Add feature for disabling parallel executors in bevy_ecs

crates/bevy_ecs/Cargo.toml

@@ -11,6 +11,7 @@ categories = ["game-engines", "data-structures"]
 
 [features]
 trace = []
+single-threaded = []
 default = ["bevy_reflect"]
 
 [dependencies]

crates/bevy_ecs/src/query/state.rs

@@ -833,6 +833,28 @@ impl<Q: WorldQuery, F: ReadOnlyWorldQuery> QueryState<Q, F> {
         );
     }
 
+    #[inline]
+    #[cfg(feature = "single-threaded")]
+    pub fn par_for_each<'w, FN: Fn(ROQueryItem<'w, Q>) + Send + Sync + Clone>(
+        &mut self,
+        world: &'w World,
+        _batch_size: usize,
+        func: FN,
+    ) {
+        self.for_each(world, func);
+    }
+
+    #[inline]
+    #[cfg(feature = "single-threaded")]
+    pub fn par_for_each_mut<'w, FN: Fn(Q::Item<'w>) + Send + Sync + Clone>(
+        &mut self,
+        world: &'w mut World,
+        _batch_size: usize,
+        func: FN,
+    ) {
+        self.for_each_mut(world, func);
+    }
+
     /// Runs `func` on each query result in parallel.
     ///
     /// This can only be called for read-only queries, see [`Self::par_for_each_mut`] for
@@ -842,6 +864,7 @@ impl<Q: WorldQuery, F: ReadOnlyWorldQuery> QueryState<Q, F> {
     /// The [`ComputeTaskPool`] is not initialized. If using this from a query that is being
     /// initialized and run from the ECS scheduler, this should never panic.
     #[inline]
+    #[cfg(not(feature = "single-threaded"))]
     pub fn par_for_each<'w, FN: Fn(ROQueryItem<'w, Q>) + Send + Sync + Clone>(
         &mut self,
         world: &'w World,
@@ -867,6 +890,7 @@ impl<Q: WorldQuery, F: ReadOnlyWorldQuery> QueryState<Q, F> {
     /// The [`ComputeTaskPool`] is not initialized. If using this from a query that is being
     /// initialized and run from the ECS scheduler, this should never panic.
     #[inline]
+    #[cfg(not(feature = "single-threaded"))]
     pub fn par_for_each_mut<'w, FN: Fn(Q::Item<'w>) + Send + Sync + Clone>(
         &mut self,
         world: &'w mut World,

crates/bevy_ecs/src/schedule/stage.rs

@@ -6,12 +6,14 @@ use crate::{
     schedule::{
         graph_utils::{self, DependencyGraphError},
         BoxedRunCriteria, DuplicateLabelStrategy, ExclusiveInsertionPoint, GraphNode,
-        ParallelExecutor, ParallelSystemExecutor, RunCriteriaContainer, RunCriteriaDescriptor,
+        ParallelSystemExecutor, RunCriteriaContainer, RunCriteriaDescriptor,
         RunCriteriaDescriptorOrLabel, RunCriteriaInner, RunCriteriaLabelId, ShouldRun,
         SingleThreadedExecutor, SystemContainer, SystemDescriptor, SystemLabelId, SystemSet,
     },
     world::{World, WorldId},
 };
+#[cfg(not(feature = "single-threaded"))]
+use crate::schedule::ParallelExecutor;
 use bevy_ecs_macros::Resource;
 use bevy_utils::{tracing::warn, HashMap, HashSet};
 use core::fmt::Debug;
@@ -136,7 +138,14 @@ impl SystemStage {
     }
 
     pub fn parallel() -> Self {
-        Self::new(Box::<ParallelExecutor>::default())
+        #[cfg(not(feature = "single-threaded"))]
+        {
+            Self::new(Box::<ParallelExecutor>::default())
+        }
+        #[cfg(feature = "single-threaded")]
+        {
+            Self::single_threaded()
+        }
     }
 
     pub fn get_executor<T: ParallelSystemExecutor>(&self) -> Option<&T> {