bevyengine/bevy · error · syn::Error

Asset derive currently doesn't work on unions

Error message

Asset derive currently doesn't work on unions

What it means

Compile-time panic from the Asset derive macro machinery (derive_dependency_visitor_internal): the derive was applied to a union. The generated dependency-visitor code only supports structs and enums, so unions are rejected outright by the proc macro.

Source

Thrown at crates/bevy_asset/macros/src/lib.rs:89

            let cases = cases.map(|variant| {
                let ident = &variant.ident;
                let field_members = variant
                    .fields
                    .iter()
                    .enumerate()
                    .filter(|(_, f)| field_has_dep(f))
                    .map(|(i, field)| as_member(field.ident.as_ref(), i));
                let field_locals = field_members.clone().map(|m| format_ident!("__self_{}", m));
                let field_visitors = field_locals.clone().map(|i| visit_dep(quote!(#i)));
                quote!(Self::#ident {#(#field_members: #field_locals,)* ..} => {
                    #(#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 body.is_none() {
        quote! { _visit }
    } else {
        quote! { visit }
    };

    Ok(quote! {
        impl #impl_generics #bevy_asset_path::VisitAssetDependencies for #struct_name #type_generics #where_clause {
            fn visit_dependencies(&self, #visit: &mut impl ::core::ops::FnMut(#bevy_asset_path::UntypedAssetId)) {
                #body
            }

View on GitHub (pinned to 396ca72708)

Solutions

1. Replace the union with a struct or enum so the Asset derive can generate code
2. Implement the Asset trait (and any dependency visitor) manually for the union type if unions are truly required

When it happens

Trigger: Thrown at crates/bevy_asset/macros/src/lib.rs:89 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/24c421b00e39748e. Report an issue: GitHub.