bevy_reflect: Allow #[reflect(default)] on enum variant fields

crates/bevy_reflect/bevy_reflect_derive/src/enum_utility.rs

@@ -1,10 +1,13 @@
-use crate::fq_std::FQDefault;
+use crate::derive_data::StructField;
+use crate::field_attributes::DefaultBehavior;
+use crate::fq_std::{FQDefault, FQOption};
 use crate::{
     derive_data::{EnumVariantFields, ReflectEnum},
     utility::ident_or_index,
 };
 use proc_macro2::Ident;
 use quote::{quote, ToTokens};
+use syn::Member;
 
 /// Contains all data needed to construct all variants within an enum.
 pub(crate) struct EnumVariantConstructors {
@@ -30,7 +33,7 @@ pub(crate) fn get_variant_constructors(
         let name = ident.to_string();
         let variant_constructor = reflect_enum.get_unit(ident);
 
-        let fields = match &variant.fields {
+        let fields: &[StructField] = match &variant.fields {
             EnumVariantFields::Unit => &[],
             EnumVariantFields::Named(fields) | EnumVariantFields::Unnamed(fields) => {
                 fields.as_slice()
@@ -39,35 +42,59 @@ pub(crate) fn get_variant_constructors(
         let mut reflect_index: usize = 0;
         let constructor_fields = fields.iter().enumerate().map(|(declare_index, field)| {
             let field_ident = ident_or_index(field.data.ident.as_ref(), declare_index);
+            let field_ty = &field.data.ty;
+
             let field_value = if field.attrs.ignore.is_ignored() {
-                quote! { #FQDefault::default() }
+                match &field.attrs.default {
+                    DefaultBehavior::Func(path) => quote! { #path() },
+                    _ => quote! { #FQDefault::default() }
+                }
             } else {
-                let error_repr = field.data.ident.as_ref().map_or_else(
-                    || format!("at index {reflect_index}"),
-                    |name| format!("`{name}`"),
-                );
-                let unwrapper = if can_panic {
-                    let type_err_message = format!(
-                        "the field {error_repr} should be of type `{}`",
-                        field.data.ty.to_token_stream()
-                    );
-                    quote!(.expect(#type_err_message))
+                let (resolve_error, resolve_missing) = if can_panic {
+                    let field_ref_str = match &field_ident {
+                        Member::Named(ident) => format!("the field `{ident}`"),
+                        Member::Unnamed(index) => format!("the field at index {}", index.index)
+                    };
+                    let ty = field.data.ty.to_token_stream();
+
+                    let on_error = format!("{field_ref_str} should be of type `{ty}`");
+                    let on_missing = format!("{field_ref_str} is required but could not be found");
+
+                    (quote!(.expect(#on_error)), quote!(.expect(#on_missing)))
                 } else {
-                    quote!(?)
+                    (quote!(?), quote!(?))
                 };
+
                 let field_accessor = match &field.data.ident {
                     Some(ident) => {
                         let name = ident.to_string();
-                        quote!(.field(#name))
+                        quote!(#ref_value.field(#name))
                     }
-                    None => quote!(.field_at(#reflect_index)),
+                    None => quote!(#ref_value.field_at(#reflect_index)),
                 };
                 reflect_index += 1;
-                let missing_field_err_message = format!("the field {error_repr} was not declared");
-                let accessor = quote!(#field_accessor .expect(#missing_field_err_message));
-                quote! {
-                    #bevy_reflect_path::FromReflect::from_reflect(#ref_value #accessor)
-                    #unwrapper
+
+                match &field.attrs.default {
+                    DefaultBehavior::Func(path) => quote! {
+                        if let #FQOption::Some(field) = #field_accessor {
+                            <#field_ty as #bevy_reflect_path::FromReflect>::from_reflect(field)
+                            #resolve_error
+                        } else {
+                            #path()
+                        }
+                    },
+                    DefaultBehavior::Default => quote! {
+                        if let #FQOption::Some(field) = #field_accessor {
+                            <#field_ty as #bevy_reflect_path::FromReflect>::from_reflect(field)
+                            #resolve_error
+                        } else {
+                            #FQDefault::default()
+                        }
+                    },
+                    DefaultBehavior::Required => quote! {
+                        <#field_ty as #bevy_reflect_path::FromReflect>::from_reflect(#field_accessor #resolve_missing)
+                        #resolve_error
+                    },
                 }
             };
             quote! { #field_ident : #field_value }

crates/bevy_reflect/bevy_reflect_derive/src/lib.rs

@@ -87,7 +87,8 @@ pub(crate) static REFLECT_VALUE_ATTRIBUTE_NAME: &str = "reflect_value";
 ///   to improve performance and/or robustness.
 ///   An example of where this is used is in the [`FromReflect`] derive macro,
 ///   where adding this attribute will cause the `FromReflect` implementation to create
-///   a base value using its [`Default`] implementation avoiding issues with ignored fields.
+///   a base value using its [`Default`] implementation avoiding issues with ignored fields
+///   (for structs and tuple structs only).
 ///
 /// ## `#[reflect_value]`
 ///

crates/bevy_reflect/src/lib.rs

@@ -751,6 +751,39 @@ mod tests {
         assert_eq!(Some(expected), my_struct);
     }
 
+    #[test]
+    fn from_reflect_should_use_default_variant_field_attributes() {
+        #[derive(Reflect, FromReflect, Eq, PartialEq, Debug)]
+        enum MyEnum {
+            Foo(#[reflect(default)] String),
+            Bar {
+                #[reflect(default = "get_baz_default")]
+                #[reflect(ignore)]
+                baz: usize,
+            },
+        }
+
+        fn get_baz_default() -> usize {
+            123
+        }
+
+        let expected = MyEnum::Foo(String::default());
+
+        let dyn_enum = DynamicEnum::new("Foo", DynamicTuple::default());
+        let my_enum = <MyEnum as FromReflect>::from_reflect(&dyn_enum);
+
+        assert_eq!(Some(expected), my_enum);
+
+        let expected = MyEnum::Bar {
+            baz: get_baz_default(),
+        };
+
+        let dyn_enum = DynamicEnum::new("Bar", DynamicStruct::default());
+        let my_enum = <MyEnum as FromReflect>::from_reflect(&dyn_enum);
+
+        assert_eq!(Some(expected), my_enum);
+    }
+
     #[test]
     fn from_reflect_should_use_default_container_attribute() {
         #[derive(Reflect, FromReflect, Eq, PartialEq, Debug)]