Make StartupSet a base set (#7574)

# Objective

Closes #7573

- Make `StartupSet` a base set

## Solution

- Add `#[system_set(base)]` to the enum declaration
- Replace `.in_set(StartupSet::...)` with `.in_base_set(StartupSet::...)`

**Note**: I don't really know what I'm doing and what exactly the difference between base and non-base sets are. I mostly opened this PR based on discussion in Discord. I also don't really know how to test that I didn't break everything. Your reviews are appreciated!

---

## Changelog

- `StartupSet` is now a base set

## Migration Guide

`StartupSet` is now a base set. This means that you have to use `.in_base_set` instead of `.in_set`:

### Before

```rs
app.add_system(foo.in_set(StartupSet::PreStartup))
```

### After

```rs
app.add_system(foo.in_base_set(StartupSet::PreStartup))
```

crates/bevy_app/src/lib.rs

@@ -166,6 +166,7 @@ impl CoreSet {
 /// that runs immediately after the matching system set.
 /// These can be useful for ordering, but you almost never want to add your systems to these sets.
 #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
+#[system_set(base)]
 pub enum StartupSet {
     /// Runs once before [`StartupSet::Startup`].
     PreStartup,
@@ -189,11 +190,12 @@ impl StartupSet {
     pub fn base_schedule() -> Schedule {
         use StartupSet::*;
         let mut schedule = Schedule::new();
+        schedule.set_default_base_set(Startup);
 
         // Create "stage-like" structure using buffer flushes + ordering
-        schedule.add_system(apply_system_buffers.in_set(PreStartupFlush));
-        schedule.add_system(apply_system_buffers.in_set(StartupFlush));
-        schedule.add_system(apply_system_buffers.in_set(PostStartupFlush));
+        schedule.add_system(apply_system_buffers.in_base_set(PreStartupFlush));
+        schedule.add_system(apply_system_buffers.in_base_set(StartupFlush));
+        schedule.add_system(apply_system_buffers.in_base_set(PostStartupFlush));
 
         schedule.configure_set(PreStartup.before(PreStartupFlush));
         schedule.configure_set(Startup.after(PreStartupFlush).before(StartupFlush));

crates/bevy_asset/src/diagnostic/asset_count_diagnostics_plugin.rs

@@ -18,7 +18,7 @@ impl<T: Asset> Default for AssetCountDiagnosticsPlugin<T> {
 
 impl<T: Asset> Plugin for AssetCountDiagnosticsPlugin<T> {
     fn build(&self, app: &mut App) {
-        app.add_startup_system(Self::setup_system.in_set(StartupSet::Startup))
+        app.add_startup_system(Self::setup_system)
             .add_system(Self::diagnostic_system);
     }
 }

crates/bevy_diagnostic/src/entity_count_diagnostics_plugin.rs

@@ -9,7 +9,7 @@ pub struct EntityCountDiagnosticsPlugin;
 
 impl Plugin for EntityCountDiagnosticsPlugin {
     fn build(&self, app: &mut App) {
-        app.add_startup_system(Self::setup_system.in_set(StartupSet::Startup))
+        app.add_startup_system(Self::setup_system)
             .add_system(Self::diagnostic_system);
     }
 }

crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -10,7 +10,7 @@ pub struct FrameTimeDiagnosticsPlugin;
 
 impl Plugin for FrameTimeDiagnosticsPlugin {
     fn build(&self, app: &mut bevy_app::App) {
-        app.add_startup_system(Self::setup_system.in_set(StartupSet::Startup))
+        app.add_startup_system(Self::setup_system)
             .add_system(Self::diagnostic_system);
     }
 }

crates/bevy_diagnostic/src/lib.rs

@@ -5,7 +5,6 @@ mod log_diagnostics_plugin;
 mod system_information_diagnostics_plugin;
 
 use bevy_app::prelude::*;
-use bevy_ecs::schedule::IntoSystemConfig;
 pub use diagnostic::*;
 pub use entity_count_diagnostics_plugin::EntityCountDiagnosticsPlugin;
 pub use frame_time_diagnostics_plugin::FrameTimeDiagnosticsPlugin;
@@ -18,10 +17,8 @@ pub struct DiagnosticsPlugin;
 
 impl Plugin for DiagnosticsPlugin {
     fn build(&self, app: &mut App) {
-        app.init_resource::<Diagnostics>().add_startup_system(
-            system_information_diagnostics_plugin::internal::log_system_info
-                .in_set(StartupSet::Startup),
-        );
+        app.init_resource::<Diagnostics>()
+            .add_startup_system(system_information_diagnostics_plugin::internal::log_system_info);
     }
 }
 

crates/bevy_diagnostic/src/system_information_diagnostics_plugin.rs

@@ -1,6 +1,5 @@
 use crate::DiagnosticId;
 use bevy_app::prelude::*;
-use bevy_ecs::prelude::*;
 
 /// Adds a System Information Diagnostic, specifically `cpu_usage` (in %) and `mem_usage` (in %)
 ///
@@ -15,7 +14,7 @@ use bevy_ecs::prelude::*;
 pub struct SystemInformationDiagnosticsPlugin;
 impl Plugin for SystemInformationDiagnosticsPlugin {
     fn build(&self, app: &mut App) {
-        app.add_startup_system(internal::setup_system.in_set(StartupSet::Startup))
+        app.add_startup_system(internal::setup_system)
             .add_system(internal::diagnostic_system);
     }
 }

crates/bevy_gilrs/src/lib.rs

@@ -20,7 +20,9 @@ impl Plugin for GilrsPlugin {
         {
             Ok(gilrs) => {
                 app.insert_non_send_resource(gilrs)
-                    .add_startup_system(gilrs_event_startup_system.in_set(StartupSet::PreStartup))
+                    .add_startup_system(
+                        gilrs_event_startup_system.in_base_set(StartupSet::PreStartup),
+                    )
                     .add_system(
                         gilrs_event_system
                             .before(InputSystem)

crates/bevy_render/src/camera/projection.rs

@@ -28,7 +28,7 @@ impl<T: CameraProjection + Component + GetTypeRegistration> Plugin for CameraPro
     fn build(&self, app: &mut App) {
         app.register_type::<T>()
             .edit_schedule(CoreSchedule::Startup, |schedule| {
-                schedule.configure_set(CameraUpdateSystem.in_set(StartupSet::PostStartup));
+                schedule.configure_set(CameraUpdateSystem.in_base_set(StartupSet::PostStartup));
             })
             .configure_set(CameraUpdateSystem.in_base_set(CoreSet::PostUpdate))
             .add_startup_system(

crates/bevy_transform/src/lib.rs

@@ -97,7 +97,7 @@ impl Plugin for TransformPlugin {
             .configure_set(TransformSystem::TransformPropagate.in_base_set(CoreSet::PostUpdate))
             .edit_schedule(CoreSchedule::Startup, |schedule| {
                 schedule.configure_set(
-                    TransformSystem::TransformPropagate.in_set(StartupSet::PostStartup),
+                    TransformSystem::TransformPropagate.in_base_set(StartupSet::PostStartup),
                 );
             })
             // FIXME: https://github.com/bevyengine/bevy/issues/4381