bevyengine#4231: panic when App::run() is called from Plugin::build()

crates/bevy_app/src/app.rs

@@ -57,6 +57,9 @@ pub struct App {
     /// A container of [`Stage`]s set to be run in a linear order.
     pub schedule: Schedule,
     sub_apps: HashMap<Box<dyn AppLabel>, SubApp>,
+    /// A private marker to prevent incorrect calls to `App::run()` from `Plugin::build()`
+    /// <https://github.com/bevyengine/bevy/issues/4231>
+    is_from_plugin_build: bool,
 }
 
 /// Each `SubApp` has its own [`Schedule`] and [`World`], enabling a separation of concerns.
@@ -100,6 +103,7 @@ impl App {
             schedule: Default::default(),
             runner: Box::new(run_once),
             sub_apps: HashMap::default(),
+            is_from_plugin_build: false,
         }
     }
 
@@ -126,6 +130,9 @@ impl App {
         let _bevy_app_run_span = info_span!("bevy_app").entered();
 
         let mut app = std::mem::replace(self, App::empty());
+        if app.is_from_plugin_build {
+            panic!("App::Run() is called from a Plugin::Build(), which might result in other initialization code not run.");
+        }
         let runner = std::mem::replace(&mut app.runner, Box::new(run_once));
         (runner)(app);
     }
@@ -768,7 +775,9 @@ impl App {
         T: Plugin,
     {
         debug!("added plugin: {}", plugin.name());
+        self.is_from_plugin_build = true;
         plugin.build(self);
+        self.is_from_plugin_build = false;
         self
     }