As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.14 to 0.15, 0.15 to 0.16.

I apologize for the inconvenience. I will update the page as soon as I find the time.

Obscure Rust compiler errors

You can get scary-looking compiler errors when you try to add systems to your Bevy app.

Common beginner mistakes

• Using commands: &mut Commands instead of mut commands: 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 a bundle type in a query. You want individual components.
• Using other arbitrary types in your function.

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

Error adding function as system

The errors can look like this:

error[E0277]: `for<'a, 'b, 'c> fn(…) {my_system}` does not describe a valid system configuration
   --> src/main.rs:11:30
    |
11  |         .add_systems(Update, my_system)
    |          -----------         ^^^^^^^^^ invalid system configuration
    |          |
    |          required by a bound introduced by this call
    |
    = help: the trait `IntoSystem<(), (), _>` is not implemented for fn item `for<'a, 'b, 'c> fn(…) {my_system}`, which is required by `for<'a, 'b, 'c> fn(…) {my_system}: IntoSystemConfigs<_>`
    = help: the following other types implement trait `IntoSystemConfigs<Marker>`:
              <(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)>>
              <(S0, S1, S2, S3, S4, S5, S6) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6)>>
              <(S0, S1, S2, S3, S4, S5, S6, S7) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6, P7)>>
              <(S0, S1, S2, S3, S4, S5, S6, S7, S8) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6, P7, P8)>>
            and 14 others
    = note: required for `for<'a, 'b, 'c> fn(…) {my_system}` to implement `IntoSystemConfigs<_>`
note: required by a bound in `bevy::prelude::App::add_systems`
   --> /home/iyes/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_app-0.14.0-rc.2/src/app.rs:287:23
    |
284 |     pub fn add_systems<M>(
    |            ----------- required by a bound in this associated function
...
287 |         systems: impl IntoSystemConfigs<M>,
    |                       ^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::add_systems`

The error (confusingly) points to the place in your code where you try to add the system, but in reality, the problem is actually in the fn function definition!

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

Error on malformed queries

You might also errors that look like this:

error[E0277]: `bevy::prelude::AnimationPlayer` is not valid to request as data in a `Query`
   --> src/main.rs:60:18
    |
60  |     mut players: Query<AnimationPlayer, &Transform>,
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid `Query` data
    |
    = help: the trait `QueryData` is not implemented for `bevy::prelude::AnimationPlayer`
    = help: the following other types implement trait `QueryData`:
              &'__w mut T
              &Archetype
              &T
              ()
              (F0, F1)
              (F0, F1, F2)
              (F0, F1, F2, F3)
              (F0, F1, F2, F3, F4)
            and 41 others
note: required by a bound in `bevy::prelude::Query`
   --> /home/iyes/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.14.0-rc.2/src/system/query.rs:349:37
    |
349 | pub struct Query<'world, 'state, D: QueryData, F: QueryFilter = ()> {
    |                                     ^^^^^^^^^ required by this bound in `Query`

error[E0277]: `&bevy::prelude::Transform` is not a valid `Query` filter
   --> src/main.rs:60:18
    |
60  |     mut query: Query<AnimationPlayer, &Transform>,
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid `Query` filter
    |
    = help: the trait `QueryFilter` is not implemented for `&bevy::prelude::Transform`
    = note: a `QueryFilter` typically uses a combination of `With<T>` and `Without<T>` statements
    = help: the following other types implement trait `QueryFilter`:
              ()
              (F0, F1)
              (F0, F1, F2)
              (F0, F1, F2, F3)
              (F0, F1, F2, F3, F4)
              (F0, F1, F2, F3, F4, F5)
              (F0, F1, F2, F3, F4, F5, F6)
              (F0, F1, F2, F3, F4, F5, F6, F7)
            and 28 others
note: required by a bound in `bevy::prelude::Query`
   --> /home/iyes/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.14.0-rc.2/src/system/query.rs:349:51
    |
349 | pub struct Query<'world, 'state, D: QueryData, F: QueryFilter = ()> {
    |                                                   ^^^^^^^^^^^ required by this bound in `Query`

error[E0107]: struct takes at most 2 generic arguments but 3 generic arguments were supplied
   --> src/main.rs:60:18
    |
60  |     mut query: Query<AnimationPlayer, &Transform, &mut GlobalTransform>,
    |                ^^^^^                              -------------------- help: remove this generic argument
    |                |
    |                expected at most 2 generic arguments
    |
note: struct defined here, with at most 2 generic parameters: `D`, `F`
   --> /home/iyes/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.14.0-rc.2/src/system/query.rs:349:12
    |
349 | pub struct Query<'world, 'state, D: QueryData, F: QueryFilter = ()> {
    |            ^^^^^                 -             -------------------

To access your components, you need to use reference syntax (& or &mut).

When you want to query for multiple components, you need to put them in a tuple: Query<(&mut Transform, &Camera, &MyComponent)>.