bevy/errors/B0001.md

# B0001

To keep [Rust rules on references](https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html#the-rules-of-references) (either one mutable reference or any number of immutable references) on a component, it is not possible to have two queries on the same component when one request mutable access to it in the same system.

Erroneous code example:

```rust,should_panic
use bevy::prelude::*;

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Enemy;

fn move_enemies_to_player(
    mut enemies: Query<&mut Transform, With<Enemy>>,
    player: Query<&Transform, With<Player>>,
) {
    // ...
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(move_enemies_to_player)
        .run();
}
```

This will panic, as it's not possible to have both a mutable and an immutable query on `Transform` at the same time.

You have two solutions:

Solution #1: use disjoint queries using [`Without`](https://docs.rs/bevy/*/bevy/ecs/query/struct.Without.html)

As a `Player` entity won't be an `Enemy` at the same time, those two queries will acutally never target the same entity. This can be encoded in the query filter with [`Without`](https://docs.rs/bevy/*/bevy/ecs/query/struct.Without.html):

```rust,no_run
use bevy::prelude::*;

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Enemy;

fn move_enemies_to_player(
    mut enemies: Query<&mut Transform, With<Enemy>>,
    player: Query<&Transform, (With<Player>, Without<Enemy>)>,
) {
    // ...
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(move_enemies_to_player)
        .run();
}
```

Solution #2: use a [`QuerySet`](https://docs.rs/bevy/*/bevy/ecs/system/struct.QuerySet.html)

A [`QuerySet`](https://docs.rs/bevy/*/bevy/ecs/system/struct.QuerySet.html) will let you have conflicting queries as a parameter, but you will still be responsible of not using them at the same time in your system.

```rust,no_run
use bevy::prelude::*;

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Enemy;

fn move_enemies_to_player(
    mut transforms: QuerySet<(
        QueryState<&mut Transform, With<Enemy>>,
        QueryState<&Transform, With<Player>>,
    )>,
) {
    // ...
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(move_enemies_to_player)
        .run();
}
```