bevy_derive: Add derives for Deref and DerefMut (bevyengine#4328)

# Objective

A common pattern in Rust is the [newtype](https://doc.rust-lang.org/rust-by-example/generics/new_types.html). This is an especially useful pattern in Bevy as it allows us to give common/foreign types different semantics (such as allowing it to implement `Component` or `FromWorld`) or to simply treat them as a "new type" (clever). For example, it allows us to wrap a common `Vec<String>` and do things like:

```rust
#[derive(Component)]
struct Items(Vec<String>);

fn give_sword(query: Query<&mut Items>) { 
  query.single_mut().0.push(String::from("Flaming Poisoning Raging Sword of Doom"));
}
```

> We could then define another struct that wraps `Vec<String>` without anything clashing in the query.

However, one of the worst parts of this pattern is the ugly `.0` we have to write in order to access the type we actually care about. This is why people often implement `Deref` and `DerefMut` in order to get around this.

Since it's such a common pattern, especially for Bevy, it makes sense to add a derive macro to automatically add those implementations.


## Solution

Added a derive macro for `Deref` and another for `DerefMut` (both exported into the prelude). This works on all structs (including tuple structs) as long as they only contain a single field:

```rust
#[derive(Deref)]
struct Foo(String);

#[derive(Deref, DerefMut)]
struct Bar {
  name: String,
}
```

This allows us to then remove that pesky `.0`:

```rust
#[derive(Component, Deref, DerefMut)]
struct Items(Vec<String>);

fn give_sword(query: Query<&mut Items>) { 
  query.single_mut().push(String::from("Flaming Poisoning Raging Sword of Doom"));
}
```

### Alternatives

There are other alternatives to this such as by using the [`derive_more`](https://crates.io/crates/derive_more) crate. However, it doesn't seem like we need an entire crate just yet since we only need `Deref` and `DerefMut` (for now).

### Considerations

One thing to consider is that the Rust std library recommends _not_ using `Deref` and `DerefMut` for things like this: "`Deref` should only be implemented for smart pointers to avoid confusion" ([reference](https://doc.rust-lang.org/std/ops/trait.Deref.html)). Personally, I believe it makes sense to use it in the way described above, but others may disagree.

### Additional Context

Discord: https://discord.com/channels/691052431525675048/692572690833473578/956648422163746827 (controversiality discussed [here](https://discord.com/channels/691052431525675048/692572690833473578/956711911481835630))

---

## Changelog

- Add `Deref` derive macro (exported to prelude)
- Add `DerefMut` derive macro (exported to prelude)
- Updated most newtypes in examples to use one or both derives

Co-authored-by: MrGVSV <49806985+MrGVSV@users.noreply.github.com>

Author: MrGVSV

crates/bevy_derive/src/derefs.rs

@@ -0,0 +1,69 @@
+use proc_macro::{Span, TokenStream};
+use quote::quote;
+use syn::{parse_macro_input, Data, DeriveInput, Index, Member, Type};
+
+pub fn derive_deref(input: TokenStream) -> TokenStream {
+    let ast = parse_macro_input!(input as DeriveInput);
+
+    let ident = &ast.ident;
+    let (field_member, field_type) = match get_inner_field(&ast, false) {
+        Ok(items) => items,
+        Err(err) => {
+            return err.into_compile_error().into();
+        }
+    };
+    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
+
+    TokenStream::from(quote! {
+        impl #impl_generics ::std::ops::Deref for #ident #ty_generics #where_clause {
+            type Target = #field_type;
+
+            fn deref(&self) -> &Self::Target {
+                &self.#field_member
+            }
+        }
+    })
+}
+
+pub fn derive_deref_mut(input: TokenStream) -> TokenStream {
+    let ast = parse_macro_input!(input as DeriveInput);
+
+    let ident = &ast.ident;
+    let (field_member, _) = match get_inner_field(&ast, true) {
+        Ok(items) => items,
+        Err(err) => {
+            return err.into_compile_error().into();
+        }
+    };
+    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
+
+    TokenStream::from(quote! {
+        impl #impl_generics ::std::ops::DerefMut for #ident #ty_generics #where_clause {
+            fn deref_mut(&mut self) -> &mut Self::Target {
+                &mut self.#field_member
+            }
+        }
+    })
+}
+
+fn get_inner_field(ast: &DeriveInput, is_mut: bool) -> syn::Result<(Member, &Type)> {
+    match &ast.data {
+        Data::Struct(data_struct) if data_struct.fields.len() == 1 => {
+            let field = data_struct.fields.iter().next().unwrap();
+            let member = field
+                .ident
+                .as_ref()
+                .map(|name| Member::Named(name.clone()))
+                .unwrap_or_else(|| Member::Unnamed(Index::from(0)));
+            Ok((member, &field.ty))
+        }
+        _ => {
+            let msg = if is_mut {
+                "DerefMut can only be derived for structs with a single field"
+            } else {
+                "Deref can only be derived for structs with a single field"
+            };
+            Err(syn::Error::new(Span::call_site().into(), msg))
+        }
+    }
+}

crates/bevy_derive/src/lib.rs

@@ -2,6 +2,7 @@ extern crate proc_macro;
 
 mod app_plugin;
 mod bevy_main;
+mod derefs;
 mod enum_variant_meta;
 mod modules;
 
@@ -15,6 +16,61 @@ pub fn derive_dynamic_plugin(input: TokenStream) -> TokenStream {
     app_plugin::derive_dynamic_plugin(input)
 }
 
+/// Implements [`Deref`] for _single-item_ structs. This is especially useful when
+/// utilizing the [newtype] pattern.
+///
+/// If you need [`DerefMut`] as well, consider using the other [derive] macro alongside
+/// this one.
+///
+/// # Example
+///
+/// ```
+/// use bevy_derive::Deref;
+///
+/// #[derive(Deref)]
+/// struct MyNewtype(String);
+///
+/// let foo = MyNewtype(String::from("Hello"));
+/// assert_eq!(5, foo.len());
+/// ```
+///
+/// [`Deref`]: std::ops::Deref
+/// [newtype]: https://doc.rust-lang.org/rust-by-example/generics/new_types.html
+/// [`DerefMut`]: std::ops::DerefMut
+/// [derive]: crate::derive_deref_mut
+#[proc_macro_derive(Deref)]
+pub fn derive_deref(input: TokenStream) -> TokenStream {
+    derefs::derive_deref(input)
+}
+
+/// Implements [`DerefMut`] for _single-item_ structs. This is especially useful when
+/// utilizing the [newtype] pattern.
+///
+/// [`DerefMut`] requires a [`Deref`] implementation. You can implement it manually or use
+/// Bevy's [derive] macro for convenience.
+///
+/// # Example
+///
+/// ```
+/// use bevy_derive::{Deref, DerefMut};
+///
+/// #[derive(Deref, DerefMut)]
+/// struct MyNewtype(String);
+///
+/// let mut foo = MyNewtype(String::from("Hello"));
+/// foo.push_str(" World!");
+/// assert_eq!("Hello World!", *foo);
+/// ```
+///
+/// [`DerefMut`]: std::ops::DerefMut
+/// [newtype]: https://doc.rust-lang.org/rust-by-example/generics/new_types.html
+/// [`Deref`]: std::ops::Deref
+/// [derive]: crate::derive_deref
+#[proc_macro_derive(DerefMut)]
+pub fn derive_deref_mut(input: TokenStream) -> TokenStream {
+    derefs::derive_deref_mut(input)
+}
+
 #[proc_macro_attribute]
 pub fn bevy_main(attr: TokenStream, item: TokenStream) -> TokenStream {
     bevy_main::bevy_main(attr, item)

crates/bevy_internal/src/prelude.rs

@@ -5,7 +5,7 @@ pub use crate::{
     transform::prelude::*, utils::prelude::*, window::prelude::*, DefaultPlugins, MinimalPlugins,
 };
 
-pub use bevy_derive::bevy_main;
+pub use bevy_derive::{bevy_main, Deref, DerefMut};
 
 #[doc(hidden)]
 #[cfg(feature = "bevy_audio")]

examples/2d/contributors.rs

@@ -27,6 +27,7 @@ struct ContributorSelection {
     idx: usize,
 }
 
+#[derive(Deref, DerefMut)]
 struct SelectTimer(Timer);
 
 #[derive(Component)]
@@ -161,7 +162,7 @@ fn select_system(
     mut query: Query<(&Contributor, &mut Sprite, &mut Transform)>,
     time: Res<Time>,
 ) {
-    if !timer.0.tick(time.delta()).just_finished() {
+    if !timer.tick(time.delta()).just_finished() {
         return;
     }
 

examples/2d/many_sprites.rs

@@ -74,6 +74,7 @@ fn move_camera(time: Res<Time>, mut camera_query: Query<&mut Transform, With<Cam
         * Transform::from_translation(Vec3::X * CAMERA_SPEED * time.delta_seconds());
 }
 
+#[derive(Deref, DerefMut)]
 struct PrintingTimer(Timer);
 
 impl Default for PrintingTimer {
@@ -84,9 +85,9 @@ impl Default for PrintingTimer {
 
 // System for printing the number of sprites on every tick of the timer
 fn print_sprite_count(time: Res<Time>, mut timer: Local<PrintingTimer>, sprites: Query<&Sprite>) {
-    timer.0.tick(time.delta());
+    timer.tick(time.delta());
 
-    if timer.0.just_finished() {
+    if timer.just_finished() {
         info!("Sprites: {}", sprites.iter().count(),);
     }
 }

examples/2d/sprite_sheet.rs

@@ -8,7 +8,7 @@ fn main() {
         .run();
 }
 
-#[derive(Component)]
+#[derive(Component, Deref, DerefMut)]
 struct AnimationTimer(Timer);
 
 fn animate_sprite(
@@ -21,8 +21,8 @@ fn animate_sprite(
     )>,
 ) {
     for (mut timer, mut sprite, texture_atlas_handle) in query.iter_mut() {
-        timer.0.tick(time.delta());
-        if timer.0.just_finished() {
+        timer.tick(time.delta());
+        if timer.just_finished() {
             let texture_atlas = texture_atlases.get(texture_atlas_handle).unwrap();
             sprite.index = (sprite.index + 1) % texture_atlas.textures.len();
         }

examples/3d/many_cubes.rs

@@ -145,9 +145,9 @@ fn print_mesh_count(
     mut timer: Local<PrintingTimer>,
     sprites: Query<(&Handle<Mesh>, &ComputedVisibility)>,
 ) {
-    timer.0.tick(time.delta());
+    timer.tick(time.delta());
 
-    if timer.0.just_finished() {
+    if timer.just_finished() {
         info!(
             "Meshes: {} - Visible Meshes {}",
             sprites.iter().len(),
@@ -156,6 +156,7 @@ fn print_mesh_count(
     }
 }
 
+#[derive(Deref, DerefMut)]
 struct PrintingTimer(Timer);
 
 impl Default for PrintingTimer {

examples/async_tasks/async_compute.rs

@@ -22,7 +22,9 @@ fn main() {
 // Number of cubes to spawn across the x, y, and z axis
 const NUM_CUBES: u32 = 6;
 
+#[derive(Deref)]
 struct BoxMeshHandle(Handle<Mesh>);
+#[derive(Deref)]
 struct BoxMaterialHandle(Handle<StandardMaterial>);
 
 /// Startup system which runs only once and generates our Box Mesh
@@ -84,8 +86,8 @@ fn handle_tasks(
         if let Some(transform) = future::block_on(future::poll_once(&mut *task)) {
             // Add our new PbrBundle of components to our tagged entity
             commands.entity(entity).insert_bundle(PbrBundle {
-                mesh: box_mesh_handle.0.clone(),
-                material: box_material_handle.0.clone(),
+                mesh: box_mesh_handle.clone(),
+                material: box_material_handle.clone(),
                 transform,
                 ..default()
             });

examples/async_tasks/external_source_external_thread.rs

@@ -15,9 +15,11 @@ fn main() {
         .run();
 }
 
+#[derive(Deref)]
 struct StreamReceiver(Receiver<u32>);
 struct StreamEvent(u32);
 
+#[derive(Deref)]
 struct LoadedFont(Handle<Font>);
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
@@ -43,7 +45,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
 
 // This system reads from the receiver and sends events to Bevy
 fn read_stream(receiver: ResMut<StreamReceiver>, mut events: EventWriter<StreamEvent>) {
-    for from_stream in receiver.0.try_iter() {
+    for from_stream in receiver.try_iter() {
         events.send(StreamEvent(from_stream));
     }
 }
@@ -54,7 +56,7 @@ fn spawn_text(
     loaded_font: Res<LoadedFont>,
 ) {
     let text_style = TextStyle {
-        font: loaded_font.0.clone(),
+        font: loaded_font.clone(),
         font_size: 20.0,
         color: Color::WHITE,
     };

examples/ecs/generic_system.rs

@@ -20,7 +20,7 @@ enum AppState {
 #[derive(Component)]
 struct TextToPrint(String);
 
-#[derive(Component)]
+#[derive(Component, Deref, DerefMut)]
 struct PrinterTick(bevy::prelude::Timer);
 
 #[derive(Component)]
@@ -67,7 +67,7 @@ fn setup_system(mut commands: Commands) {
 
 fn print_text_system(time: Res<Time>, mut query: Query<(&mut PrinterTick, &TextToPrint)>) {
     for (mut timer, text) in query.iter_mut() {
-        if timer.0.tick(time.delta()).just_finished() {
+        if timer.tick(time.delta()).just_finished() {
             info!("{}", text.0);
         }
     }