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# Objective - Fixes #12377 ## Solution Added simple `#[diagnostic::on_unimplemented(...)]` attributes to some critical public traits providing a more approachable initial error message. Where appropriate, a `note` is added indicating that a `derive` macro is available. ## Examples <details> <summary>Examples hidden for brevity</summary> Below is a collection of examples showing the new error messages produced by this change. In general, messages will start with a more Bevy-centric error message (e.g., _`MyComponent` is not a `Component`_), and a note directing the user to an available derive macro where appropriate. ### Missing `#[derive(Resource)]` <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; struct MyResource; fn main() { App::new() .insert_resource(MyResource) .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: `MyResource` is not a `Resource` --> examples/app/empty.rs:7:26 | 7 | .insert_resource(MyResource) | --------------- ^^^^^^^^^^ invalid `Resource` | | | required by a bound introduced by this call | = help: the trait `Resource` is not implemented for `MyResource` = note: consider annotating `MyResource` with `#[derive(Resource)]` = help: the following other types implement trait `Resource`: AccessibilityRequested ManageAccessibilityUpdates bevy::bevy_a11y::Focus DiagnosticsStore FrameCount bevy::prelude::State<S> SystemInfo bevy::prelude::Axis<T> and 141 others note: required by a bound in `bevy::prelude::App::insert_resource` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_app\src\app.rs:419:31 | 419 | pub fn insert_resource<R: Resource>(&mut self, resource: R) -> &mut Self { | ^^^^^^^^ required by this bound in `App::insert_resource` ``` </details> ### Putting A `QueryData` in a `QueryFilter` Slot <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; #[derive(Component)] struct A; #[derive(Component)] struct B; fn my_system(_query: Query<&A, &B>) {} fn main() { App::new() .add_systems(Update, my_system) .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: `&B` is not a valid `Query` filter --> examples/app/empty.rs:9:22 | 9 | fn my_system(_query: Query<&A, &B>) {} | ^^^^^^^^^^^^^ invalid `Query` filter | = help: the trait `QueryFilter` is not implemented for `&B` = help: the following other types implement trait `QueryFilter`: With<T> Without<T> bevy::prelude::Or<()> bevy::prelude::Or<(F0,)> bevy::prelude::Or<(F0, F1)> bevy::prelude::Or<(F0, F1, F2)> bevy::prelude::Or<(F0, F1, F2, F3)> bevy::prelude::Or<(F0, F1, F2, F3, F4)> and 28 others note: required by a bound in `bevy::prelude::Query` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_ecs\src\system\query.rs:349:51 | 349 | pub struct Query<'world, 'state, D: QueryData, F: QueryFilter = ()> { | ^^^^^^^^^^^ required by this bound in `Query` ``` </details> ### Missing `#[derive(Component)]` <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; struct A; fn my_system(mut commands: Commands) { commands.spawn(A); } fn main() { App::new() .add_systems(Startup, my_system) .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: `A` is not a `Bundle` --> examples/app/empty.rs:6:20 | 6 | commands.spawn(A); | ----- ^ invalid `Bundle` | | | required by a bound introduced by this call | = help: the trait `bevy::prelude::Component` is not implemented for `A`, which is required by `A: Bundle` = note: consider annotating `A` with `#[derive(Component)]` or `#[derive(Bundle)]` = help: the following other types implement trait `Bundle`: TransformBundle SceneBundle DynamicSceneBundle AudioSourceBundle<Source> SpriteBundle SpriteSheetBundle Text2dBundle MaterialMesh2dBundle<M> and 34 others = note: required for `A` to implement `Bundle` note: required by a bound in `bevy::prelude::Commands::<'w, 's>::spawn` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_ecs\src\system\commands\mod.rs:243:21 | 243 | pub fn spawn<T: Bundle>(&mut self, bundle: T) -> EntityCommands { | ^^^^^^ required by this bound in `Commands::<'w, 's>::spawn` ``` </details> ### Missing `#[derive(Asset)]` <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; struct A; fn main() { App::new() .init_asset::<A>() .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: `A` is not an `Asset` --> examples/app/empty.rs:7:23 | 7 | .init_asset::<A>() | ---------- ^ invalid `Asset` | | | required by a bound introduced by this call | = help: the trait `Asset` is not implemented for `A` = note: consider annotating `A` with `#[derive(Asset)]` = help: the following other types implement trait `Asset`: Font AnimationGraph DynamicScene Scene AudioSource Pitch bevy::bevy_gltf::Gltf GltfNode and 17 others note: required by a bound in `init_asset` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_asset\src\lib.rs:307:22 | 307 | fn init_asset<A: Asset>(&mut self) -> &mut Self; | ^^^^^ required by this bound in `AssetApp::init_asset` ``` </details> ### Mismatched Input and Output on System Piping <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; fn producer() -> u32 { 123 } fn consumer(_: In<u16>) {} fn main() { App::new() .add_systems(Update, producer.pipe(consumer)) .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: `fn(bevy::prelude::In<u16>) {consumer}` is not a valid system with input `u32` and output `_` --> examples/app/empty.rs:11:44 | 11 | .add_systems(Update, producer.pipe(consumer)) | ---- ^^^^^^^^ invalid system | | | required by a bound introduced by this call | = help: the trait `bevy::prelude::IntoSystem<u32, _, _>` is not implemented for fn item `fn(bevy::prelude::In<u16>) {consumer}` = note: expecting a system which consumes `u32` and produces `_` note: required by a bound in `pipe` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_ecs\src\system\mod.rs:168:12 | 166 | fn pipe<B, Final, MarkerB>(self, system: B) -> PipeSystem<Self::System, B::System> | ---- required by a bound in this associated function 167 | where 168 | B: IntoSystem<Out, Final, MarkerB>, | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `IntoSystem::pipe` ``` </details> ### Missing Reflection <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; #[derive(Component)] struct MyComponent; fn main() { App::new() .register_type::<MyComponent>() .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: `MyComponent` does not provide type registration information --> examples/app/empty.rs:8:26 | 8 | .register_type::<MyComponent>() | ------------- ^^^^^^^^^^^ the trait `GetTypeRegistration` is not implemented for `MyComponent` | | | required by a bound introduced by this call | = note: consider annotating `MyComponent` with `#[derive(Reflect)]` = help: the following other types implement trait `GetTypeRegistration`: bool char isize i8 i16 i32 i64 i128 and 443 others note: required by a bound in `bevy::prelude::App::register_type` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_app\src\app.rs:619:29 | 619 | pub fn register_type<T: bevy_reflect::GetTypeRegistration>(&mut self) -> &mut Self { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::register_type` ``` </details> ### Missing `#[derive(States)]` Implementation <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; #[derive(Debug, Clone, Copy, Default, Eq, PartialEq, Hash)] enum AppState { #[default] Menu, InGame { paused: bool, turbo: bool, }, } fn main() { App::new() .init_state::<AppState>() .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: the trait bound `AppState: FreelyMutableState` is not satisfied --> examples/app/empty.rs:15:23 | 15 | .init_state::<AppState>() | ---------- ^^^^^^^^ the trait `FreelyMutableState` is not implemented for `AppState` | | | required by a bound introduced by this call | = note: consider annotating `AppState` with `#[derive(States)]` note: required by a bound in `bevy::prelude::App::init_state` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_app\src\app.rs:282:26 | 282 | pub fn init_state<S: FreelyMutableState + FromWorld>(&mut self) -> &mut Self { | ^^^^^^^^^^^^^^^^^^ required by this bound in `App::init_state` ``` </details> ### Adding a `System` with Unhandled Output <details> <summary>Example Code</summary> ```rust use bevy::prelude::*; fn producer() -> u32 { 123 } fn main() { App::new() .add_systems(Update, consumer) .run(); } ``` </details> <details> <summary>Error Generated</summary> ```error error[E0277]: `fn() -> u32 {producer}` does not describe a valid system configuration --> examples/app/empty.rs:9:30 | 9 | .add_systems(Update, producer) | ----------- ^^^^^^^^ invalid system configuration | | | required by a bound introduced by this call | = help: the trait `IntoSystem<(), (), _>` is not implemented for fn item `fn() -> u32 {producer}`, which is required by `fn() -> u32 {producer}: IntoSystemConfigs<_>` = help: the following other types implement trait `IntoSystemConfigs<Marker>`: <Box<(dyn bevy::prelude::System<In = (), Out = ()> + 'static)> as IntoSystemConfigs<()>> <NodeConfigs<Box<(dyn bevy::prelude::System<In = (), Out = ()> + 'static)>> as IntoSystemConfigs<()>> <(S0,) as IntoSystemConfigs<(SystemConfigTupleMarker, P0)>> <(S0, S1) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1)>> <(S0, S1, S2) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2)>> <(S0, S1, S2, S3) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3)>> <(S0, S1, S2, S3, S4) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4)>> <(S0, S1, S2, S3, S4, S5) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5)>> and 14 others = note: required for `fn() -> u32 {producer}` to implement `IntoSystemConfigs<_>` note: required by a bound in `bevy::prelude::App::add_systems` --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_app\src\app.rs:342:23 | 339 | pub fn add_systems<M>( | ----------- required by a bound in this associated function ... 342 | systems: impl IntoSystemConfigs<M>, | ^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::add_systems` ``` </details> </details> ## Testing CI passed locally. ## Migration Guide Upgrade to version 1.78 (or higher) of Rust. ## Future Work - Currently, hints are not supported in this diagnostic. Ideally, suggestions like _"consider using ..."_ would be in a hint rather than a note, but that is the best option for now. - System chaining and other `all_tuples!(...)`-based traits have bad error messages due to the slightly different error message format. --------- Co-authored-by: Jamie Ridding <Themayu@users.noreply.github.com> Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> Co-authored-by: BD103 <59022059+BD103@users.noreply.github.com> |
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