Make initial StateTransition run before PreStartup (#14208)

# Objective

- Fixes #14206 

## Solution

- Run initial `StateTransition` as a startup schedule before
`PreStartup`, instead of running it inside `Startup` as an exclusive
system.

Related discord discussion:

https://discord.com/channels/691052431525675048/692572690833473578/1259543775668207678

## Testing

Reproduction now works correctly:

```rs
use bevy::prelude::*;

#[derive(Debug, Clone, Copy, Default, Eq, PartialEq, Hash, States)]
enum AppState {
    #[default]
    Menu,
    InGame,
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_state::<AppState>()
        .add_systems(Startup, setup)
        .add_systems(OnEnter(AppState::Menu), enter_menu_state)
        .run();
}

fn setup(mut next_state: ResMut<NextState<AppState>>) {
    next_state.set(AppState::Menu);
}

fn enter_menu_state() {
    println!("Entered menu state");
}
```


![image](https://github.com/bevyengine/bevy/assets/13040204/96d7a533-c439-4c0b-8f15-49f620903ce1)


---

## Changelog

- Initial `StateTransition` runs before `PreStartup` instead of inside
`Startup`.

crates/bevy_state/src/app.rs

@@ -1,9 +1,5 @@
-use bevy_app::{App, MainScheduleOrder, Plugin, PreUpdate, Startup, SubApp};
-use bevy_ecs::{
-    event::Events,
-    schedule::{IntoSystemConfigs, ScheduleLabel},
-    world::FromWorld,
-};
+use bevy_app::{App, MainScheduleOrder, Plugin, PreStartup, PreUpdate, SubApp};
+use bevy_ecs::{event::Events, schedule::IntoSystemConfigs, world::FromWorld};
 use bevy_utils::{tracing::warn, warn_once};
 
 use crate::state::{
@@ -221,7 +217,8 @@ impl Plugin for StatesPlugin {
     fn build(&self, app: &mut App) {
         let mut schedule = app.world_mut().resource_mut::<MainScheduleOrder>();
         schedule.insert_after(PreUpdate, StateTransition);
-        setup_state_transitions_in_world(app.world_mut(), Some(Startup.intern()));
+        schedule.insert_startup_before(PreStartup, StateTransition);
+        setup_state_transitions_in_world(app.world_mut());
     }
 }
 

crates/bevy_state/src/state/mod.rs

@@ -19,7 +19,6 @@ pub use transitions::*;
 mod tests {
     use bevy_ecs::event::EventRegistry;
     use bevy_ecs::prelude::*;
-    use bevy_ecs::schedule::ScheduleLabel;
     use bevy_state_macros::States;
     use bevy_state_macros::SubStates;
 
@@ -64,7 +63,7 @@ mod tests {
 
         world.insert_resource(schedules);
 
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
 
         world.run_schedule(StateTransition);
         assert_eq!(world.resource::<State<SimpleState>>().0, SimpleState::A);
@@ -120,7 +119,7 @@ mod tests {
 
         world.insert_resource(schedules);
 
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
 
         world.run_schedule(StateTransition);
         assert_eq!(world.resource::<State<SimpleState>>().0, SimpleState::A);
@@ -180,7 +179,7 @@ mod tests {
 
         world.insert_resource(schedules);
 
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
 
         world.run_schedule(StateTransition);
         assert_eq!(world.resource::<State<SimpleState>>().0, SimpleState::A);
@@ -275,7 +274,7 @@ mod tests {
 
         world.insert_resource(schedules);
 
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
 
         world.run_schedule(StateTransition);
         assert_eq!(world.resource::<State<SimpleState>>().0, SimpleState::A);
@@ -354,9 +353,6 @@ mod tests {
         }
     }
 
-    #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
-    struct Startup;
-
     #[test]
     fn computed_state_transitions_are_produced_correctly() {
         let mut world = World::new();
@@ -367,7 +363,7 @@ mod tests {
         world.init_resource::<State<SimpleState2>>();
         world.init_resource::<Schedules>();
 
-        setup_state_transitions_in_world(&mut world, Some(Startup.intern()));
+        setup_state_transitions_in_world(&mut world);
 
         let mut schedules = world
             .get_resource_mut::<Schedules>()
@@ -431,7 +427,7 @@ mod tests {
 
         world.init_resource::<ComputedStateTransitionCounter>();
 
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
 
         assert_eq!(world.resource::<State<SimpleState>>().0, SimpleState::A);
         assert_eq!(world.resource::<State<SimpleState2>>().0, SimpleState2::A1);
@@ -508,7 +504,7 @@ mod tests {
     #[test]
     fn same_state_transition_should_emit_event_and_not_run_schedules() {
         let mut world = World::new();
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
         EventRegistry::register_event::<StateTransitionEvent<SimpleState>>(&mut world);
         world.init_resource::<State<SimpleState>>();
         let mut schedules = world.resource_mut::<Schedules>();
@@ -568,7 +564,7 @@ mod tests {
         SubState::register_sub_state_systems(&mut apply_changes);
         schedules.insert(apply_changes);
         world.insert_resource(schedules);
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
 
         world.insert_resource(NextState::Pending(SimpleState::B(true)));
         world.run_schedule(StateTransition);
@@ -599,7 +595,7 @@ mod tests {
         TestComputedState::register_computed_state_systems(&mut apply_changes);
         schedules.insert(apply_changes);
         world.insert_resource(schedules);
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
 
         world.insert_resource(NextState::Pending(SimpleState::B(true)));
         world.run_schedule(StateTransition);
@@ -651,7 +647,7 @@ mod tests {
     #[test]
     fn check_transition_orders() {
         let mut world = World::new();
-        setup_state_transitions_in_world(&mut world, None);
+        setup_state_transitions_in_world(&mut world);
         EventRegistry::register_event::<StateTransitionEvent<SimpleState>>(&mut world);
         EventRegistry::register_event::<StateTransitionEvent<SubState>>(&mut world);
         EventRegistry::register_event::<StateTransitionEvent<TransitionTestingComputedState>>(

crates/bevy_state/src/state/transitions.rs

@@ -2,9 +2,7 @@ use std::{marker::PhantomData, mem};
 
 use bevy_ecs::{
     event::{Event, EventReader, EventWriter},
-    schedule::{
-        InternedScheduleLabel, IntoSystemSetConfigs, Schedule, ScheduleLabel, Schedules, SystemSet,
-    },
+    schedule::{IntoSystemSetConfigs, Schedule, ScheduleLabel, Schedules, SystemSet},
     system::{Commands, In, ResMut},
     world::World,
 };
@@ -182,10 +180,7 @@ pub(crate) fn internal_apply_state_transition<S: States>(
 ///
 /// Runs automatically when using `App` to insert states, but needs to
 /// be added manually in other situations.
-pub fn setup_state_transitions_in_world(
-    world: &mut World,
-    startup_label: Option<InternedScheduleLabel>,
-) {
+pub fn setup_state_transitions_in_world(world: &mut World) {
     let mut schedules = world.get_resource_or_insert_with(Schedules::default);
     if schedules.contains(StateTransition) {
         return;
@@ -201,12 +196,6 @@ pub fn setup_state_transitions_in_world(
             .chain(),
     );
     schedules.insert(schedule);
-
-    if let Some(startup) = startup_label {
-        schedules.add_systems(startup, |world: &mut World| {
-            let _ = world.try_run_schedule(StateTransition);
-        });
-    }
 }
 
 /// Returns the latest state transition event of type `S`, if any are available.