add detailed errors (bevyengine#2994)

# Objective

- Improve error descriptions and help understand how to fix them
- I noticed one today that could be expanded, it seemed like a good starting point

## Solution

- Start something like https://github.com/rust-lang/rust/tree/master/compiler/rustc_error_codes/src/error_codes
- Remove sentence about Rust mutability rules which is not very helpful in the error message

I decided to start the error code with B for Bevy so that they're not confused with error code from rust (which starts with E)


Longer term, there are a few more evolutions that can continue this:
- the code samples should be compiled check, and even executed for some of them to check they have the correct error code in a panic
- the error could be build on a page in the website like https://doc.rust-lang.org/error-index.html
- most panic should have their own error code

Cargo.toml

@@ -13,7 +13,7 @@ repository = "https://github.com/bevyengine/bevy"
 
 [workspace]
 exclude = ["benches"]
-members = ["crates/*", "examples/ios", "tools/ci"]
+members = ["crates/*", "examples/ios", "tools/ci", "errors"]
 
 [features]
 default = [

crates/bevy_ecs/src/system/system_param.rs

@@ -185,7 +185,7 @@ fn assert_component_access_compatibility(
         .map(|component_id| world.components.get_info(component_id).unwrap().name())
         .collect::<Vec<&str>>();
     let accesses = conflicting_components.join(", ");
-    panic!("Query<{}, {}> in system {} accesses component(s) {} in a way that conflicts with a previous system parameter. Allowing this would break Rust's mutability rules. Consider using `Without<T>` to create disjoint Queries or merging conflicting Queries into a `QuerySet`.",
+    panic!("error[B0001]: Query<{}, {}> in system {} accesses component(s) {} in a way that conflicts with a previous system parameter. Consider using `Without<T>` to create disjoint Queries or merging conflicting Queries into a `QuerySet`.",
                 query_type, filter_type, system_name, accesses);
 }
 
@@ -287,7 +287,7 @@ unsafe impl<T: Resource> SystemParamState for ResState<T> {
         let combined_access = system_meta.component_access_set.combined_access_mut();
         if combined_access.has_write(component_id) {
             panic!(
-                "Res<{}> in system {} conflicts with a previous ResMut<{0}> access. Allowing this would break Rust's mutability rules. Consider removing the duplicate access.",
+                "error[B0002]: Res<{}> in system {} conflicts with a previous ResMut<{0}> access. Consider removing the duplicate access.",
                 std::any::type_name::<T>(), system_meta.name);
         }
         combined_access.add_read(component_id);
@@ -398,11 +398,11 @@ unsafe impl<T: Resource> SystemParamState for ResMutState<T> {
         let combined_access = system_meta.component_access_set.combined_access_mut();
         if combined_access.has_write(component_id) {
             panic!(
-                "ResMut<{}> in system {} conflicts with a previous ResMut<{0}> access. Allowing this would break Rust's mutability rules. Consider removing the duplicate access.",
+                "error[B0002]: ResMut<{}> in system {} conflicts with a previous ResMut<{0}> access. Consider removing the duplicate access.",
                 std::any::type_name::<T>(), system_meta.name);
         } else if combined_access.has_read(component_id) {
             panic!(
-                "ResMut<{}> in system {} conflicts with a previous Res<{0}> access. Allowing this would break Rust's mutability rules. Consider removing the duplicate access.",
+                "error[B0002]: ResMut<{}> in system {} conflicts with a previous Res<{0}> access. Consider removing the duplicate access.",
                 std::any::type_name::<T>(), system_meta.name);
         }
         combined_access.add_write(component_id);
@@ -775,7 +775,7 @@ unsafe impl<T: 'static> SystemParamState for NonSendState<T> {
         let combined_access = system_meta.component_access_set.combined_access_mut();
         if combined_access.has_write(component_id) {
             panic!(
-                "NonSend<{}> in system {} conflicts with a previous mutable resource access ({0}). Allowing this would break Rust's mutability rules. Consider removing the duplicate access.",
+                "error[B0002]: NonSend<{}> in system {} conflicts with a previous mutable resource access ({0}). Consider removing the duplicate access.",
                 std::any::type_name::<T>(), system_meta.name);
         }
         combined_access.add_read(component_id);
@@ -891,11 +891,11 @@ unsafe impl<T: 'static> SystemParamState for NonSendMutState<T> {
         let combined_access = system_meta.component_access_set.combined_access_mut();
         if combined_access.has_write(component_id) {
             panic!(
-                "NonSendMut<{}> in system {} conflicts with a previous mutable resource access ({0}). Allowing this would break Rust's mutability rules. Consider removing the duplicate access.",
+                "error[B0002]: NonSendMut<{}> in system {} conflicts with a previous mutable resource access ({0}). Consider removing the duplicate access.",
                 std::any::type_name::<T>(), system_meta.name);
         } else if combined_access.has_read(component_id) {
             panic!(
-                "NonSendMut<{}> in system {} conflicts with a previous immutable resource access ({0}). Allowing this would break Rust's mutability rules. Consider removing the duplicate access.",
+                "error[B0002]: NonSendMut<{}> in system {} conflicts with a previous immutable resource access ({0}). Consider removing the duplicate access.",
                 std::any::type_name::<T>(), system_meta.name);
         }
         combined_access.add_write(component_id);

errors/B0001.md

@@ -0,0 +1,91 @@
+# 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();
+}
+```

errors/B0002.md

@@ -0,0 +1,44 @@
+# B0002
+
+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 resource, it is not possible to have more than one resource of a kind if one is mutable in the same system. This can happen between [`Res<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.Res.html) and [`ResMut<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.ResMut.html) for the same `T`, or between [`NonSend<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.NonSend.html) and [`NonSendMut<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.NonSendMut.html) for the same `T`.
+
+Erroneous code example:
+
+```rust,should_panic
+use bevy::prelude::*;
+
+fn update_materials(
+    mut material_updater: ResMut<Assets<ColorMaterial>>,
+    current_materials: Res<Assets<ColorMaterial>>,
+) {
+    // ...
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_system(update_materials)
+        .run();
+}
+```
+
+This will panic, as it's not possible to have both a mutable and an immutable resource on `State` at the same time.
+
+As a mutable resource already provide access to the current resource value, you can remove the immutable resource.
+
+```rust,no_run
+use bevy::prelude::*;
+
+fn update_materials(
+    mut material_updater: ResMut<Assets<ColorMaterial>>,
+) {
+    // ...
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_system(update_materials)
+        .run();
+}
+```

errors/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "errors"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+bevy = { path = "../" }

errors/src/lib.rs

@@ -0,0 +1,5 @@
+#[doc = include_str!("../B0001.md")]
+pub struct B0001;
+
+#[doc = include_str!("../B0002.md")]
+pub struct B0002;