Make clippy happy, add small test.

Author: moonheart08

crates/bevy_app/src/app.rs

@@ -157,7 +157,7 @@ impl App {
         self.sub_apps.update();
     }
 
-    /// Sets the update function to use for [`update`](Self::update) on the [SubApps] collection to the provided function.
+    /// Sets the update function to use for [`update`](Self::update) on the [`SubApps`] collection to the provided function.
     pub fn set_update_fn(&mut self, func: impl Fn(&mut SubApps) + 'static) {
         self.sub_apps.update_fn = Some(Box::new(func));
     }
@@ -1441,8 +1441,8 @@ impl Termination for AppExit {
 
 #[cfg(test)]
 mod tests {
-    use core::marker::PhantomData;
-    use std::sync::Mutex;
+    use core::{marker::PhantomData, sync::atomic::AtomicBool};
+    use std::{boxed::Box, sync::Mutex};
 
     use bevy_ecs::{
         change_detection::{DetectChanges, ResMut},
@@ -1848,4 +1848,23 @@ mod tests {
         assert_eq!(test_events.len(), 2); // Events are double-buffered, so we see 2 + 0 = 2
         assert_eq!(test_events.iter_current_update_events().count(), 0);
     }
+
+    #[test]
+    fn custom_update_fn_invoked() {
+        let invocation_check = &*Box::leak(Box::new(AtomicBool::new(false)));
+
+        let mut app = App::new();
+
+        app.set_update_fn(|_sub_apps| {
+            invocation_check.fetch_or(true, core::sync::atomic::Ordering::SeqCst);
+        });
+
+        app.update();
+
+        assert_eq!(
+            invocation_check.load(core::sync::atomic::Ordering::SeqCst),
+            true,
+            "Failed to run the app's custom update function."
+        );
+    }
 }

crates/bevy_app/src/sub_app.rs

@@ -496,7 +496,7 @@ pub struct SubApps {
     /// Other, labeled sub-apps.
     pub sub_apps: HashMap<InternedAppLabel, SubApp>,
     /// The function called by [`update`](SubApps::update) to update all subapps in the collection.
-    /// Replacing the update function can be used to customize how and when [SubApp::update] and [SubApp::extract] are called.
+    /// Replacing the update function can be used to customize how and when [`SubApp::update`] and [`SubApp::extract`] are called.
     pub update_fn: Option<UpdateFn>,
 }
 
@@ -543,7 +543,7 @@ impl Default for SubApps {
 /// Calls [`update`](SubApp::update) for the main sub-app, and then calls
 /// [`extract`](SubApp::extract) and [`update`](SubApp::update) for the rest.
 ///
-/// This serves as the default for [SubApps::update] if none is provided.
+/// This serves as the default for [`SubApps::update`] if none is provided.
 pub fn update_subapps(sub_apps: &mut SubApps) {
     #[cfg(feature = "trace")]
     let _bevy_update_span = info_span!("update").entered();