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)

• 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][cb::paramset]
• 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
• &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
• tuples containing any of these types, with up to 16 members

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.