Base Sets (bevyengine#7466)

# Objective

NOTE: This depends on bevyengine#7267 and should not be merged until bevyengine#7267 is merged. If you are reviewing this before that is merged, I highly recommend viewing the Base Sets commit instead of trying to find my changes amongst those from bevyengine#7267.

"Default sets" as described by the [Stageless RFC](bevyengine/rfcs#45) have some [unfortunate consequences](bevyengine#7365).

## Solution

This adds "base sets" as a variant of `SystemSet`:

A set is a "base set" if `SystemSet::is_base` returns `true`. Typically this will be opted-in to using the `SystemSet` derive:

```rust
#[derive(SystemSet, Clone, Hash, Debug, PartialEq, Eq)]
#[system_set(base)]
enum MyBaseSet {
  A,
  B,
}
``` 

**Base sets are exclusive**: a system can belong to at most one "base set". Adding a system to more than one will result in an error. When possible we fail immediately during system-config-time with a nice file + line number. For the more nested graph-ey cases, this will fail at the final schedule build. 

**Base sets cannot belong to other sets**: this is where the word "base" comes from

Systems and Sets can only be added to base sets using `in_base_set`. Calling `in_set` with a base set will fail. As will calling `in_base_set` with a normal set.

```rust
app.add_system(foo.in_base_set(MyBaseSet::A))
       // X must be a normal set ... base sets cannot be added to base sets
       .configure_set(X.in_base_set(MyBaseSet::A))
```

Base sets can still be configured like normal sets:

```rust
app.add_system(MyBaseSet::B.after(MyBaseSet::Ap))
``` 

The primary use case for base sets is enabling a "default base set":

```rust
schedule.set_default_base_set(CoreSet::Update)
  // this will belong to CoreSet::Update by default
  .add_system(foo)
  // this will override the default base set with PostUpdate
  .add_system(bar.in_base_set(CoreSet::PostUpdate))
```

This allows us to build apis that work by default in the standard Bevy style. This is a rough analog to the "default stage" model, but it use the new "stageless sets" model instead, with all of the ordering flexibility (including exclusive systems) that it provides.

---

## Changelog

- Added "base sets" and ported CoreSet to use them.

## Migration Guide

TODO

Author: cart

Diff for: crates/bevy_animation/src/lib.rs

@@ -552,7 +552,7 @@ impl Plugin for AnimationPlugin {
             .register_type::<AnimationPlayer>()
             .add_system(
                 animation_player
-                    .in_set(CoreSet::PostUpdate)
+                    .in_base_set(CoreSet::PostUpdate)
                     .before(TransformSystem::TransformPropagate),
             );
     }

Diff for: crates/bevy_app/src/app.rs

@@ -328,14 +328,14 @@ impl App {
                 apply_state_transition::<S>,
             )
                 .chain()
-                .in_set(CoreSet::StateTransitions),
+                .in_base_set(CoreSet::StateTransitions),
         );
 
         let main_schedule = self.get_schedule_mut(CoreSchedule::Main).unwrap();
         for variant in S::variants() {
             main_schedule.configure_set(
                 OnUpdate(variant.clone())
-                    .in_set(CoreSet::StateTransitions)
+                    .in_base_set(CoreSet::StateTransitions)
                     .run_if(state_equals(variant))
                     .after(apply_state_transition::<S>),
             );
@@ -580,7 +580,7 @@ impl App {
     {
         if !self.world.contains_resource::<Events<T>>() {
             self.init_resource::<Events<T>>()
-                .add_system(Events::<T>::update_system.in_set(CoreSet::First));
+                .add_system(Events::<T>::update_system.in_base_set(CoreSet::First));
         }
         self
     }

Diff for: crates/bevy_app/src/lib.rs

@@ -29,8 +29,8 @@ pub mod prelude {
 
 use bevy_ecs::{
     schedule_v3::{
-        apply_system_buffers, IntoSystemConfig, IntoSystemSetConfig, Schedule, ScheduleLabel,
-        SystemSet,
+        apply_system_buffers, IntoSystemConfig, IntoSystemSetConfig, IntoSystemSetConfigs,
+        Schedule, ScheduleLabel, SystemSet,
     },
     system::Local,
     world::World,
@@ -90,6 +90,7 @@ impl CoreSchedule {
 /// 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 CoreSet {
     /// Runs before all other members of this set.
     First,
@@ -129,20 +130,30 @@ impl CoreSet {
         let mut schedule = Schedule::new();
 
         // Create "stage-like" structure using buffer flushes + ordering
-        schedule.add_system(apply_system_buffers.in_set(FirstFlush));
-        schedule.add_system(apply_system_buffers.in_set(PreUpdateFlush));
-        schedule.add_system(apply_system_buffers.in_set(UpdateFlush));
-        schedule.add_system(apply_system_buffers.in_set(PostUpdateFlush));
-        schedule.add_system(apply_system_buffers.in_set(LastFlush));
-
-        schedule.configure_set(First.before(FirstFlush));
-        schedule.configure_set(PreUpdate.after(FirstFlush).before(PreUpdateFlush));
-        schedule.configure_set(StateTransitions.after(PreUpdateFlush).before(FixedUpdate));
-        schedule.configure_set(FixedUpdate.after(StateTransitions).before(Update));
-        schedule.configure_set(Update.after(FixedUpdate).before(UpdateFlush));
-        schedule.configure_set(PostUpdate.after(UpdateFlush).before(PostUpdateFlush));
-        schedule.configure_set(Last.after(PostUpdateFlush).before(LastFlush));
-
+        schedule
+            .set_default_base_set(Update)
+            .add_system(apply_system_buffers.in_base_set(FirstFlush))
+            .add_system(apply_system_buffers.in_base_set(PreUpdateFlush))
+            .add_system(apply_system_buffers.in_base_set(UpdateFlush))
+            .add_system(apply_system_buffers.in_base_set(PostUpdateFlush))
+            .add_system(apply_system_buffers.in_base_set(LastFlush))
+            .configure_sets(
+                (
+                    First,
+                    FirstFlush,
+                    PreUpdate,
+                    PreUpdateFlush,
+                    StateTransitions,
+                    FixedUpdate,
+                    Update,
+                    UpdateFlush,
+                    PostUpdate,
+                    PostUpdateFlush,
+                    Last,
+                    LastFlush,
+                )
+                    .chain(),
+            );
         schedule
     }
 }

Diff for: crates/bevy_asset/src/asset_server.rs

@@ -644,7 +644,7 @@ pub fn free_unused_assets_system(asset_server: Res<AssetServer>) {
 mod test {
     use super::*;
     use crate::{loader::LoadedAsset, update_asset_storage_system};
-    use bevy_app::{App, CoreSet};
+    use bevy_app::App;
     use bevy_ecs::prelude::*;
     use bevy_reflect::TypeUuid;
     use bevy_utils::BoxedFuture;
@@ -852,16 +852,8 @@ mod test {
         let mut app = App::new();
         app.insert_resource(assets);
         app.insert_resource(asset_server);
-        app.add_system(
-            free_unused_assets_system
-                .in_set(FreeUnusedAssets)
-                .in_set(CoreSet::Update),
-        );
-        app.add_system(
-            update_asset_storage_system::<PngAsset>
-                .after(FreeUnusedAssets)
-                .in_set(CoreSet::Update),
-        );
+        app.add_system(free_unused_assets_system.in_set(FreeUnusedAssets));
+        app.add_system(update_asset_storage_system::<PngAsset>.after(FreeUnusedAssets));
 
         fn load_asset(path: AssetPath, world: &World) -> HandleUntyped {
             let asset_server = world.resource::<AssetServer>();

Diff for: crates/bevy_asset/src/assets.rs

@@ -331,8 +331,8 @@ impl AddAsset for App {
         };
 
         self.insert_resource(assets)
-            .add_system(Assets::<T>::asset_event_system.in_set(AssetSet::AssetEvents))
-            .add_system(update_asset_storage_system::<T>.in_set(AssetSet::LoadAssets))
+            .add_system(Assets::<T>::asset_event_system.in_base_set(AssetSet::AssetEvents))
+            .add_system(update_asset_storage_system::<T>.in_base_set(AssetSet::LoadAssets))
             .register_type::<Handle<T>>()
             .add_event::<AssetEvent<T>>()
     }

Diff for: crates/bevy_asset/src/debug_asset_server.rs

@@ -2,7 +2,7 @@
 //!
 //! Internal assets (e.g. shaders) are bundled directly into an application and can't be hot
 //! reloaded using the conventional API.
-use bevy_app::{App, CoreSet, Plugin};
+use bevy_app::{App, Plugin};
 use bevy_ecs::{prelude::*, system::SystemState};
 use bevy_tasks::{IoTaskPool, TaskPoolBuilder};
 use bevy_utils::HashMap;
@@ -75,7 +75,7 @@ impl Plugin for DebugAssetServerPlugin {
             watch_for_changes: true,
         });
         app.insert_non_send_resource(DebugAssetApp(debug_asset_app));
-        app.add_system(run_debug_asset_app.in_set(CoreSet::Update));
+        app.add_system(run_debug_asset_app);
     }
 }
 

Diff for: crates/bevy_asset/src/diagnostic/asset_count_diagnostics_plugin.rs

@@ -19,7 +19,7 @@ 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))
-            .add_system(Self::diagnostic_system.in_set(CoreSet::Update));
+            .add_system(Self::diagnostic_system);
     }
 }
 

Diff for: crates/bevy_asset/src/lib.rs

@@ -51,6 +51,7 @@ use bevy_ecs::prelude::*;
 
 /// [`SystemSet`]s for asset loading in an [`App`] schedule.
 #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
+#[system_set(base)]
 pub enum AssetSet {
     /// Asset storages are updated.
     LoadAssets,
@@ -109,22 +110,20 @@ impl Plugin for AssetPlugin {
 
         app.configure_set(
             AssetSet::LoadAssets
-                .no_default_set()
                 .before(CoreSet::PreUpdate)
                 .after(CoreSet::First),
         )
         .configure_set(
             AssetSet::AssetEvents
-                .no_default_set()
                 .after(CoreSet::PostUpdate)
                 .before(CoreSet::Last),
         )
-        .add_system(asset_server::free_unused_assets_system.in_set(CoreSet::PreUpdate));
+        .add_system(asset_server::free_unused_assets_system.in_base_set(CoreSet::PreUpdate));
 
         #[cfg(all(
             feature = "filesystem_watcher",
             all(not(target_arch = "wasm32"), not(target_os = "android"))
         ))]
-        app.add_system(io::filesystem_watcher_system.in_set(AssetSet::LoadAssets));
+        app.add_system(io::filesystem_watcher_system.in_base_set(AssetSet::LoadAssets));
     }
 }

Diff for: crates/bevy_audio/src/lib.rs

@@ -56,7 +56,7 @@ impl Plugin for AudioPlugin {
             .add_asset::<AudioSource>()
             .add_asset::<AudioSink>()
             .init_resource::<Audio<AudioSource>>()
-            .add_system(play_queued_audio_system::<AudioSource>.in_set(CoreSet::PostUpdate));
+            .add_system(play_queued_audio_system::<AudioSource>.in_base_set(CoreSet::PostUpdate));
 
         #[cfg(any(feature = "mp3", feature = "flac", feature = "wav", feature = "vorbis"))]
         app.init_asset_loader::<AudioLoader>();
@@ -71,6 +71,6 @@ impl AddAudioSource for App {
         self.add_asset::<T>()
             .init_resource::<Audio<T>>()
             .init_resource::<AudioOutput<T>>()
-            .add_system(play_queued_audio_system::<T>.in_set(CoreSet::PostUpdate))
+            .add_system(play_queued_audio_system::<T>.in_base_set(CoreSet::PostUpdate))
     }
 }

Diff for: crates/bevy_core/src/lib.rs

@@ -107,7 +107,7 @@ impl Plugin for TaskPoolPlugin {
         self.task_pool_options.create_default_pools();
 
         #[cfg(not(target_arch = "wasm32"))]
-        app.add_system(tick_global_task_pools.in_set(bevy_app::CoreSet::Last));
+        app.add_system(tick_global_task_pools.in_base_set(bevy_app::CoreSet::Last));
     }
 }
 /// A dummy type that is [`!Send`](Send), to force systems to run on the main thread.
@@ -142,7 +142,7 @@ pub struct FrameCountPlugin;
 impl Plugin for FrameCountPlugin {
     fn build(&self, app: &mut App) {
         app.init_resource::<FrameCount>();
-        app.add_system(update_frame_count.in_set(CoreSet::Last));
+        app.add_system(update_frame_count.in_base_set(CoreSet::Last));
     }
 }
 

Diff for: crates/bevy_diagnostic/src/entity_count_diagnostics_plugin.rs

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

Diff for: crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -11,7 +11,7 @@ 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))
-            .add_system(Self::diagnostic_system.in_set(CoreSet::Update));
+            .add_system(Self::diagnostic_system);
     }
 }
 

Diff for: crates/bevy_diagnostic/src/log_diagnostics_plugin.rs

@@ -37,9 +37,9 @@ impl Plugin for LogDiagnosticsPlugin {
         });
 
         if self.debug {
-            app.add_system(Self::log_diagnostics_debug_system.in_set(CoreSet::PostUpdate));
+            app.add_system(Self::log_diagnostics_debug_system.in_base_set(CoreSet::PostUpdate));
         } else {
-            app.add_system(Self::log_diagnostics_system.in_set(CoreSet::PostUpdate));
+            app.add_system(Self::log_diagnostics_system.in_base_set(CoreSet::PostUpdate));
         }
     }
 }

Diff for: crates/bevy_diagnostic/src/system_information_diagnostics_plugin.rs

@@ -16,7 +16,7 @@ pub struct SystemInformationDiagnosticsPlugin;
 impl Plugin for SystemInformationDiagnosticsPlugin {
     fn build(&self, app: &mut App) {
         app.add_startup_system(internal::setup_system.in_set(StartupSet::Startup))
-            .add_system(internal::diagnostic_system.in_set(CoreSet::Update));
+            .add_system(internal::diagnostic_system);
     }
 }
 

Diff for: crates/bevy_ecs/macros/src/lib.rs

@@ -2,9 +2,10 @@ extern crate proc_macro;
 
 mod component;
 mod fetch;
+mod set;
 
-use crate::fetch::derive_world_query_impl;
-use bevy_macro_utils::{derive_boxed_label, derive_set, get_named_struct_fields, BevyManifest};
+use crate::{fetch::derive_world_query_impl, set::derive_set};
+use bevy_macro_utils::{derive_boxed_label, get_named_struct_fields, BevyManifest};
 use proc_macro::TokenStream;
 use proc_macro2::Span;
 use quote::{format_ident, quote};
@@ -537,7 +538,7 @@ pub fn derive_schedule_label(input: TokenStream) -> TokenStream {
 }
 
 /// Derive macro generating an impl of the trait `SystemSet`.
-#[proc_macro_derive(SystemSet)]
+#[proc_macro_derive(SystemSet, attributes(system_set))]
 pub fn derive_system_set(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
     let mut trait_path = bevy_ecs_path();

Diff for: crates/bevy_ecs/macros/src/set.rs

@@ -0,0 +1,84 @@
+use proc_macro::TokenStream;
+use quote::{quote, ToTokens};
+use syn::parse::{Parse, ParseStream};
+
+pub static SYSTEM_SET_ATTRIBUTE_NAME: &str = "system_set";
+pub static BASE_ATTRIBUTE_NAME: &str = "base";
+
+/// Derive a label trait
+///
+/// # Args
+///
+/// - `input`: The [`syn::DeriveInput`] for struct that is deriving the label trait
+/// - `trait_path`: The path [`syn::Path`] to the label trait
+pub fn derive_set(input: syn::DeriveInput, trait_path: &syn::Path) -> TokenStream {
+    let ident = input.ident;
+
+    let mut is_base = false;
+    for attr in &input.attrs {
+        if !attr
+            .path
+            .get_ident()
+            .map_or(false, |ident| ident == SYSTEM_SET_ATTRIBUTE_NAME)
+        {
+            continue;
+        }
+
+        attr.parse_args_with(|input: ParseStream| {
+            let meta = input.parse_terminated::<syn::Meta, syn::token::Comma>(syn::Meta::parse)?;
+            for meta in meta {
+                let ident = meta.path().get_ident().unwrap_or_else(|| {
+                    panic!(
+                        "Unrecognized attribute: `{}`",
+                        meta.path().to_token_stream()
+                    )
+                });
+                if ident == BASE_ATTRIBUTE_NAME {
+                    if let syn::Meta::Path(_) = meta {
+                        is_base = true;
+                    } else {
+                        panic!(
+                            "The `{BASE_ATTRIBUTE_NAME}` attribute is expected to have no value or arguments",
+                        );
+                    }
+                } else {
+                    panic!(
+                        "Unrecognized attribute: `{}`",
+                        meta.path().to_token_stream()
+                    );
+                }
+            }
+            Ok(())
+        })
+        .unwrap_or_else(|_| panic!("Invalid `{SYSTEM_SET_ATTRIBUTE_NAME}` attribute format"));
+    }
+
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut where_clause = where_clause.cloned().unwrap_or_else(|| syn::WhereClause {
+        where_token: Default::default(),
+        predicates: Default::default(),
+    });
+    where_clause.predicates.push(
+        syn::parse2(quote! {
+            Self: 'static + Send + Sync + Clone + Eq + ::std::fmt::Debug + ::std::hash::Hash
+        })
+        .unwrap(),
+    );
+
+    (quote! {
+        impl #impl_generics #trait_path for #ident #ty_generics #where_clause {
+            fn is_system_type(&self) -> bool {
+                false
+            }
+
+            fn is_base(&self) -> bool {
+                #is_base
+            }
+
+            fn dyn_clone(&self) -> std::boxed::Box<dyn #trait_path> {
+                std::boxed::Box::new(std::clone::Clone::clone(self))
+            }
+        }
+    })
+    .into()
+}