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:

In exclusive 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.