Populated (query) system param (#15488)

# Objective

Add a `Populated` system parameter that acts like `Query`, but prevents
system from running if there are no matching entities.

Fixes: #15302

## Solution

Implement the system param which newtypes the `Query`.
The only change is new validation, which fails if query is empty.

The new system param is used in `fallible_params` example.

## Testing

Ran `fallible_params` example.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_ecs/src/lib.rs

@@ -58,8 +58,8 @@ pub mod prelude {
         },
         system::{
             Commands, Deferred, EntityCommand, EntityCommands, In, InMut, InRef, IntoSystem, Local,
-            NonSend, NonSendMut, ParallelCommands, ParamSet, Query, ReadOnlySystem, Res, ResMut,
+            NonSend, NonSendMut, ParallelCommands, ParamSet, Populated, Query, ReadOnlySystem, Res,
-            Resource, Single, System, SystemIn, SystemInput, SystemParamBuilder,
+            ResMut, Resource, Single, System, SystemIn, SystemInput, SystemParamBuilder,
             SystemParamFunction,
         },
         world::{

crates/bevy_ecs/src/system/query.rs

@@ -33,6 +33,15 @@ use core::{
 ///
 /// [`World`]: crate::world::World
 ///
+/// # Similar parameters
+///
+/// [`Query`] has few sibling [`SystemParam`](crate::system::system_param::SystemParam)s, which perform additional validation:
+/// - [`Single`] - Exactly one matching query item.
+/// - [`Option<Single>`] - Zero or one matching query item.
+/// - [`Populated`] - At least one matching query item.
+///
+/// Those parameters will prevent systems from running if their requirements aren't met.
+///
 /// # System parameter declaration
 ///
 /// A query should always be declared as a system parameter.
@@ -1667,3 +1676,36 @@ impl<'w, D: QueryData, F: QueryFilter> Single<'w, D, F> {
         self.item
     }
 }
+
+/// [System parameter] that works very much like [`Query`] except it always contains at least one matching entity.
+///
+/// This [`SystemParam`](crate::system::SystemParam) fails validation if no matching entities exist.
+/// This will cause systems that use this parameter to be skipped.
+///
+/// Much like [`Query::is_empty`] the worst case runtime will be `O(n)` where `n` is the number of *potential* matches.
+/// This can be notably expensive for queries that rely on non-archetypal filters such as [`Added`](crate::query::Added) or [`Changed`](crate::query::Changed)
+/// which must individually check each query result for a match.
+///
+/// See [`Query`] for more details.
+pub struct Populated<'w, 's, D: QueryData, F: QueryFilter = ()>(pub(crate) Query<'w, 's, D, F>);
+
+impl<'w, 's, D: QueryData, F: QueryFilter> Deref for Populated<'w, 's, D, F> {
+    type Target = Query<'w, 's, D, F>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<D: QueryData, F: QueryFilter> DerefMut for Populated<'_, '_, D, F> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<'w, 's, D: QueryData, F: QueryFilter> Populated<'w, 's, D, F> {
+    /// Returns the inner item with ownership.
+    pub fn into_inner(self) -> Query<'w, 's, D, F> {
+        self.0
+    }
+}

crates/bevy_ecs/src/system/system_param.rs

@@ -26,6 +26,8 @@ use core::{
     ops::{Deref, DerefMut},
 };
 
+use super::Populated;
+
 /// A parameter that can be used in a [`System`](super::System).
 ///
 /// # Derive
@@ -497,6 +499,61 @@ unsafe impl<'a, D: ReadOnlyQueryData + 'static, F: QueryFilter + 'static> ReadOn
 {
 }
 
+// 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<D: QueryData + 'static, F: QueryFilter + 'static> SystemParam
+    for Populated<'_, '_, D, F>
+{
+    type State = QueryState<D, F>;
+    type Item<'w, 's> = Populated<'w, 's, 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> {
+        // SAFETY: Delegate to existing `SystemParam` implementations.
+        let query = unsafe { Query::get_param(state, system_meta, world, change_tick) };
+        Populated(query)
+    }
+
+    #[inline]
+    unsafe fn validate_param(
+        state: &Self::State,
+        system_meta: &SystemMeta,
+        world: UnsafeWorldCell,
+    ) -> bool {
+        state.validate_world(world.id());
+        // SAFETY:
+        // - We have read-only access to the components accessed by query.
+        // - The world has been validated.
+        !unsafe {
+            state.is_empty_unsafe_world_cell(world, system_meta.last_run, world.change_tick())
+        }
+    }
+}
+
+// SAFETY: QueryState is constrained to read-only fetches, so it only reads World.
+unsafe impl<'w, 's, D: ReadOnlyQueryData + 'static, F: QueryFilter + 'static> ReadOnlySystemParam
+    for Populated<'w, 's, 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

examples/ecs/fallible_params.rs

@@ -2,9 +2,10 @@
 //! from running if their acquiry conditions aren't met.
 //!
 //! Fallible parameters include:
-//! - [`Res<R>`], [`ResMut<R>`] - If resource doesn't exist.
+//! - [`Res<R>`], [`ResMut<R>`] - Resource has to exist.
-//! - [`Single<D, F>`] - If there is no or more than one entities matching.
+//! - [`Single<D, F>`] - There must be exactly one matching entity.
-//! - [`Option<Single<D, F>>`] - If there are more than one entities matching.
+//! - [`Option<Single<D, F>>`] - There must be zero or one matching entity.
+//! - [`Populated<D, F>`] - There must be at least one matching entity.
 
 use bevy::prelude::*;
 use rand::Rng;
@@ -105,9 +106,9 @@ fn user_input(
 }
 
 // System that moves the enemies in a circle.
-// TODO: Use [`NonEmptyQuery`] when it exists.
+// Only runs if there are enemies.
-fn move_targets(mut enemies: Query<(&mut Transform, &mut Enemy)>, time: Res<Time>) {
+fn move_targets(mut enemies: Populated<(&mut Transform, &mut Enemy)>, time: Res<Time>) {
-    for (mut transform, mut target) in &mut enemies {
+    for (mut transform, mut target) in &mut *enemies {
         target.rotation += target.rotation_speed * time.delta_seconds();
         transform.rotation = Quat::from_rotation_z(target.rotation);
         let offset = transform.right() * target.radius;