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https://github.com/bevyengine/bevy
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# 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
44 lines
1.5 KiB
Markdown
44 lines
1.5 KiB
Markdown
# B0002
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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`.
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Erroneous code example:
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```rust,should_panic
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use bevy::prelude::*;
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fn update_materials(
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mut material_updater: ResMut<Assets<ColorMaterial>>,
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current_materials: Res<Assets<ColorMaterial>>,
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) {
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// ...
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}
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_system(update_materials)
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.run();
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}
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```
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This will panic, as it's not possible to have both a mutable and an immutable resource on `State` at the same time.
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As a mutable resource already provide access to the current resource value, you can remove the immutable resource.
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```rust,no_run
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use bevy::prelude::*;
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fn update_materials(
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mut material_updater: ResMut<Assets<ColorMaterial>>,
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) {
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// ...
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}
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_system(update_materials)
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.run();
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}
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```
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