add a more helpful error to help debug panicking command on despawned…

… entity (#5198)

# Objective

- Help users fix issue when their app panic when executing a command on a despawned entity

## Solution

- Add an error code and a page describing how to debug the issue

crates/bevy_ecs/src/system/commands/mod.rs

@@ -712,9 +712,7 @@ pub struct Despawn {
 impl Command for Despawn {
     fn write(self, world: &mut World) {
         if !world.despawn(self.entity) {
-            warn!("Could not despawn entity {:?} because it doesn't exist in this World.\n\
-                    If this command was added to a newly spawned entity, ensure that you have not despawned that entity within the same stage.\n\
-                    This may have occurred due to system order ambiguity, or if the spawning system has multiple command buffers", self.entity);
+            warn!("error[B0003]: Could not despawn entity {:?} because it doesn't exist in this World.", self.entity);
         }
     }
 }
@@ -732,9 +730,7 @@ where
         if let Some(mut entity) = world.get_entity_mut(self.entity) {
             entity.insert_bundle(self.bundle);
         } else {
-            panic!("Could not insert a bundle (of type `{}`) for entity {:?} because it doesn't exist in this World.\n\
-                    If this command was added to a newly spawned entity, ensure that you have not despawned that entity within the same stage.\n\
-                    This may have occurred due to system order ambiguity, or if the spawning system has multiple command buffers", std::any::type_name::<T>(), self.entity);
+            panic!("error[B0003]: Could not insert a bundle (of type `{}`) for entity {:?} because it doesn't exist in this World.", std::any::type_name::<T>(), self.entity);
         }
     }
 }
@@ -753,9 +749,7 @@ where
         if let Some(mut entity) = world.get_entity_mut(self.entity) {
             entity.insert(self.component);
         } else {
-            panic!("Could not add a component (of type `{}`) to entity {:?} because it doesn't exist in this World.\n\
-                    If this command was added to a newly spawned entity, ensure that you have not despawned that entity within the same stage.\n\
-                    This may have occurred due to system order ambiguity, or if the spawning system has multiple command buffers", std::any::type_name::<T>(), self.entity);
+            panic!("error[B0003]: Could not add a component (of type `{}`) to entity {:?} because it doesn't exist in this World.", std::any::type_name::<T>(), self.entity);
         }
     }
 }

errors/B0003.md

@@ -0,0 +1,91 @@
+# B0003
+
+As commands are executed asynchronously, it is possible to issue a command on an entity that will no longer exist at the time of the command execution.
+
+Erroneous code example:
+
+```rust,should_panic
+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .add_system(despawning)
+        .add_system(use_entity.after(despawning))
+        .run();
+}
+
+struct MyEntity(Entity);
+
+#[derive(Component)]
+struct Hello;
+
+fn setup(mut commands: Commands) {
+    let entity = commands.spawn().id();
+    commands.insert_resource(MyEntity(entity));
+}
+
+fn despawning(mut commands: Commands, entity: Option<Res<MyEntity>>) {
+    if let Some(my_entity) = entity {
+        commands.entity(my_entity.0).despawn();
+        commands.remove_resource::<MyEntity>();
+    }
+}
+
+fn use_entity(mut commands: Commands, entity: Option<Res<MyEntity>>) {
+    if let Some(my_entity) = entity {
+        commands.entity(my_entity.0).insert(Hello);
+    }
+}
+```
+
+This will panic, as system `use_entity` is executed after system `despawning`. Without the system ordering specified here, the ordering would be random and this code would panic half the time.
+
+The default panic message is telling you the entity id (`0v0`) and the command that failed (adding a component `Hello`):
+
+```text
+thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13
+```
+
+But you don't know which system tried to add a component, and which system despawned the entity.
+
+To get the system that created the command that panics, you can enable the `trace` feature of Bevy. This will add a panic handler that will print more informations:
+
+```text
+   0: bevy_ecs::schedule::stage::system_commands
+           with name="use_entity_after_despawn::use_entity"
+             at crates/bevy_ecs/src/schedule/stage.rs:880
+   1: bevy_ecs::schedule::stage
+           with name=Update
+             at crates/bevy_ecs/src/schedule/mod.rs:337
+   2: bevy_app::app::frame
+             at crates/bevy_app/src/app.rs:113
+   3: bevy_app::app::bevy_app
+             at crates/bevy_app/src/app.rs:126
+thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13
+```
+
+From the first two lines, you now know that it panics while executing a command from the system `use_entity`.
+
+To get the system that created the despawn command, you can enable DEBUG logs for crate `bevy_ecs`, for example by setting the environment variable `RUST_LOG=bevy_ecs=debug`. This will log:
+
+```text
+DEBUG stage{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world: Despawning entity 0v0
+thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13
+```
+
+From the first line, you know the entity `0v0` was despawned when executing a command from system `despawning`. In a real case, you could have many log lines, you will need to search for the exact entity from the panic message.
+
+Combining those two, you should get enough informations to understand why this panic is happening and how to fix it:
+
+```text
+DEBUG stage{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world: Despawning entity 0v0
+   0: bevy_ecs::schedule::stage::system_commands
+           with name="use_entity_after_despawn::use_entity"
+             at crates/bevy_ecs/src/schedule/stage.rs:880
+   1: bevy_ecs::schedule::stage
+           with name=Update
+             at crates/bevy_ecs/src/schedule/mod.rs:337
+thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13
+```

errors/src/lib.rs

@@ -3,3 +3,6 @@ pub struct B0001;
 
 #[doc = include_str!("../B0002.md")]
 pub struct B0002;
+
+#[doc = include_str!("../B0003.md")]
+pub struct B0003;