Obscure Rust compiler errors

You can get confusing 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 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]: the trait bound `for<'a, 'b, 'c> fn(...) {system}: IntoSystem<(), (), _>` is not satisfied
   --> src/main.rs:5:21
    |
5   |         .add_system(my_system)
    |          ---------- ^^^^^^^^^ the trait `IntoSystem<(), (), _>` is not implemented for fn item `for<'a, 'b, 'c> fn(...) {system}`
    |          |
    |          required by a bound introduced by this call
    |
    = help: the trait `IntoSystemAppConfig<()>` is implemented for `SystemAppConfig`
    = note: required for `for<'a, 'b, 'c> fn(...) {system}` to implement `IntoSystemConfig<_>`
    = note: required for `for<'a, 'b, 'c> fn(...) {system}` to implement `IntoSystemAppConfig<_>`

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]: the trait bound `Transform: WorldQuery` is not satisfied
   --> src/main.rs:10:12
    |
10  |     query: Query<Transform>,
    |            ^^^^^^^^^^^^^^^ the trait `WorldQuery` is not implemented for `Transform`
    |
    = help: the following other types implement trait `WorldQuery`:
              &'__w mut T
              &T
              ()
              (F0, F1)
              (F0, F1, F2)
              (F0, F1, F2, F3)
              (F0, F1, F2, F3, F4)
              (F0, F1, F2, F3, F4, F5)
            and 54 others
note: required by a bound in `bevy::prelude::Query`
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.10.0/src/system/query.rs:276:37
    |
276 | pub struct Query<'world, 'state, Q: WorldQuery, F: ReadOnlyWorldQuery = ()> {
    |                                     ^^^^^^^^^^ required by this bound in `Query`

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

error[E0107]: struct takes at most 2 generic arguments but 3 generic arguments were supplied
   --> src/main.rs:10:12
    |
10  |     query: Query<&Transform, &Camera, &GlobalTransform>,
    |            ^^^^^                      ---------------- help: remove this generic argument
    |            |
    |            expected at most 2 generic arguments
    |
note: struct defined here, with at most 2 generic parameters: `Q`, `F`
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.10.0/src/system/query.rs:276:12
    |
276 | pub struct Query<'world, 'state, Q: WorldQuery, F: ReadOnlyWorldQuery = ()> {
    |            ^^^^^                 -              --------------------------

When you want to query for multiple components, you need to put them in a tuple: (&Transform, &Camera, &GlobalTransform).