6183b56b5d
# Objective The goal with this PR is to allow the use of types that don't implement `Reflect` within the reflection API. Rust's [orphan rule](https://doc.rust-lang.org/book/ch10-02-traits.html#implementing-a-trait-on-a-type) prevents implementing a trait on an external type when neither type nor trait are owned by the implementor. This means that if a crate, `cool_rust_lib`, defines a type, `Foo`, then a user cannot use it with reflection. What this means is that we have to ignore it most of the time: ```rust #[derive(Reflect)] struct SomeStruct { #[reflect(ignore)] data: cool_rust_lib::Foo } ``` Obviously, it's impossible to implement `Reflect` on `Foo`. But does it *have* to be? Most of reflection doesn't deal with concrete types— it's almost all using `dyn Reflect`. And being very metadata-driven, it should theoretically be possible. I mean, [`serde`](https://serde.rs/remote-derive.html) does it. ## Solution > Special thanks to @danielhenrymantilla for their help reviewing this PR and offering wisdom wrt safety. Taking a page out of `serde`'s book, this PR adds the ability to easily use "remote types" with reflection. In this context, a "remote type" is the external type for which we have no ability to implement `Reflect`. This adds the `#[reflect_remote(...)]` attribute macro, which is used to generate "remote type wrappers". All you have to do is define the wrapper exactly the same as the remote type's definition: ```rust // Pretend this is our external crate mod cool_rust_lib { #[derive(Default)] struct Foo { pub value: String } } #[reflect_remote(cool_rust_lib::Foo)] struct FooWrapper { pub value: String } ``` > **Note:** All fields in the external type *must* be public. This could be addressed with a separate getter/setter attribute either in this PR or in another one. The macro takes this user-defined item and transforms it into a newtype wrapper around the external type, marking it as `#[repr(transparent)]`. The fields/variants defined by the user are simply used to build out the reflection impls. Additionally, it generates an implementation of the new trait, `ReflectRemote`, which helps prevent accidental misuses of this API. Therefore, the output generated by the macro would look something like: ```rust #[repr(transparent)] struct FooWrapper(pub cool_rust_lib::Foo); impl ReflectRemote for FooWrapper { type Remote = cool_rust_lib::Foo; // transmutation methods... } // reflection impls... // these will acknowledge and make use of the `value` field ``` Internally, the reflection API will pass around the `FooWrapper` and [transmute](https://doc.rust-lang.org/std/mem/fn.transmute.html) it where necessary. All we have to do is then tell `Reflect` to do that. So rather than ignoring the field, we tell `Reflect` to use our wrapper using the `#[reflect(remote = ...)]` field attribute: ```rust #[derive(Reflect)] struct SomeStruct { #[reflect(remote = FooWrapper)] data: cool_rust_lib::Foo } ``` #### Other Macros & Type Data Because this macro consumes the defined item and generates a new one, we can't just put our macros anywhere. All macros that should be passed to the generated struct need to come *below* this macro. For example, to derive `Default` and register its associated type data: ```rust // ✅ GOOD #[reflect_remote(cool_rust_lib::Foo)] #[derive(Default)] #[reflect(Default)] struct FooWrapper { pub value: String } // ❌ BAD #[derive(Default)] #[reflect_remote(cool_rust_lib::Foo)] #[reflect(Default)] struct FooWrapper { pub value: String } ``` #### Generics Generics are forwarded to the generated struct as well. They should also be defined in the same order: ```rust #[reflect_remote(RemoteGeneric<'a, T1, T2>)] struct GenericWrapper<'a, T1, T2> { pub foo: &'a T1, pub bar: &'a T2, } ``` > Naming does *not* need to match the original definition's. Only order matters here. > Also note that the code above is just a demonstration and doesn't actually compile since we'd need to enforce certain bounds (e.g. `T1: Reflect`, `'a: 'static`, etc.) #### Nesting And, yes, you can nest remote types: ```rust #[reflect_remote(RemoteOuter)] struct OuterWrapper { #[reflect(remote = InnerWrapper)] pub inner: RemoteInner } #[reflect_remote(RemoteInner)] struct InnerWrapper(usize); ``` #### Assertions This macro will also generate some compile-time assertions to ensure that the correct types are used. It's important we catch this early so users don't have to wait for something to panic. And it also helps keep our `unsafe` a little safer. For example, a wrapper definition that does not match its corresponding remote type will result in an error: ```rust mod external_crate { pub struct TheirStruct(pub u32); } #[reflect_remote(external_crate::TheirStruct)] struct MyStruct(pub String); // ERROR: expected type `u32` but found `String` ``` <details> <summary>Generated Assertion</summary> ```rust const _: () = { #[allow(non_snake_case)] #[allow(unused_variables)] #[allow(unused_assignments)] #[allow(unreachable_patterns)] #[allow(clippy::multiple_bound_locations)] fn assert_wrapper_definition_matches_remote_type( mut __remote__: external_crate::TheirStruct, ) { __remote__.0 = (|| -> ::core::option::Option<String> { None })().unwrap(); } }; ``` </details> Additionally, using the incorrect type in a `#[reflect(remote = ...)]` attribute should result in an error: ```rust mod external_crate { pub struct TheirFoo(pub u32); pub struct TheirBar(pub i32); } #[reflect_remote(external_crate::TheirFoo)] struct MyFoo(pub u32); #[reflect_remote(external_crate::TheirBar)] struct MyBar(pub i32); #[derive(Reflect)] struct MyStruct { #[reflect(remote = MyBar)] // ERROR: expected type `TheirFoo` but found struct `TheirBar` foo: external_crate::TheirFoo } ``` <details> <summary>Generated Assertion</summary> ```rust const _: () = { struct RemoteFieldAssertions; impl RemoteFieldAssertions { #[allow(non_snake_case)] #[allow(clippy::multiple_bound_locations)] fn assert__foo__is_valid_remote() { let _: <MyBar as bevy_reflect::ReflectRemote>::Remote = (|| -> ::core::option::Option<external_crate::TheirFoo> { None })().unwrap(); } } }; ``` </details> ### Discussion There are a couple points that I think still need discussion or validation. - [x] 1. `Any` shenanigans ~~If we wanted to downcast our remote type from a `dyn Reflect`, we'd have to first downcast to the wrapper then extract the inner type. This PR has a [commit](b840db9f74cb6d357f951cb11b150d46bac89ee2) that addresses this by making all the `Reflect::*any` methods return the inner type rather than the wrapper type. This allows us to downcast directly to our remote type.~~ ~~However, I'm not sure if this is something we want to do. For unknowing users, it could be confusing and seemingly inconsistent. Is it worth keeping? Or should this behavior be removed?~~ I think this should be fine. The remote wrapper is an implementation detail and users should not need to downcast to the wrapper type. Feel free to let me know if there are other opinions on this though! - [x] 2. Implementing `Deref/DerefMut` and `From` ~~We don't currently do this, but should we implement other traits on the generated transparent struct? We could implement `Deref`/`DerefMut` to easily access the inner type. And we could implement `From` for easier conversion between the two types (e.g. `T: Into<Foo>`).~~ As mentioned in the comments, we probably don't need to do this. Again, the remote wrapper is an implementation detail, and should generally not be used directly. - [x] 3. ~~Should we define a getter/setter field attribute in this PR as well or leave it for a future one?~~ I think this should be saved for a future PR - [ ] 4. Any foreseeable issues with this implementation? #### Alternatives One alternative to defining our own `ReflectRemote` would be to use [bytemuck's `TransparentWrapper`](https://docs.rs/bytemuck/1.13.1/bytemuck/trait.TransparentWrapper.html) (as suggested by @danielhenrymantilla). This is definitely a viable option, as `ReflectRemote` is pretty much the same thing as `TransparentWrapper`. However, the cost would be bringing in a new crate— though, it is already in use in a few other sub-crates like bevy_render. I think we're okay just defining `ReflectRemote` ourselves, but we can go the bytemuck route if we'd prefer offloading that work to another crate. --- ## Changelog * Added the `#[reflect_remote(...)]` attribute macro to allow `Reflect` to be used on remote types * Added `ReflectRemote` trait for ensuring proper remote wrapper usage |
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What is Bevy?
Bevy is a refreshingly simple data-driven game engine built in Rust. It is free and open-source forever!
WARNING
Bevy is still in the early stages of development. Important features are missing. Documentation is sparse. A new version of Bevy containing breaking changes to the API is released approximately once every 3 months. We provide migration guides, but we can't guarantee migrations will always be easy. Use only if you are willing to work in this environment.
MSRV: Bevy relies heavily on improvements in the Rust language and compiler. As a result, the Minimum Supported Rust Version (MSRV) is generally close to "the latest stable release" of Rust.
Design Goals
- Capable: Offer a complete 2D and 3D feature set
- Simple: Easy for newbies to pick up, but infinitely flexible for power users
- Data Focused: Data-oriented architecture using the Entity Component System paradigm
- Modular: Use only what you need. Replace what you don't like
- Fast: App logic should run quickly, and when possible, in parallel
- Productive: Changes should compile quickly ... waiting isn't fun
About
- Features: A quick overview of Bevy's features.
- News: A development blog that covers our progress, plans and shiny new features.
Docs
- Quick Start Guide: Bevy's official Quick Start Guide. The best place to start learning Bevy.
- Bevy Rust API Docs: Bevy's Rust API docs, which are automatically generated from the doc comments in this repo.
- Official Examples: Bevy's dedicated, runnable examples, which are great for digging into specific concepts.
- Community-Made Learning Resources: More tutorials, documentation, and examples made by the Bevy community.
Community
Before contributing or participating in discussions with the community, you should familiarize yourself with our Code of Conduct.
- Discord: Bevy's official discord server.
- Reddit: Bevy's official subreddit.
- GitHub Discussions: The best place for questions about Bevy, answered right here!
- Bevy Assets: A collection of awesome Bevy projects, tools, plugins and learning materials.
Contributing
If you'd like to help build Bevy, check out the Contributor's Guide. For simple problems, feel free to open an issue or PR and tackle it yourself!
For more complex architecture decisions and experimental mad science, please open an RFC (Request For Comments) so we can brainstorm together effectively!
Getting Started
We recommend checking out the Quick Start Guide for a brief introduction.
Follow the Setup guide to ensure your development environment is set up correctly. Once set up, you can quickly try out the examples by cloning this repo and running the following commands:
# Switch to the correct version (latest release, default is main development branch)
git checkout latest
# Runs the "breakout" example
cargo run --example breakout
To draw a window with standard functionality enabled, use:
use bevy::prelude::*;
fn main(){
App::new()
.add_plugins(DefaultPlugins)
.run();
}
Fast Compiles
Bevy can be built just fine using default configuration on stable Rust. However for really fast iterative compiles, you should enable the "fast compiles" setup by following the instructions here.
Bevy Cargo Features
This list outlines the different cargo features supported by Bevy. These allow you to customize the Bevy feature set for your use-case.
Thanks
Bevy is the result of the hard work of many people. A huge thanks to all Bevy contributors, the many open source projects that have come before us, the Rust gamedev ecosystem, and the many libraries we build on.
A huge thanks to Bevy's generous sponsors. Bevy will always be free and open source, but it isn't free to make. Please consider sponsoring our work if you like what we're building.
This project is tested with BrowserStack.
License
Bevy is free, open source and permissively licensed! Except where noted (below and/or in individual files), all code in this repository is dual-licensed under either:
- MIT License (LICENSE-MIT or http://opensource.org/licenses/MIT)
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
at your option. This means you can select the license you prefer! This dual-licensing approach is the de-facto standard in the Rust ecosystem and there are very good reasons to include both.
Some of the engine's code carries additional copyright notices and license terms due to their external origins.
These are generally BSD-like, but exact details vary by crate:
If the README of a crate contains a 'License' header (or similar), the additional copyright notices and license terms applicable to that crate will be listed.
The above licensing requirement still applies to contributions to those crates, and sections of those crates will carry those license terms.
The license field of each crate will also reflect this.
For example, bevy_mikktspace has code under the Zlib license (as well as a copyright notice when choosing the MIT license).
The assets included in this repository (for our examples) typically fall under different open licenses. These will not be included in your game (unless copied in by you), and they are not distributed in the published bevy crates. See CREDITS.md for the details of the licenses of those files.
Your contributions
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.