bevy/errors/B0003.md

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# 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_systems(Startup, setup)
.add_systems(Update, (use_0_and_despawn_1, use_1_and_despawn_0))
.run();
}
Make `Resource` trait opt-in, requiring `#[derive(Resource)]` V2 (#5577) *This PR description is an edited copy of #5007, written by @alice-i-cecile.* # Objective Follow-up to https://github.com/bevyengine/bevy/pull/2254. The `Resource` trait currently has a blanket implementation for all types that meet its bounds. While ergonomic, this results in several drawbacks: * it is possible to make confusing, silent mistakes such as inserting a function pointer (Foo) rather than a value (Foo::Bar) as a resource * it is challenging to discover if a type is intended to be used as a resource * we cannot later add customization options (see the [RFC](https://github.com/bevyengine/rfcs/blob/main/rfcs/27-derive-component.md) for the equivalent choice for Component). * dependencies can use the same Rust type as a resource in invisibly conflicting ways * raw Rust types used as resources cannot preserve privacy appropriately, as anyone able to access that type can read and write to internal values * we cannot capture a definitive list of possible resources to display to users in an editor ## Notes to reviewers * Review this commit-by-commit; there's effectively no back-tracking and there's a lot of churn in some of these commits. *ira: My commits are not as well organized :')* * I've relaxed the bound on Local to Send + Sync + 'static: I don't think these concerns apply there, so this can keep things simple. Storing e.g. a u32 in a Local is fine, because there's a variable name attached explaining what it does. * I think this is a bad place for the Resource trait to live, but I've left it in place to make reviewing easier. IMO that's best tackled with https://github.com/bevyengine/bevy/issues/4981. ## Changelog `Resource` is no longer automatically implemented for all matching types. Instead, use the new `#[derive(Resource)]` macro. ## Migration Guide Add `#[derive(Resource)]` to all types you are using as a resource. If you are using a third party type as a resource, wrap it in a tuple struct to bypass orphan rules. Consider deriving `Deref` and `DerefMut` to improve ergonomics. `ClearColor` no longer implements `Component`. Using `ClearColor` as a component in 0.8 did nothing. Use the `ClearColorConfig` in the `Camera3d` and `Camera2d` components instead. Co-authored-by: Alice <alice.i.cecile@gmail.com> Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> Co-authored-by: devil-ira <justthecooldude@gmail.com> Co-authored-by: Carter Anderson <mcanders1@gmail.com>
2022-08-08 21:36:35 +00:00
#[derive(Resource)]
struct MyEntities(Entity, Entity);
#[derive(Component)]
struct Hello;
fn setup(mut commands: Commands) {
let entity1 = commands.spawn_empty().id();
let entity2 = commands.spawn_empty().id();
commands.insert_resource(MyEntities(entity1, entity2));
}
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_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 the system that is executed first will despawn the entity used by the second.
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 /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`!
```
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 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 `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.