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Update B0003 error docs to stageless #8736

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Jun 2, 2023
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Update B0003 error docs to stageless #8736

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56 changes: 36 additions & 20 deletions errors/B0003.md
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Original file line number Diff line number Diff line change
Expand Up @@ -43,50 +43,66 @@ fn use_entity(mut commands: Commands, entity: Option<Res<MyEntity>>) {

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`):
The default panic message is telling you the entity id (`1v0`) 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
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
```

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::schedule::stage::system_commands
0: bevy_ecs::system::commands::system_commands
with name="use_entity_after_despawn::use_entity"
at crates/bevy_ecs/src/schedule/stage.rs:880
1: bevy_ecs::schedule::stage
at crates/bevy_ecs/src/system/commands/mod.rs:117
1: bevy_ecs::world::schedule
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
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:

```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
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
```

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.
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 stage{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world: Despawning entity 0v0
0: bevy_ecs::schedule::stage::system_commands
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/schedule/stage.rs:880
1: bevy_ecs::schedule::stage
at crates/bevy_ecs/src/system/commands/mod.rs:117
1: bevy_ecs::world::schedule
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
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
```