Error adding function as system

You can sometimes get confusing arcane compiler errors when you try to add systems to your Bevy app.

The errors can look like this:

the trait bound `for<'r, 's, 't0> fn(bevy::prelude::Query<'r, 's, (&'t0 Param)) {my_system}: IntoSystem<(), (), _>` is not satisfied

This is caused by your function having incompatible parameters. Bevy can only accept special types as system parameters.

You might also errors that look like this:

the trait bound `Component: WorldQuery` is not satisfied
the trait `WorldQuery` is not implemented for `Component`

this struct takes at most 2 type arguments but 3 type arguments were supplied

These errors are caused by a malformed query.

Common beginner mistakes

• Using &mut Commands (bevy 0.4 syntax) instead of Commands.
• Using Query<MyStuff> instead of Query<&MyStuff> or Query<&mut MyStuff>.
• Using Query<&ComponentA, &ComponentB> instead of Query<(&ComponentA, &ComponentB)> (forgetting the tuple)
• Using your resource types directly without Res or ResMut.
• Using your component types directly without putting them in a Query.
• Using other arbitrary types in your function.

Note that Query<Entity> is correct, because the Entity ID is special; it is not a component.

Supported types

Only the following types are supported as system parameters:

(List in API Docs)

In regular systems:

• Commands: Manipulate the ECS using commands
• Res<T>: Shared access to a resource
• ResMut<T>: Exclusive (mutable) access to a resource
• Option<Res<T>>: Shared access to a resource that may not exist
• Option<ResMut<T>>: Exclusive (mutable) access to a resource that may not exist
• Query<T, F = ()> (can contain tuples of up to 15 types): Access to entities and components
• ParamSet (with up to 8 params): Resolve conflicts between incompatible system parameters
• Local<T>: Data local to the system
• EventReader<T>: Receive events
• EventWriter<T>: Send events
• RemovedComponents<T>: Removal detection
• NonSend<T>: Shared access to Non-Send (main thread only) data
• NonSendMut<T>: Mut access to Non-Send (main thread only) data
• ParallelCommands: Abstraction to help use Commands when you will do your own parallelism
• SystemName: The name (string) of the system, may be useful for debugging
• &World: Read-only direct access to the ECS World
• Entities: Low-level ECS metadata: All entities
• Components: Low-level ECS metadata: All components
• Bundles: Low-level ECS metadata: All bundles
• Archetypes: Low-level ECS metadata: All archetypes
• SystemChangeTick: Low-level ECS metadata: Tick used for change detection
• StaticSystemParam: Helper for generic system abstractions, to avoid lifetime annotations
• tuples containing any of these types, with up to 16 members

In exclusive systems:

• &mut World: Full direct access to the ECS World
• Local<T>: Data local to the system
• SystemState<P>: Emulates a regular system, allowing you to easily access data from the World. P are the system parameters.
• QueryState<Q, F = ()>: Allows you to perform queries on the World, similar to a Query in regular systems.

Your function can have a maximum of 16 total parameters. If you need more, group them into tuples to work around the limit. Tuples can contain up to 16 members, but can be nested indefinitely.

Systems running during the Extract stage can also use Extract<T>, to access data from the Main World instead of the Render World. T can be any other system parameter type.