fix B0003 example and update logs (#11162)

# Objective

- Example in error B0003 is not failing anymore after #9822

## Solution

- Update the example code so that is always fail
- Also update logs and instructions on how to debug as it's easier now

---------

Co-authored-by: Alice Cecile <[email protected]>

Author: mockersf

errors/B0003.md

@@ -11,98 +11,52 @@ fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
         .add_systems(Startup, setup)
-        .add_systems(Update, despawning)
-        .add_systems(Update, use_entity.after(despawning))
+        .add_systems(Update, (use_0_and_despawn_1, use_1_and_despawn_0))
         .run();
 }
 
 #[derive(Resource)]
-struct MyEntity(Entity);
+struct MyEntities(Entity, Entity);
 
 #[derive(Component)]
 struct Hello;
 
 fn setup(mut commands: Commands) {
-    let entity = commands.spawn_empty().id();
-    commands.insert_resource(MyEntity(entity));
+    let entity1 = commands.spawn_empty().id();
+    let entity2 = commands.spawn_empty().id();
+    commands.insert_resource(MyEntities(entity1, entity2));
 }
 
-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_0_and_despawn_1(mut commands: Commands, my_entities: Res<MyEntities>) {
+    commands.entity(my_entities.0).insert(Hello);
+    commands.entity(my_entities.1).despawn();
 }
 
-fn use_entity(mut commands: Commands, entity: Option<Res<MyEntity>>) {
-    if let Some(my_entity) = entity {
-        commands.entity(my_entity.0).insert(Hello);
-    }
+fn use_1_and_despawn_0(mut commands: Commands, my_entities: Res<MyEntities>) {
+    commands.entity(my_entities.1).insert(Hello);
+    commands.entity(my_entities.0).despawn();
 }
 ```
 
-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.
+This will panic, as the system that is executed first will despawn the entity used by the second.
 
-The default panic message is telling you the entity id (`1v0`) and the command that failed (adding a component `Hello`):
+The default panic message is telling you which entity doesn't exist (`2v0` in the example log just below), the command that failed (adding a component `Hello`) and the system from which it originated (`use_1_and_despawn_0`):
 
 ```text
-thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
+thread 'main' panicked at /bevy/crates/bevy_ecs/src/system/commands/mod.rs:1097:13:
+error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 2v0 because it doesn't exist in this World.
+Encountered a panic when applying buffers for system `use_entity_after_despawn::use_1_and_despawn_0`!
+Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
 ```
 
-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 information:
-
-```text
-   0: bevy_ecs::system::commands::system_commands
-           with name="use_entity_after_despawn::use_entity"
-             at crates/bevy_ecs/src/system/commands/mod.rs:117
-   1: bevy_ecs::world::schedule
-           with name=Update
-             at crates/bevy_ecs/src/world/mod.rs:1755
-   2: bevy_ecs::schedule::executor::single_threaded::system
-           with name="bevy_app::main_schedule::Main::run_main"
-             at crates/bevy_ecs/src/schedule/executor/single_threaded.rs:98
-   3: bevy_ecs::world::schedule
-           with name=Main
-             at crates/bevy_ecs/src/world/mod.rs:1755
-   4: bevy_app::app::main app
-             at crates/bevy_app/src/app.rs:249
-   5: bevy_app::app::update
-             at crates/bevy_app/src/app.rs:246
-   6: bevy_winit::winit event_handler
-             at crates/bevy_winit/src/lib.rs:338
-   7: bevy_app::app::bevy_app
-             at crates/bevy_app/src/app.rs:290
-thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
-```
-
-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:
+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` or by configuring the `LogPlugin`. This will log:
 
 ```text
-DEBUG schedule{name=Main}:system{name="bevy_app::main_schedule::Main::run_main"}:schedule{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world::entity_ref: Despawning entity 1v0
-thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
+DEBUG system_commands{name="use_entity_after_despawn::use_0_and_despawn_1"}: bevy_ecs::world::entity_ref: Despawning entity 2v0
+thread 'main' panicked at /bevy/crates/bevy_ecs/src/system/commands/mod.rs:1097:13:
+error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 2v0 because it doesn't exist in this World.
+Encountered a panic when applying buffers for system `use_entity_after_despawn::use_1_and_despawn_0`!
+Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
 ```
 
-From the first line, you know the entity `1v0` 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 information to understand why this panic is happening and how to fix it:
-
-```text
-DEBUG schedule{name=Main}:system{name="bevy_app::main_schedule::Main::run_main"}:schedule{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world::entity_ref: Despawning entity 1v0
-   0: bevy_ecs::system::commands::system_commands
-           with name="use_entity_after_despawn::use_entity"
-             at crates/bevy_ecs/src/system/commands/mod.rs:117
-   1: bevy_ecs::world::schedule
-           with name=Update
-             at crates/bevy_ecs/src/world/mod.rs:1755
-   2: bevy_ecs::schedule::executor::single_threaded::system
-           with name="bevy_app::main_schedule::Main::run_main"
-             at crates/bevy_ecs/src/schedule/executor/single_threaded.rs:98
-   3: bevy_ecs::world::schedule
-           with name=Main
-             at crates/bevy_ecs/src/world/mod.rs:1755
-thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
-```
+From the first line, you know the entity `2v0` was despawned when executing a command from system `use_0_and_despawn_1`. In a real case, you could have many log lines, you will need to search for the exact entity from the panic message.