a scoped version of `EntityMut` where it does
not provide `&mut World` access nor allow for structural changes to the
entity is feasible as well. This could be done as a safe alternative to
exclusive system when structural mutation isn't required or the target
set of entities is scoped.

---

## Changelog
Added: `Access::has_any_write`
Added: `EntityRef` now implements `WorldQuery`. Allows read-only access
to the entire entity, incompatible with any other mutable access, can be
mixed with `With`/`Without` filters for more targeted use.
Added: `EntityRef` to `bevy::ecs::prelude`.
Removed: `EntityRef::world`

## Migration Guide
TODO

---------

Co-authored-by: Carter Weinberg <weinbergcarter@gmail.com>
Co-authored-by: Jakob Hellermann <jakob.hellermann@protonmail.com>
Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_ecs/src/lib.rs

@@ -50,7 +50,7 @@ pub mod prelude {
             Commands, Deferred, In, IntoSystem, Local, NonSend, NonSendMut, ParallelCommands,
             ParamSet, Query, ReadOnlySystem, Res, ResMut, Resource, System, SystemParamFunction,
         },
-        world::{FromWorld, World},
+        world::{EntityRef, FromWorld, World},
     };
 }
 
@@ -70,7 +70,7 @@ mod tests {
         entity::Entity,
         query::{Added, Changed, FilteredAccess, ReadOnlyWorldQuery, With, Without},
         system::Resource,
-        world::{Mut, World},
+        world::{EntityRef, Mut, World},
     };
     use bevy_tasks::{ComputeTaskPool, TaskPool};
     use std::{
@@ -1323,13 +1323,27 @@ mod tests {
         world.query::<(&A, &mut A)>();
     }
 
+    #[test]
+    #[should_panic]
+    fn entity_ref_and_mut_query_panic() {
+        let mut world = World::new();
+        world.query::<(EntityRef, &mut A)>();
+    }
+
     #[test]
     #[should_panic]
     fn mut_and_ref_query_panic() {
         let mut world = World::new();
         world.query::<(&mut A, &A)>();
     }
 
+    #[test]
+    #[should_panic]
+    fn mut_and_entity_ref_query_panic() {
+        let mut world = World::new();
+        world.query::<(&mut A, EntityRef)>();
+    }
+
     #[test]
     #[should_panic]
     fn mut_and_mut_query_panic() {

crates/bevy_ecs/src/query/access.rs

@@ -121,6 +121,11 @@ impl<T: SparseSetIndex> Access<T> {
         self.writes.contains(index.sparse_set_index())
     }
 
+    /// Returns `true` if this accesses anything mutably.
+    pub fn has_any_write(&self) -> bool {
+        !self.writes.is_clear()
+    }
+
     /// Sets this as having access to all indexed elements (i.e. `&World`).
     pub fn read_all(&mut self) {
         self.reads_all = true;

crates/bevy_ecs/src/query/fetch.rs

@@ -5,7 +5,7 @@ use crate::{
     entity::Entity,
     query::{Access, DebugCheckedUnwrap, FilteredAccess},
     storage::{ComponentSparseSet, Table, TableRow},
-    world::{unsafe_world_cell::UnsafeWorldCell, Mut, Ref, World},
+    world::{unsafe_world_cell::UnsafeWorldCell, EntityRef, Mut, Ref, World},
 };
 pub use bevy_ecs_macros::WorldQuery;
 use bevy_ptr::{ThinSlicePtr, UnsafeCellDeref};
@@ -536,6 +536,89 @@ unsafe impl WorldQuery for Entity {
 /// SAFETY: access is read only
 unsafe impl ReadOnlyWorldQuery for Entity {}
 
+/// SAFETY: `Self` is the same as `Self::ReadOnly`
+unsafe impl<'a> WorldQuery for EntityRef<'a> {
+    type Fetch<'w> = &'w World;
+    type Item<'w> = EntityRef<'w>;
+    type ReadOnly = Self;
+    type State = ();
+
+    fn shrink<'wlong: 'wshort, 'wshort>(item: Self::Item<'wlong>) -> Self::Item<'wshort> {
+        item
+    }
+
+    const IS_DENSE: bool = true;
+
+    const IS_ARCHETYPAL: bool = true;
+
+    unsafe fn init_fetch<'w>(
+        world: UnsafeWorldCell<'w>,
+        _state: &Self::State,
+        _last_run: Tick,
+        _this_run: Tick,
+    ) -> Self::Fetch<'w> {
+        // SAFE: EntityRef has permission to access the whole world immutably thanks to update_component_access and update_archetype_component_access
+        world.world()
+    }
+
+    unsafe fn clone_fetch<'w>(world: &Self::Fetch<'w>) -> Self::Fetch<'w> {
+        world
+    }
+
+    #[inline]
+    unsafe fn set_archetype<'w>(
+        _fetch: &mut Self::Fetch<'w>,
+        _state: &Self::State,
+        _archetype: &'w Archetype,
+        _table: &Table,
+    ) {
+    }
+
+    #[inline]
+    unsafe fn set_table<'w>(_fetch: &mut Self::Fetch<'w>, _state: &Self::State, _table: &'w Table) {
+    }
+
+    #[inline(always)]
+    unsafe fn fetch<'w>(
+        world: &mut Self::Fetch<'w>,
+        entity: Entity,
+        _table_row: TableRow,
+    ) -> Self::Item<'w> {
+        // SAFETY: `fetch` must be called with an entity that exists in the world
+        unsafe { world.get_entity(entity).debug_checked_unwrap() }
+    }
+
+    fn update_component_access(_state: &Self::State, access: &mut FilteredAccess<ComponentId>) {
+        assert!(
+            !access.access().has_any_write(),
+            "EntityRef conflicts with a previous access in this query. Shared access cannot coincide with exclusive access.",
+        );
+        access.read_all();
+    }
+
+    fn update_archetype_component_access(
+        _state: &Self::State,
+        archetype: &Archetype,
+        access: &mut Access<ArchetypeComponentId>,
+    ) {
+        for component_id in archetype.components() {
+            access.add_read(archetype.get_archetype_component_id(component_id).unwrap());
+        }
+    }
+
+    fn init_state(_world: &mut World) {}
+
+    fn matches_component_set(
+        _state: &Self::State,
+        _set_contains_id: &impl Fn(ComponentId) -> bool,
+    ) -> bool {
+        true
+    }
+}
+
+/// SAFETY: access is read only
+unsafe impl<'a> ReadOnlyWorldQuery for EntityRef<'a> {}
+
 #[doc(hidden)]
 pub struct ReadFetch<'w, T> {
     // T::Storage = TableStorage

crates/bevy_ecs/src/system/mod.rs

@@ -125,71 +125,6 @@ pub use system_param::*;
 
 use crate::world::World;
 
-/// Ensure that a given function is a [system](System).
-///
-/// This should be used when writing doc examples,
-/// to confirm that systems used in an example are
-/// valid systems.
-///
-/// # Examples
-///
-/// The following example will panic when run since the
-/// system's parameters mutably access the same component
-/// multiple times.
-///
-/// ```should_panic
-/// # use bevy_ecs::{prelude::*, system::assert_is_system};
-/// #
-/// # #[derive(Component)]
-/// # struct Transform;
-/// #
-/// fn my_system(query1: Query<&mut Transform>, query2: Query<&mut Transform>) {
-///     // ...
-/// }
-///
-/// assert_is_system(my_system);
-/// ```
-pub fn assert_is_system<In: 'static, Out: 'static, Marker>(
-    system: impl IntoSystem<In, Out, Marker>,
-) {
-    let mut system = IntoSystem::into_system(system);
-
-    // Initialize the system, which will panic if the system has access conflicts.
-    let mut world = World::new();
-    system.initialize(&mut world);
-}
-
-/// Ensure that a given function is a [read-only system](ReadOnlySystem).
-///
-/// This should be used when writing doc examples,
-/// to confirm that systems used in an example are
-/// valid systems.
-///
-/// # Examples
-///
-/// The following example will fail to compile
-/// since the system accesses a component mutably.
-///
-/// ```compile_fail
-/// # use bevy_ecs::{prelude::*, system::assert_is_read_only_system};
-/// #
-/// # #[derive(Component)]
-/// # struct Transform;
-/// #
-/// fn my_system(query: Query<&mut Transform>) {
-///     // ...
-/// }
-///
-/// assert_is_read_only_system(my_system);
-/// ```
-pub fn assert_is_read_only_system<In: 'static, Out: 'static, Marker, S>(system: S)
-where
-    S: IntoSystem<In, Out, Marker>,
-    S::System: ReadOnlySystem,
-{
-    assert_is_system(system);
-}
-
 /// Conversion trait to turn something into a [`System`].
 ///
 /// Use this to get a system from a function. Also note that every system implements this trait as
@@ -477,6 +412,83 @@ pub mod adapter {
     pub fn ignore<T>(In(_): In<T>) {}
 }
 
+/// Ensure that a given function is a [system](System).
+///
+/// This should be used when writing doc examples,
+/// to confirm that systems used in an example are
+/// valid systems.
+///
+/// # Examples
+///
+/// The following example will panic when run since the
+/// system's parameters mutably access the same component
+/// multiple times.
+///
+/// ```should_panic
+/// # use bevy_ecs::{prelude::*, system::assert_is_system};
+/// #
+/// # #[derive(Component)]
+/// # struct Transform;
+/// #
+/// fn my_system(query1: Query<&mut Transform>, query2: Query<&mut Transform>) {
+///     // ...
+/// }
+///
+/// assert_is_system(my_system);
+/// ```
+pub fn assert_is_system<In: 'static, Out: 'static, Marker>(
+    system: impl IntoSystem<In, Out, Marker>,
+) {
+    let mut system = IntoSystem::into_system(system);
+
+    // Initialize the system, which will panic if the system has access conflicts.
+    let mut world = World::new();
+    system.initialize(&mut world);
+}
+
+/// Ensure that a given function is a [read-only system](ReadOnlySystem).
+///
+/// This should be used when writing doc examples,
+/// to confirm that systems used in an example are
+/// valid systems.
+///
+/// # Examples
+///
+/// The following example will fail to compile
+/// since the system accesses a component mutably.
+///
+/// ```compile_fail
+/// # use bevy_ecs::{prelude::*, system::assert_is_read_only_system};
+/// #
+/// # #[derive(Component)]
+/// # struct Transform;
+/// #
+/// fn my_system(query: Query<&mut Transform>) {
+///     // ...
+/// }
+///
+/// assert_is_read_only_system(my_system);
+/// ```
+pub fn assert_is_read_only_system<In: 'static, Out: 'static, Marker, S>(system: S)
+where
+    S: IntoSystem<In, Out, Marker>,
+    S::System: ReadOnlySystem,
+{
+    assert_is_system(system);
+}
+
+/// Ensures that the provided system doesn't with itself.
+///
+/// This function will  panic if the provided system conflict with itself.
+///
+/// Note: this will run the system on an empty world.
+pub fn assert_system_does_not_conflict<Out, Params, S: IntoSystem<(), Out, Params>>(sys: S) {
+    let mut world = World::new();
+    let mut system = IntoSystem::into_system(sys);
+    system.initialize(&mut world);
+    system.run((), &mut world);
+}
+
 #[cfg(test)]
 mod tests {
     use std::any::TypeId;
@@ -1739,6 +1751,13 @@ mod tests {
         query.iter();
     }
 
+    #[test]
+    #[should_panic]
+    fn assert_system_does_not_conflict() {
+        fn system(_query: Query<(&mut W<u32>, &mut W<u32>)>) {}
+        super::assert_system_does_not_conflict(system);
+    }
+
     #[test]
     #[should_panic]
     fn panic_inside_system() {

crates/bevy_ecs/src/system/query.rs

@@ -183,6 +183,52 @@ use std::{any::TypeId, borrow::Borrow, fmt::Debug};
 ///
 /// An alternative to this idiom is to wrap the conflicting queries into a [`ParamSet`](super::ParamSet).
 ///
+/// ## Whole Entity Access
+///
+/// [`EntityRef`]s can be fetched from a query. This will give read-only access to any component on the entity,
+/// and can be use to dynamically fetch any component without baking it into the query type. Due to this global
+/// access to the entity, this will block any other system from parallelizing with it. As such these queries
+/// should be sparingly used.
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// # #[derive(Component)]
+/// # struct ComponentA;
+/// # fn system(
+/// query: Query<(EntityRef, &ComponentA)>
+/// # ) {}
+/// # bevy_ecs::system::assert_is_system(system);
+/// ```
+///
+/// As `EntityRef` can read any component on an entity, a query using it will conflict with *any* mutable
+/// access. It is strongly advised to couple `EntityRef` queries with the use of either `With`/`Without`
+/// filters or `ParamSets`. This also limits the scope of the query, which will improve iteration performance
+/// and also allows it to parallelize with other non-conflicting systems.
+///
+/// ```should_panic
+/// # use bevy_ecs::prelude::*;
+/// # #[derive(Component)]
+/// # struct ComponentA;
+/// # fn system(
+/// // This will panic!
+/// query: Query<(EntityRef, &mut ComponentA)>
+/// # ) {}
+/// # bevy_ecs::system::assert_system_does_not_conflict(system);
+/// ```
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// # #[derive(Component)]
+/// # struct ComponentA;
+/// # #[derive(Component)]
+/// # struct ComponentB;
+/// # fn system(
+/// // This will not panic.
+/// query_a: Query<EntityRef, With<ComponentA>>,
+/// query_b: Query<&mut ComponentB, Without<ComponentA>>,
+/// # ) {}
+/// # bevy_ecs::system::assert_system_does_not_conflict(system);
+/// ```
+///
 /// # Accessing query items
 ///
 /// The following table summarizes the behavior of the safe methods that can be used to get query items.
@@ -248,6 +294,7 @@ use std::{any::TypeId, borrow::Borrow, fmt::Debug};
 /// [`Changed`]: crate::query::Changed
 /// [components]: crate::component::Component
 /// [entity identifiers]: crate::entity::Entity
+/// [`EntityRef`]: crate::world::EntityRef
 /// [`for_each`]: Self::for_each
 /// [`for_each_mut`]: Self::for_each_mut
 /// [`get`]: Self::get