deprecate SimpleExecutor (#18753)

# Objective

Contributes to #18741 and #18453.

## Solution

Deprecate `SimpleExecutor`. If users run into migration issues, we can
backtrack. Otherwise, we follow this up with #18741

We can't easily deprecate the module too because of
[this](https://github.com/rust-lang/rust/issues/47238).

## Testing

CI

---------

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

benches/benches/bevy_ecs/scheduling/schedule.rs

@@ -137,6 +137,7 @@ pub fn empty_schedule_run(criterion: &mut Criterion) {
     });
 
     let mut schedule = Schedule::default();
+    #[expect(deprecated, reason = "We still need to test/bench this.")]
     schedule.set_executor_kind(bevy_ecs::schedule::ExecutorKind::Simple);
     group.bench_function("Simple", |bencher| {
         bencher.iter(|| schedule.run(app.world_mut()));

crates/bevy_ecs/src/schedule/executor/mod.rs

@@ -6,6 +6,7 @@ mod single_threaded;
 use alloc::{borrow::Cow, vec, vec::Vec};
 use core::any::TypeId;
 
+#[expect(deprecated, reason = "We still need to support this.")]
 pub use self::{simple::SimpleExecutor, single_threaded::SingleThreadedExecutor};
 
 #[cfg(feature = "std")]
@@ -53,6 +54,10 @@ pub enum ExecutorKind {
     SingleThreaded,
     /// Like [`SingleThreaded`](ExecutorKind::SingleThreaded) but calls [`apply_deferred`](crate::system::System::apply_deferred)
     /// immediately after running each system.
+    #[deprecated(
+        since = "0.17.0",
+        note = "Use SingleThreaded instead. See https://github.com/bevyengine/bevy/issues/18453 for motivation."
+    )]
     Simple,
     /// Runs the schedule using a thread pool. Non-conflicting systems can run in parallel.
     #[cfg(feature = "std")]
@@ -346,6 +351,7 @@ mod tests {
     struct TestComponent;
 
     const EXECUTORS: [ExecutorKind; 3] = [
+        #[expect(deprecated, reason = "We still need to test this.")]
         ExecutorKind::Simple,
         ExecutorKind::SingleThreaded,
         ExecutorKind::MultiThreaded,
@@ -406,6 +412,7 @@ mod tests {
         let mut world = World::new();
         let mut schedule = Schedule::default();
 
+        #[expect(deprecated, reason = "We still need to test this.")]
         schedule.set_executor_kind(ExecutorKind::Simple);
         schedule.add_systems(look_for_missing_resource);
         schedule.run(&mut world);

crates/bevy_ecs/src/schedule/executor/simple.rs

@@ -1,3 +1,5 @@
+#![expect(deprecated, reason = "Everything here is deprecated")]
+
 use core::panic::AssertUnwindSafe;
 use fixedbitset::FixedBitSet;
 
@@ -20,6 +22,10 @@ use super::__rust_begin_short_backtrace;
 /// A variant of [`SingleThreadedExecutor`](crate::schedule::SingleThreadedExecutor) that calls
 /// [`apply_deferred`](crate::system::System::apply_deferred) immediately after running each system.
 #[derive(Default)]
+#[deprecated(
+    since = "0.17.0",
+    note = "Use SingleThreadedExecutor instead. See https://github.com/bevyengine/bevy/issues/18453 for motivation."
+)]
 pub struct SimpleExecutor {
     /// Systems sets whose conditions have been evaluated.
     evaluated_sets: FixedBitSet,
@@ -165,7 +171,10 @@ impl SimpleExecutor {
         }
     }
 }
-
+#[deprecated(
+    since = "0.17.0",
+    note = "Use SingleThreadedExecutor instead. See https://github.com/bevyengine/bevy/issues/18453 for motivation."
+)]
 fn evaluate_and_fold_conditions(conditions: &mut [BoxedCondition], world: &mut World) -> bool {
     let error_handler = default_error_handler();
 

crates/bevy_ecs/src/schedule/mod.rs

@@ -1237,6 +1237,7 @@ mod tests {
 
         /// verify the [`SimpleExecutor`] supports stepping
         #[test]
+        #[expect(deprecated, reason = "We still need to test this.")]
         fn simple_executor() {
             assert_executor_supports_stepping!(ExecutorKind::Simple);
         }

crates/bevy_ecs/src/schedule/schedule.rs

@@ -218,6 +218,7 @@ impl Schedules {
 
 fn make_executor(kind: ExecutorKind) -> Box<dyn SystemExecutor> {
     match kind {
+        #[expect(deprecated, reason = "We still need to support this.")]
         ExecutorKind::Simple => Box::new(SimpleExecutor::new()),
         ExecutorKind::SingleThreaded => Box::new(SingleThreadedExecutor::new()),
         #[cfg(feature = "std")]

release-content/migration-guides/simple_executor_going_away.md

@@ -0,0 +1,25 @@
+---
+title: Deprecated Simple Executor
+pull_requests: [18753]
+---
+
+Bevy has deprecated `SimpleExecutor`, one of the `SystemExecutor`s in Bevy alongside `SingleThreadedExecutor` and `MultiThreadedExecutor` (which aren't going anywhere any time soon).
+
+The `SimpleExecutor` leaves performance on the table compared to the other executors in favor of simplicity.
+Specifically, `SimpleExecutor` applies any commands a system produces right after it finishes, so every system starts with a clean `World` with no pending commands.
+As a result, the default `SimpleExecutor` runs all systems in the order they are added to the schedule, though more ordering constraints can be applied, like `before`, `after`, `chain`, etc.
+In other executors, these ordering onstraints also inform the executor exactly where to apply commands.
+For example, if system `A` produces commands and runs `before` system `B`, `A`'s commands will be applied before `B` starts.
+However, the `before` ordering is implicit in `SimpleExecutor` if `A` is added to the schedule before `B`.
+
+The dueling behavior between ordering systems based on when they were added to a schedule as opposed to using ordering constraints is difficult to maintain and can be confusing, especially for new users.
+But, if you have a strong preference for the existing behavior of `SimpleExecutor`, please make an issue and we can discuss your needs.
+
+If you were using `SimpleExecutor`, consider upgrading to `SingleThreadedExecutor` instead, or try `MultiThreadedExecutor` if it fits the schedule.
+The `MultiThreadedExecutor` is great at large schedules and async heavy work, and the `SingleThreadedExecutor` is good at smaller schedules or schedules that have fewer parallelizable systems.
+So what was `SimpleExecutor` good at? Not much. That's why we plan to remove it. Removing it will reduce some maintenance and consistency burdens, allowing us to focus on more exciting features!
+
+When migrating, you might uncover bugs where one system depends on another's commands but is not ordered to reflect that.
+These bugs can be fixed by making those implicit orderings explicit via constraints like `before`, `after`, `chain`, etc.
+If finding all of those implicit but necessary orderings is unrealistic, `chain` can also be used to mimic the behavior of the `SimpleExecutor`.
+Again, if you run into trouble migrating, feel free to open an issue!