Add on_unimplemented Diagnostics to Most Public Traits (#13347)

# 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 <[email protected]>
Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: BD103 <[email protected]>

Author: 4 people

Cargo.toml

@@ -10,7 +10,7 @@ keywords = ["game", "engine", "gamedev", "graphics", "bevy"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/bevyengine/bevy"
 documentation = "https://docs.rs/bevy"
-rust-version = "1.77.0"
+rust-version = "1.78.0"
 
 [workspace]
 exclude = [

crates/bevy_app/src/plugin.rs

@@ -54,7 +54,7 @@ use std::any::Any;
 ///     }
 /// }
 /// # fn damp_flickering() {}
-/// ````
+/// ```
 pub trait Plugin: Downcast + Any + Send + Sync {
     /// Configures the [`App`] to which this plugin is added.
     fn build(&self, app: &mut App);

crates/bevy_asset/src/lib.rs

@@ -223,6 +223,11 @@ impl Plugin for AssetPlugin {
     }
 }
 
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not an `Asset`",
+    label = "invalid `Asset`",
+    note = "consider annotating `{Self}` with `#[derive(Asset)]`"
+)]
 pub trait Asset: VisitAssetDependencies + TypePath + Send + Sync + 'static {}
 
 pub trait VisitAssetDependencies {

crates/bevy_ecs/src/bundle.rs

@@ -141,6 +141,11 @@ use std::ptr::NonNull;
 // bundle, in the _exact_ order that [`DynamicBundle::get_components`] is called.
 // - [`Bundle::from_components`] must call `func` exactly once for each [`ComponentId`] returned by
 //   [`Bundle::component_ids`].
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a `Bundle`",
+    label = "invalid `Bundle`",
+    note = "consider annotating `{Self}` with `#[derive(Component)]` or `#[derive(Bundle)]`"
+)]
 pub unsafe trait Bundle: DynamicBundle + Send + Sync + 'static {
     /// Gets this [`Bundle`]'s component ids, in the order of this bundle's [`Component`]s
     #[doc(hidden)]

crates/bevy_ecs/src/component.rs

@@ -150,6 +150,11 @@ use std::{
 ///
 /// [`SyncCell`]: bevy_utils::synccell::SyncCell
 /// [`Exclusive`]: https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a `Component`",
+    label = "invalid `Component`",
+    note = "consider annotating `{Self}` with `#[derive(Component)]`"
+)]
 pub trait Component: Send + Sync + 'static {
     /// A constant indicating the storage type used for this component.
     const STORAGE_TYPE: StorageType;

crates/bevy_ecs/src/entity/map_entities.rs

@@ -37,7 +37,6 @@ use super::EntityHashMap;
 ///     }
 /// }
 /// ```
-///
 pub trait MapEntities {
     /// Updates all [`Entity`] references stored inside using `entity_mapper`.
     ///

crates/bevy_ecs/src/event.rs

@@ -28,6 +28,11 @@ use std::{
 /// You can conveniently access events using the [`EventReader`] and [`EventWriter`] system parameter.
 ///
 /// Events must be thread-safe.
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not an `Event`",
+    label = "invalid `Event`",
+    note = "consider annotating `{Self}` with `#[derive(Event]`"
+)]
 pub trait Event: Send + Sync + 'static {}
 
 /// An `EventId` uniquely identifies an event stored in a specific [`World`].

crates/bevy_ecs/src/query/fetch.rs

@@ -268,6 +268,10 @@ use std::{cell::UnsafeCell, marker::PhantomData};
 ///
 /// [`Query`]: crate::system::Query
 /// [`ReadOnly`]: Self::ReadOnly
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not valid to request as data in a `Query`",
+    label = "invalid `Query` data"
+)]
 pub unsafe trait QueryData: WorldQuery {
     /// The read-only variant of this [`QueryData`], which satisfies the [`ReadOnlyQueryData`] trait.
     type ReadOnly: ReadOnlyQueryData<State = <Self as WorldQuery>::State>;

crates/bevy_ecs/src/query/filter.rs

@@ -70,7 +70,11 @@ use std::{cell::UnsafeCell, marker::PhantomData};
 /// [`matches_component_set`]: Self::matches_component_set
 /// [`Query`]: crate::system::Query
 /// [`State`]: Self::State
-
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a valid `Query` filter",
+    label = "invalid `Query` filter",
+    note = "a `QueryFilter` typically uses a combination of `With<T>` and `Without<T>` statements"
+)]
 pub trait QueryFilter: WorldQuery {
     /// Returns true if (and only if) this Filter relies strictly on archetypes to limit which
     /// components are accessed by the Query.
@@ -938,6 +942,11 @@ impl<T: Component> QueryFilter for Changed<T> {
 ///
 /// [`Added`] and [`Changed`] works with entities, and therefore are not archetypal. As such
 /// they do not implement [`ArchetypeFilter`].
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a valid `Query` filter based on archetype information",
+    label = "invalid `Query` filter",
+    note = "an `ArchetypeFilter` typically uses a combination of `With<T>` and `Without<T>` statements"
+)]
 pub trait ArchetypeFilter: QueryFilter {}
 
 impl<T: Component> ArchetypeFilter for With<T> {}

crates/bevy_ecs/src/schedule/config.rs

@@ -288,6 +288,10 @@ impl<T> NodeConfigs<T> {
 ///     )
 /// );
 /// ```
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` does not describe a valid system configuration",
+    label = "invalid system configuration"
+)]
 pub trait IntoSystemConfigs<Marker>
 where
     Self: Sized,
@@ -562,6 +566,10 @@ impl SystemSetConfig {
 pub type SystemSetConfigs = NodeConfigs<InternedSystemSet>;
 
 /// Types that can convert into a [`SystemSetConfigs`].
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` does not describe a valid system set configuration",
+    label = "invalid system set configuration"
+)]
 pub trait IntoSystemSetConfigs
 where
     Self: Sized,

crates/bevy_ecs/src/schedule/set.rs

@@ -149,6 +149,10 @@ impl SystemSet for AnonymousSet {
 }
 
 /// Types that can be converted into a [`SystemSet`].
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a system set",
+    label = "invalid system set"
+)]
 pub trait IntoSystemSet<Marker>: Sized {
     /// The type of [`SystemSet`] this instance converts into.
     type Set: SystemSet;

crates/bevy_ecs/src/system/adapter_system.rs

@@ -39,6 +39,10 @@ use crate::{schedule::InternedSystemSet, world::unsafe_world_cell::UnsafeWorldCe
 /// # system.initialize(&mut world);
 /// # assert!(system.run((), &mut world));
 /// ```
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not adapt a system of type `{S}`",
+    label = "invalid system adapter"
+)]
 pub trait Adapt<S: System>: Send + Sync + 'static {
     /// The [input](System::In) type for an [`AdapterSystem`].
     type In;

crates/bevy_ecs/src/system/combinator.rs

@@ -83,6 +83,11 @@ use super::{ReadOnlySystem, System};
 /// # assert!(world.resource::<RanFlag>().0);
 /// # world.resource_mut::<RanFlag>().0 = false;
 /// ```
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not combine systems `{A}` and `{B}`",
+    label = "invalid system combination",
+    note = "the inputs and outputs of `{A}` and `{B}` are not compatible with this combiner"
+)]
 pub trait Combine<A: System, B: System> {
     /// The [input](System::In) type for a [`CombinatorSystem`].
     type In;

crates/bevy_ecs/src/system/exclusive_function_system.rs

@@ -161,14 +161,18 @@ where
 ///
 /// This trait can be useful for making your own systems which accept other systems,
 /// sometimes called higher order systems.
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not an exclusive system",
+    label = "invalid system"
+)]
 pub trait ExclusiveSystemParamFunction<Marker>: Send + Sync + 'static {
     /// The input type to this system. See [`System::In`].
     type In;
 
     /// The return type of this system. See [`System::Out`].
     type Out;
 
-    /// The [`ExclusiveSystemParam`]/s defined by this system's `fn` parameters.
+    /// The [`ExclusiveSystemParam`]'s defined by this system's `fn` parameters.
     type Param: ExclusiveSystemParam;
 
     /// Executes this system once. See [`System::run`].

crates/bevy_ecs/src/system/exclusive_system_param.rs

@@ -10,6 +10,10 @@ use std::marker::PhantomData;
 
 /// A parameter that can be used in an exclusive system (a system with an `&mut World` parameter).
 /// Any parameters implementing this trait must come after the `&mut World` parameter.
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not be used as a parameter for an exclusive system",
+    label = "invalid system parameter"
+)]
 pub trait ExclusiveSystemParam: Sized {
     /// Used to store data which persists across invocations of a system.
     type State: Send + Sync + 'static;

crates/bevy_ecs/src/system/function_system.rs

@@ -626,6 +626,10 @@ where
 /// ```
 /// [`PipeSystem`]: crate::system::PipeSystem
 /// [`ParamSet`]: crate::system::ParamSet
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a valid system",
+    label = "invalid system"
+)]
 pub trait SystemParamFunction<Marker>: Send + Sync + 'static {
     /// The input type to this system. See [`System::In`].
     type In;

crates/bevy_ecs/src/system/mod.rs

@@ -147,6 +147,10 @@ use crate::world::World;
 // This trait has to be generic because we have potentially overlapping impls, in particular
 // because Rust thinks a type could impl multiple different `FnMut` combinations
 // even though none can currently
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a valid system with input `{In}` and output `{Out}`",
+    label = "invalid system"
+)]
 pub trait IntoSystem<In, Out, Marker>: Sized {
     /// The type of [`System`] that this instance converts into.
     type System: System<In = In, Out = Out>;

crates/bevy_ecs/src/system/system.rs

@@ -22,6 +22,7 @@ use super::IntoSystem;
 /// Systems are executed in parallel, in opportunistic order; data access is managed automatically.
 /// It's possible to specify explicit execution order between specific systems,
 /// see [`IntoSystemConfigs`](crate::schedule::IntoSystemConfigs).
+#[diagnostic::on_unimplemented(message = "`{Self}` is not a system", label = "invalid system")]
 pub trait System: Send + Sync + 'static {
     /// The system's input. See [`In`](crate::system::In) for
     /// [`FunctionSystem`](crate::system::FunctionSystem)s.

crates/bevy_ecs/src/system/system_param.rs

@@ -432,6 +432,11 @@ impl_param_set!();
 /// ```
 ///
 /// [`Exclusive`]: https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not a `Resource`",
+    label = "invalid `Resource`",
+    note = "consider annotating `{Self}` with `#[derive(Resource)]`"
+)]
 pub trait Resource: Send + Sync + 'static {}
 
 // SAFETY: Res only reads a single World resource

crates/bevy_reflect/compile_fail/tests/reflect_derive/generics_fail.rs

@@ -12,9 +12,9 @@ struct NoReflect(f32);
 
 fn main() {
     let mut foo: Box<dyn Struct> = Box::new(Foo::<NoReflect> { a: NoReflect(42.0) });
-    //~^ ERROR: not satisfied
+    //~^ ERROR: `NoReflect` does not provide type registration information
 
     // foo doesn't implement Reflect because NoReflect doesn't implement Reflect
     foo.get_field::<NoReflect>("a").unwrap();
-    //~^ ERROR: not satisfied
+    //~^ ERROR: `NoReflect` can not be reflected
 }

crates/bevy_reflect/compile_fail/tests/reflect_derive/generics_fail.stderr

@@ -1,11 +1,12 @@
-error[E0277]: the trait bound `NoReflect: Reflect` is not satisfied
+error[E0277]: `NoReflect` can not be reflected
    --> tests/reflect_derive/generics_fail.rs:18:21
     |
 18  |     foo.get_field::<NoReflect>("a").unwrap();
     |         ---------   ^^^^^^^^^ the trait `Reflect` is not implemented for `NoReflect`
     |         |
     |         required by a bound introduced by this call
     |
+    = note: Try using `#[derive(Reflect)]`
     = help: the following other types implement trait `Reflect`:
               bool
               char
@@ -17,17 +18,19 @@ error[E0277]: the trait bound `NoReflect: Reflect` is not satisfied
               i128
             and 74 others
 note: required by a bound in `bevy_reflect::GetField::get_field`
-   --> $BEVY_ROOT/crates/bevy_reflect/src/struct_trait.rs:242:21
+   --> $BEVY_ROOT/crates/bevy_reflect/src/struct_trait.rs:244:21
     |
-242 |     fn get_field<T: Reflect>(&self, name: &str) -> Option<&T>;
+244 |     fn get_field<T: Reflect>(&self, name: &str) -> Option<&T>;
     |                     ^^^^^^^ required by this bound in `GetField::get_field`
 
-error[E0277]: the trait bound `NoReflect: GetTypeRegistration` is not satisfied
+error[E0277]: `NoReflect` does not provide type registration information
   --> tests/reflect_derive/generics_fail.rs:14:36
    |
 14 |     let mut foo: Box<dyn Struct> = Box::new(Foo::<NoReflect> { a: NoReflect(42.0) });
-   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `GetTypeRegistration` is not implemented for `NoReflect`, which is required by `Foo<NoReflect>: bevy_reflect::Struct`
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Invalid Type
    |
+   = help: the trait `GetTypeRegistration` is not implemented for `NoReflect`, which is required by `Foo<NoReflect>: bevy_reflect::Struct`
+   = note: Try using `#[derive(Reflect)]`
    = help: the following other types implement trait `GetTypeRegistration`:
              bool
              char

crates/bevy_reflect/src/from_reflect.rs

@@ -21,6 +21,10 @@ use crate::{FromType, Reflect};
 /// [derive macro]: bevy_reflect_derive::FromReflect
 /// [`DynamicStruct`]: crate::DynamicStruct
 /// [crate-level documentation]: crate
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not be created through reflection",
+    note = "consider annotating `{Self}` with `#[derive(FromReflect)]`"
+)]
 pub trait FromReflect: Reflect + Sized {
     /// Constructs a concrete instance of `Self` from a reflected value.
     fn from_reflect(reflect: &dyn Reflect) -> Option<Self>;

crates/bevy_reflect/src/path/mod.rs

@@ -230,6 +230,10 @@ impl<'a> ReflectPath<'a> for &'a str {
 /// [`List`]: crate::List
 /// [`Array`]: crate::Array
 /// [`Enum`]: crate::Enum
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` does not provide a reflection path",
+    note = "consider annotating `{Self}` with `#[derive(Reflect)]`"
+)]
 pub trait GetPath: Reflect {
     /// Returns a reference to the value specified by `path`.
     ///

crates/bevy_reflect/src/reflect.rs

@@ -179,6 +179,10 @@ impl std::fmt::Display for ReflectKind {
 /// [`bevy_reflect`]: crate
 /// [derive macro]: bevy_reflect_derive::Reflect
 /// [crate-level documentation]: crate
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not be reflected",
+    note = "consider annotating `{Self}` with `#[derive(Reflect)]`"
+)]
 pub trait Reflect: DynamicTypePath + Any + Send + Sync {
     /// Returns the [`TypeInfo`] of the type _represented_ by this value.
     ///

crates/bevy_reflect/src/type_info.rs

@@ -70,6 +70,10 @@ use std::fmt::Debug;
 /// ```
 ///
 /// [utility]: crate::utility
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not provide type information through reflection",
+    note = "consider annotating `{Self}` with `#[derive(Reflect)]`"
+)]
 pub trait Typed: Reflect + TypePath {
     /// Returns the compile-time [info] for the underlying type.
     ///

crates/bevy_reflect/src/type_path.rs

@@ -79,6 +79,10 @@ use std::fmt;
 /// [`crate_name`]: TypePath::crate_name
 /// [`module_path`]: TypePath::module_path
 /// [`type_ident`]: TypePath::type_ident
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` does not have a type path",
+    note = "consider annotating `{Self}` with `#[derive(Reflect)]` or `#[derive(TypePath)]`"
+)]
 pub trait TypePath: 'static {
     /// Returns the fully qualified path of the underlying type.
     ///
@@ -129,6 +133,10 @@ pub trait TypePath: 'static {
 /// Since this is a supertrait of [`Reflect`] its methods can be called on a `dyn Reflect`.
 ///
 /// [`Reflect`]: crate::Reflect
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not be used as a dynamic type path",
+    note = "consider annotating `{Self}` with `#[derive(Reflect)]` or `#[derive(TypePath)]`"
+)]
 pub trait DynamicTypePath {
     /// See [`TypePath::type_path`].
     fn reflect_type_path(&self) -> &str;

crates/bevy_reflect/src/type_registry.rs

@@ -56,6 +56,10 @@ impl Debug for TypeRegistryArc {
 /// See the [crate-level documentation] for more information on type registration.
 ///
 /// [crate-level documentation]: crate
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` does not provide type registration information",
+    note = "consider annotating `{Self}` with `#[derive(Reflect)]`"
+)]
 pub trait GetTypeRegistration: 'static {
     /// Returns the default [`TypeRegistration`] for this type.
     fn get_type_registration() -> TypeRegistration;

crates/bevy_state/src/state/freely_mutable_state.rs

@@ -9,6 +9,7 @@ use super::transitions::*;
 ///
 /// While ordinary states are freely mutable (and implement this trait as part of their derive macro),
 /// computed states are not: instead, they can *only* change when the states that drive them do.
+#[diagnostic::on_unimplemented(note = "consider annotating `{Self}` with `#[derive(States)]`")]
 pub trait FreelyMutableState: States {
     /// This function registers all the necessary systems to apply state changes and run transition schedules
     fn register_state(schedule: &mut Schedule) {

crates/bevy_state/src/state/states.rs

@@ -54,6 +54,11 @@ use std::hash::Hash;
 /// app.add_systems(Update, handle_escape_pressed.run_if(in_state(GameState::MainMenu)));
 /// app.add_systems(OnEnter(GameState::SettingsMenu), open_settings_menu);
 /// ```
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not be used as a state",
+    label = "invalid state",
+    note = "consider annotating `{Self}` with `#[derive(States)]`"
+)]
 pub trait States: 'static + Send + Sync + Clone + PartialEq + Eq + Hash + Debug {
     /// How many other states this state depends on.
     /// Used to help order transitions and de-duplicate [`ComputedStates`](crate::state::ComputedStates), as well as prevent cyclical

crates/bevy_state/src/state/sub_states.rs

@@ -143,6 +143,11 @@ pub use bevy_state_macros::SubStates;
 ///
 /// impl FreelyMutableState for GamePhase {}
 /// ```
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` can not be used as a sub-state",
+    label = "invalid sub-state",
+    note = "consider annotating `{Self}` with `#[derive(SubStates)]`"
+)]
 pub trait SubStates: States + FreelyMutableState {
     /// The set of states from which the [`Self`] is derived.
     ///