Add derives for Deref and DerefMut

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 single-item structs"
+            } else {
+                "Deref can only be derived for single-item structs"
+            };
+            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")]