derive asset for enums (#10410)

# Objective

allow deriving `Asset` for enums, and for tuple structs.

## Solution

add to the proc macro, generating visitor calls to the variant's data
(if required).

supports unnamed or named field variants, and struct variants when the
struct also derives `Asset`:
```rust
#[derive(Asset, TypePath)]
pub enum MyAssetEnum {
    Unnamed (
        #[dependency]
        Handle<Image>
    ),
    Named {
        #[dependency]
        array_handle: Handle<Image>,
        #[dependency]
        atlas_handles: Vec<Handle<Image>>,
    },
    StructStyle(
        #[dependency]
        VariantStruct
    ),
    Empty,
}

#[derive(Asset, TypePath)]
pub struct VariantStruct {
    // ...
}
```

also extend the struct implementation to support tuple structs: 
```rust
#[derive(Asset, TypePath)]
pub struct MyImageNewtype(
    #[dependency]
    Handle<Image>
);
````

---------

Co-authored-by: Nicola Papale <nico@nicopap.ch>

crates/bevy_asset/macros/src/lib.rs

@@ -1,6 +1,6 @@
 use bevy_macro_utils::BevyManifest;
 use proc_macro::{Span, TokenStream};
-use quote::quote;
+use quote::{format_ident, quote};
 use syn::{parse_macro_input, Data, DeriveInput, Path};
 
 pub(crate) fn bevy_asset_path() -> syn::Path {
@@ -41,33 +41,71 @@ fn derive_dependency_visitor_internal(
     ast: &DeriveInput,
     bevy_asset_path: &Path,
 ) -> Result<proc_macro2::TokenStream, syn::Error> {
-    let mut field_visitors = Vec::new();
-    if let Data::Struct(data_struct) = &ast.data {
-        for field in &data_struct.fields {
-            if field
-                .attrs
-                .iter()
-                .any(|a| a.path().is_ident(DEPENDENCY_ATTRIBUTE))
-            {
-                if let Some(field_ident) = &field.ident {
-                    field_visitors.push(quote! {
-                        #bevy_asset_path::VisitAssetDependencies::visit_dependencies(&self.#field_ident, visit);
-                    });
-                }
-            }
-        }
-    } else {
-        return Err(syn::Error::new(
-            Span::call_site().into(),
-            "Asset derive currently only works on structs",
-        ));
-    }
-
     let struct_name = &ast.ident;
     let (impl_generics, type_generics, where_clause) = &ast.generics.split_for_impl();
 
+    let visit_dep = |to_read| quote!(#bevy_asset_path::VisitAssetDependencies::visit_dependencies(#to_read, visit););
+    let is_dep_attribute = |a: &syn::Attribute| a.path().is_ident(DEPENDENCY_ATTRIBUTE);
+    let field_has_dep = |f: &syn::Field| f.attrs.iter().any(is_dep_attribute);
+
+    let body = match &ast.data {
+        Data::Struct(data_struct) => {
+            let fields = data_struct.fields.iter();
+            let field_visitors = fields.enumerate().filter(|(_, f)| field_has_dep(f));
+            let field_visitors = field_visitors.map(|(i, field)| match &field.ident {
+                Some(ident) => visit_dep(quote!(&self.#ident)),
+                None => {
+                    let index = syn::Index::from(i);
+                    visit_dep(quote!(&self.#index))
+                }
+            });
+            Some(quote!( #(#field_visitors)* ))
+        }
+        Data::Enum(data_enum) => {
+            let variant_has_dep = |v: &syn::Variant| v.fields.iter().any(field_has_dep);
+            let any_case_required = data_enum.variants.iter().any(variant_has_dep);
+            let cases = data_enum.variants.iter().filter(|v| variant_has_dep(v));
+            let cases = cases.map(|variant| {
+                let ident = &variant.ident;
+                let fields = &variant.fields;
+
+                let field_visitors = fields.iter().enumerate().filter(|(_, f)| field_has_dep(f));
+
+                let field_visitors = field_visitors.map(|(i, field)| match &field.ident {
+                    Some(ident) => visit_dep(quote!(#ident)),
+                    None => {
+                        let ident = format_ident!("member{i}");
+                        visit_dep(quote!(#ident))
+                    }
+                });
+                let fields = match fields {
+                    syn::Fields::Named(fields) => {
+                        let named = fields.named.iter().map(|f| f.ident.as_ref());
+                        quote!({ #(#named,)* .. })
+                    }
+                    syn::Fields::Unnamed(fields) => {
+                        let named = (0..fields.unnamed.len()).map(|i| format_ident!("member{i}"));
+                        quote!( ( #(#named,)* ) )
+                    }
+                    syn::Fields::Unit => unreachable!("Can't pass filter is_dep_attribute"),
+                };
+                quote!(Self::#ident #fields => {
+                    #(#field_visitors)*
+                })
+            });
+
+            any_case_required.then(|| quote!(match self { #(#cases)*, _ => {} }))
+        }
+        Data::Union(_) => {
+            return Err(syn::Error::new(
+                Span::call_site().into(),
+                "Asset derive currently doesn't work on unions",
+            ));
+        }
+    };
+
     // prevent unused variable warning in case there are no dependencies
-    let visit = if field_visitors.is_empty() {
+    let visit = if body.is_none() {
         quote! { _visit }
     } else {
         quote! { visit }
@@ -76,7 +114,7 @@ fn derive_dependency_visitor_internal(
     Ok(quote! {
         impl #impl_generics #bevy_asset_path::VisitAssetDependencies for #struct_name #type_generics #where_clause {
             fn visit_dependencies(&self, #visit: &mut impl FnMut(#bevy_asset_path::UntypedAssetId)) {
-                #(#field_visitors)*
+                #body
             }
         }
     })

crates/bevy_asset/src/lib.rs

@@ -1180,4 +1180,35 @@ mod tests {
         // running schedule does not error on ambiguity between the 2 uses_assets systems
         app.world.run_schedule(Update);
     }
+
+    // validate the Asset derive macro for various asset types
+    #[derive(Asset, TypePath)]
+    pub struct TestAsset;
+
+    #[allow(dead_code)]
+    #[derive(Asset, TypePath)]
+    pub enum EnumTestAsset {
+        Unnamed(#[dependency] Handle<TestAsset>),
+        Named {
+            #[dependency]
+            handle: Handle<TestAsset>,
+            #[dependency]
+            vec_handles: Vec<Handle<TestAsset>>,
+            #[dependency]
+            embedded: TestAsset,
+        },
+        StructStyle(#[dependency] TestAsset),
+        Empty,
+    }
+
+    #[derive(Asset, TypePath)]
+    pub struct StructTestAsset {
+        #[dependency]
+        handle: Handle<TestAsset>,
+        #[dependency]
+        embedded: TestAsset,
+    }
+
+    #[derive(Asset, TypePath)]
+    pub struct TupleTestAsset(#[dependency] Handle<TestAsset>);
 }