Create a System Builder Syntax instead of using System Descriptors

crates/bevy_app/src/app.rs

@@ -229,6 +229,23 @@ impl App {
     }
 
     pub fn add_system_set(&mut self, set: SystemSet) -> &mut Self {
+        fn ensure_no_conflicts(set: &SystemSet) {
+            let mut conflict = false;
+            for system in set.systems.iter() {
+                if system.config().stage != set.config().stage {
+                    conflict = true;
+                }
+            }
+            for system in set.exclusive_systems.iter() {
+                if system.config().stage != set.config().stage {
+                    conflict = true;
+                }
+            }
+            if conflict {
+                panic!("There is a mismatch between a system set's stage and it's sub systems")
+            }
+        }
+
         if set.config().startup {
             let set = {
                 if set.config().stage.is_some() {
@@ -237,6 +254,7 @@ impl App {
                     set.stage(StartupStage::Startup)
                 }
             };
+            ensure_no_conflicts(&set);
             self.schedule
                 .stage(CoreStage::Startup, |schedule: &mut Schedule| {
                     schedule.add_system_set(set)
@@ -249,6 +267,7 @@ impl App {
                     set.stage(CoreStage::Update)
                 }
             };
+            ensure_no_conflicts(&set);
             self.schedule.add_system_set(set);
         }
         self

crates/bevy_ecs/src/schedule/system_set.rs

@@ -9,8 +9,8 @@ use std::{fmt::Debug, hash::Hash};
 /// A builder for describing several systems at the same time.
 #[derive(Default)]
 pub struct SystemSet {
-    pub(crate) systems: Vec<BoxedSystem>,
-    pub(crate) exclusive_systems: Vec<BoxedExclusiveSystem>,
+    pub systems: Vec<BoxedSystem>,
+    pub exclusive_systems: Vec<BoxedExclusiveSystem>,
     pub(crate) config: SystemConfig,
 }