QuerySingle family of system params (bevyengine#15476)

# Objective

Add the following system params:
- `QuerySingle<D, F>` - Valid if only one matching entity exists,
- `Option<QuerySingle<D, F>>` - Valid if zero or one matching entity
exists.

As @chescock pointed out, we don't need `Mut` variants.

Fixes: bevyengine#15264

## Solution

Implement the type and both variants of system params.
Also implement `ReadOnlySystemParam` for readonly queries.

Added a new ECS example `fallible_params` which showcases `SingleQuery`
usage.
In the future we might want to add `NonEmptyQuery`,
`NonEmptyEventReader` and `Res` to it (or maybe just stop at mentioning
it).

## Testing

Tested with the example.
There is a lot of warning spam so we might want to implement bevyengine#15391.

Cargo.toml

@@ -1892,6 +1892,17 @@ description = "Run systems only when one or multiple conditions are met"
 category = "ECS (Entity Component System)"
 wasm = false
 
+[[example]]
+name = "fallible_params"
+path = "examples/ecs/fallible_params.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.fallible_params]
+name = "Fallible System Parameters"
+description = "Systems are skipped if their parameters cannot be acquired"
+category = "ECS (Entity Component System)"
+wasm = false
+
 [[example]]
 name = "startup_system"
 path = "examples/ecs/startup_system.rs"

crates/bevy_ecs/src/change_detection.rs

@@ -543,11 +543,10 @@ impl<'w> From<TicksMut<'w>> for Ticks<'w> {
 ///
 /// If you need a unique mutable borrow, use [`ResMut`] instead.
 ///
-/// # Panics
+/// This [`SystemParam`](crate::system::SystemParam) fails validation if resource doesn't exist.
+/// This will cause systems that use this parameter to be skipped.
 ///
-/// Panics when used as a [`SystemParameter`](crate::system::SystemParam) if the resource does not exist.
-///
-/// Use `Option<Res<T>>` instead if the resource might not always exist.
+/// Use [`Option<Res<T>>`] instead if the resource might not always exist.
 pub struct Res<'w, T: ?Sized + Resource> {
     pub(crate) value: &'w T,
     pub(crate) ticks: Ticks<'w>,
@@ -622,11 +621,10 @@ impl_debug!(Res<'w, T>, Resource);
 ///
 /// If you need a shared borrow, use [`Res`] instead.
 ///
-/// # Panics
-///
-/// Panics when used as a [`SystemParam`](crate::system::SystemParam) if the resource does not exist.
+/// This [`SystemParam`](crate::system::SystemParam) fails validation if resource doesn't exist.
+/// This will cause systems that use this parameter to be skipped.
 ///
-/// Use `Option<ResMut<T>>` instead if the resource might not always exist.
+/// Use [`Option<ResMut<T>>`] instead if the resource might not always exist.
 pub struct ResMut<'w, T: ?Sized + Resource> {
     pub(crate) value: &'w mut T,
     pub(crate) ticks: TicksMut<'w>,
@@ -684,11 +682,10 @@ impl<'w, T: Resource> From<ResMut<'w, T>> for Mut<'w, T> {
 /// the scheduler to instead run the system on the main thread so that it doesn't send the resource
 /// over to another thread.
 ///
-/// # Panics
-///
-/// Panics when used as a `SystemParameter` if the resource does not exist.
+/// This [`SystemParam`](crate::system::SystemParam) fails validation if non-send resource doesn't exist.
+/// This will cause systems that use this parameter to be skipped.
 ///
-/// Use `Option<NonSendMut<T>>` instead if the resource might not always exist.
+/// Use [`Option<NonSendMut<T>>`] instead if the resource might not always exist.
 pub struct NonSendMut<'w, T: ?Sized + 'static> {
     pub(crate) value: &'w mut T,
     pub(crate) ticks: TicksMut<'w>,

crates/bevy_ecs/src/lib.rs

@@ -58,8 +58,9 @@ pub mod prelude {
         },
         system::{
             Commands, Deferred, EntityCommand, EntityCommands, In, InMut, InRef, IntoSystem, Local,
-            NonSend, NonSendMut, ParallelCommands, ParamSet, Query, ReadOnlySystem, Res, ResMut,
-            Resource, System, SystemIn, SystemInput, SystemParamBuilder, SystemParamFunction,
+            NonSend, NonSendMut, ParallelCommands, ParamSet, Query, QuerySingle, ReadOnlySystem,
+            Res, ResMut, Resource, System, SystemIn, SystemInput, SystemParamBuilder,
+            SystemParamFunction,
         },
         world::{
             Command, EntityMut, EntityRef, EntityWorldMut, FromWorld, OnAdd, OnInsert, OnRemove,

crates/bevy_ecs/src/system/query.rs

@@ -8,7 +8,11 @@ use crate::{
     },
     world::unsafe_world_cell::UnsafeWorldCell,
 };
-use core::borrow::Borrow;
+use core::{
+    borrow::Borrow,
+    marker::PhantomData,
+    ops::{Deref, DerefMut},
+};
 
 /// [System parameter] that provides selective access to the [`Component`] data stored in a [`World`].
 ///
@@ -1629,3 +1633,37 @@ impl<'w, 'q, Q: QueryData, F: QueryFilter> From<&'q mut Query<'w, '_, Q, F>>
         value.transmute_lens_filtered()
     }
 }
+
+/// [System parameter] that provides access to single entity's components, much like [`Query::single`]/[`Query::single_mut`].
+///
+/// This [`SystemParam`](crate::system::SystemParam) fails validation if zero or more than one matching entity exists.
+/// This will cause systems that use this parameter to be skipped.
+///
+/// Use [`Option<QuerySingle<D, F>>`] instead if zero or one matching entities can exist.
+///
+/// See [`Query`] for more details.
+pub struct QuerySingle<'w, D: QueryData, F: QueryFilter = ()> {
+    pub(crate) item: D::Item<'w>,
+    pub(crate) _filter: PhantomData<F>,
+}
+
+impl<'w, D: QueryData, F: QueryFilter> Deref for QuerySingle<'w, D, F> {
+    type Target = D::Item<'w>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.item
+    }
+}
+
+impl<'w, D: QueryData, F: QueryFilter> DerefMut for QuerySingle<'w, D, F> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.item
+    }
+}
+
+impl<'w, D: QueryData, F: QueryFilter> QuerySingle<'w, D, F> {
+    /// Returns the inner item with ownership.
+    pub fn into_inner(self) -> D::Item<'w> {
+        self.item
+    }
+}

crates/bevy_ecs/src/system/system.rs

@@ -104,6 +104,10 @@ pub trait System: Send + Sync + 'static {
     /// is not a strict requirement, both [`System::run`] and [`System::run_unsafe`]
     /// should provide their own safety mechanism to prevent undefined behavior.
     ///
+    /// This method has to be called directly before [`System::run_unsafe`] with no other (relevant)
+    /// world mutations inbetween. Otherwise, while it won't lead to any undefined behavior,
+    /// the validity of the param may change.
+    ///
     /// # Safety
     ///
     /// - The caller must ensure that [`world`](UnsafeWorldCell) has permission to access any world data

crates/bevy_ecs/src/system/system_param.rs

@@ -7,10 +7,10 @@ use crate::{
     entity::Entities,
     query::{
         Access, AccessConflicts, FilteredAccess, FilteredAccessSet, QueryData, QueryFilter,
-        QueryState, ReadOnlyQueryData,
+        QuerySingleError, QueryState, ReadOnlyQueryData,
     },
     storage::{ResourceData, SparseSetIndex},
-    system::{Query, SystemMeta},
+    system::{Query, QuerySingle, SystemMeta},
     world::{unsafe_world_cell::UnsafeWorldCell, DeferredWorld, FromWorld, World},
 };
 use bevy_ecs_macros::impl_param_set;
@@ -227,12 +227,21 @@ pub unsafe trait SystemParam: Sized {
     /// The [`world`](UnsafeWorldCell) can only be used to read param's data
     /// and world metadata. No data can be written.
     ///
+    /// When using system parameters that require `change_tick` you can use
+    /// [`UnsafeWorldCell::change_tick()`]. Even if this isn't the exact
+    /// same tick used for [`SystemParam::get_param`], the world access
+    /// ensures that the queried data will be the same in both calls.
+    ///
+    /// This method has to be called directly before [`SystemParam::get_param`] with no other (relevant)
+    /// world mutations inbetween. Otherwise, while it won't lead to any undefined behavior,
+    /// the validity of the param may change.
+    ///
     /// # Safety
     ///
     /// - The passed [`UnsafeWorldCell`] must have read-only access to world data
     ///   registered in [`init_state`](SystemParam::init_state).
     /// - `world` must be the same [`World`] that was used to initialize [`state`](SystemParam::init_state).
-    /// - all `world`'s archetypes have been processed by [`new_archetype`](SystemParam::new_archetype).
+    /// - All `world`'s archetypes have been processed by [`new_archetype`](SystemParam::new_archetype).
     unsafe fn validate_param(
         _state: &Self::State,
         _system_meta: &SystemMeta,
@@ -356,6 +365,140 @@ fn assert_component_access_compatibility(
     panic!("error[B0001]: Query<{query_type}, {filter_type}> in system {system_name} accesses component(s){accesses} in a way that conflicts with a previous system parameter. Consider using `Without<T>` to create disjoint Queries or merging conflicting Queries into a `ParamSet`. See: https://bevyengine.org/learn/errors/b0001");
 }
 
+// SAFETY: Relevant query ComponentId and ArchetypeComponentId access is applied to SystemMeta. If
+// this Query conflicts with any prior access, a panic will occur.
+unsafe impl<'a, D: QueryData + 'static, F: QueryFilter + 'static> SystemParam
+    for QuerySingle<'a, D, F>
+{
+    type State = QueryState<D, F>;
+    type Item<'w, 's> = QuerySingle<'w, D, F>;
+
+    fn init_state(world: &mut World, system_meta: &mut SystemMeta) -> Self::State {
+        Query::init_state(world, system_meta)
+    }
+
+    unsafe fn new_archetype(
+        state: &mut Self::State,
+        archetype: &Archetype,
+        system_meta: &mut SystemMeta,
+    ) {
+        // SAFETY: Delegate to existing `SystemParam` implementations.
+        unsafe { Query::new_archetype(state, archetype, system_meta) };
+    }
+
+    #[inline]
+    unsafe fn get_param<'w, 's>(
+        state: &'s mut Self::State,
+        system_meta: &SystemMeta,
+        world: UnsafeWorldCell<'w>,
+        change_tick: Tick,
+    ) -> Self::Item<'w, 's> {
+        state.validate_world(world.id());
+        // SAFETY: State ensures that the components it accesses are not accessible somewhere elsewhere.
+        let result =
+            unsafe { state.get_single_unchecked_manual(world, system_meta.last_run, change_tick) };
+        let single =
+            result.expect("The query was expected to contain exactly one matching entity.");
+        QuerySingle {
+            item: single,
+            _filter: PhantomData,
+        }
+    }
+
+    #[inline]
+    unsafe fn validate_param(
+        state: &Self::State,
+        system_meta: &SystemMeta,
+        world: UnsafeWorldCell,
+    ) -> bool {
+        state.validate_world(world.id());
+        // SAFETY: State ensures that the components it accesses are not mutably accessible elsewhere
+        // and the query is read only.
+        let result = unsafe {
+            state.as_readonly().get_single_unchecked_manual(
+                world,
+                system_meta.last_run,
+                world.change_tick(),
+            )
+        };
+        result.is_ok()
+    }
+}
+
+// SAFETY: Relevant query ComponentId and ArchetypeComponentId access is applied to SystemMeta. If
+// this Query conflicts with any prior access, a panic will occur.
+unsafe impl<'a, D: QueryData + 'static, F: QueryFilter + 'static> SystemParam
+    for Option<QuerySingle<'a, D, F>>
+{
+    type State = QueryState<D, F>;
+    type Item<'w, 's> = Option<QuerySingle<'w, D, F>>;
+
+    fn init_state(world: &mut World, system_meta: &mut SystemMeta) -> Self::State {
+        QuerySingle::init_state(world, system_meta)
+    }
+
+    unsafe fn new_archetype(
+        state: &mut Self::State,
+        archetype: &Archetype,
+        system_meta: &mut SystemMeta,
+    ) {
+        // SAFETY: Delegate to existing `SystemParam` implementations.
+        unsafe { QuerySingle::new_archetype(state, archetype, system_meta) };
+    }
+
+    #[inline]
+    unsafe fn get_param<'w, 's>(
+        state: &'s mut Self::State,
+        system_meta: &SystemMeta,
+        world: UnsafeWorldCell<'w>,
+        change_tick: Tick,
+    ) -> Self::Item<'w, 's> {
+        state.validate_world(world.id());
+        // SAFETY: State ensures that the components it accesses are not accessible elsewhere.
+        let result =
+            unsafe { state.get_single_unchecked_manual(world, system_meta.last_run, change_tick) };
+        match result {
+            Ok(single) => Some(QuerySingle {
+                item: single,
+                _filter: PhantomData,
+            }),
+            Err(QuerySingleError::NoEntities(_)) => None,
+            Err(QuerySingleError::MultipleEntities(e)) => panic!("{}", e),
+        }
+    }
+
+    #[inline]
+    unsafe fn validate_param(
+        state: &Self::State,
+        system_meta: &SystemMeta,
+        world: UnsafeWorldCell,
+    ) -> bool {
+        state.validate_world(world.id());
+        // SAFETY: State ensures that the components it accesses are not mutably accessible elsewhere
+        // and the query is read only.
+        let result = unsafe {
+            state.as_readonly().get_single_unchecked_manual(
+                world,
+                system_meta.last_run,
+                world.change_tick(),
+            )
+        };
+        !matches!(result, Err(QuerySingleError::MultipleEntities(_)))
+    }
+}
+
+// SAFETY: QueryState is constrained to read-only fetches, so it only reads World.
+unsafe impl<'a, D: ReadOnlyQueryData + 'static, F: QueryFilter + 'static> ReadOnlySystemParam
+    for QuerySingle<'a, D, F>
+{
+}
+
+// SAFETY: QueryState is constrained to read-only fetches, so it only reads World.
+unsafe impl<'a, D: ReadOnlyQueryData + 'static, F: QueryFilter + 'static> ReadOnlySystemParam
+    for Option<QuerySingle<'a, D, F>>
+{
+}
+
 /// A collection of potentially conflicting [`SystemParam`]s allowed by disjoint access.
 ///
 /// Allows systems to safely access and interact with up to 8 mutually exclusive [`SystemParam`]s, such as
@@ -1172,11 +1315,10 @@ unsafe impl<T: SystemBuffer> SystemParam for Deferred<'_, T> {
 /// the scheduler to instead run the system on the main thread so that it doesn't send the resource
 /// over to another thread.
 ///
-/// # Panics
-///
-/// Panics when used as a `SystemParameter` if the resource does not exist.
+/// This [`SystemParam`] fails validation if non-send resource doesn't exist.
+/// This will cause systems that use this parameter to be skipped.
 ///
-/// Use `Option<NonSend<T>>` instead if the resource might not always exist.
+/// Use [`Option<NonSend<T>>`] instead if the resource might not always exist.
 pub struct NonSend<'w, T: 'static> {
     pub(crate) value: &'w T,
     ticks: ComponentTicks,

examples/README.md

@@ -288,6 +288,7 @@ Example | Description
 [Dynamic ECS](../examples/ecs/dynamic.rs) | Dynamically create components, spawn entities with those components and query those components
 [ECS Guide](../examples/ecs/ecs_guide.rs) | Full guide to Bevy's ECS
 [Event](../examples/ecs/event.rs) | Illustrates event creation, activation, and reception
+[Fallible System Parameters](../examples/ecs/fallible_params.rs) | Systems are skipped if their parameters cannot be acquired
 [Fixed Timestep](../examples/ecs/fixed_timestep.rs) | Shows how to create systems that run every fixed timestep, rather than every tick
 [Generic System](../examples/ecs/generic_system.rs) | Shows how to create systems that can be reused with different types
 [Hierarchy](../examples/ecs/hierarchy.rs) | Creates a hierarchy of parents and children entities