tracel-ai/burn · error

Only struct can be derived

Error message

Only struct can be derived

What it means

Derive-macro input guard in the Module codegen: `parse_module_fields` only handles struct ASTs, so applying the Module derive to an enum or union reaches the non-struct arm and fails at compile time. The offending input is the non-struct item carrying the derive attribute.

Source

Thrown at crates/burn-derive/src/module/codegen_struct.rs:340

    generics: &mut ModuleGenerics,
) -> syn::Result<Vec<ModuleField>> {
    let mut fields = Vec::new();
    let mut module_generics = HashSet::new();
    let mut skip_generics = HashSet::new();

    match &ast.data {
        syn::Data::Struct(struct_data) => {
            for field in struct_data.fields.iter() {
                let field_type = parse_module_field_type(field, generics)?;
                if field_type.is_module {
                    module_generics.extend(field_type.generic_idents.iter().cloned());
                } else if matches!(field_type.attr, Some(ModuleFieldAttribute::Skip)) {
                    skip_generics.extend(field_type.generic_idents.iter().cloned());
                }
                fields.push(ModuleField::new(field.clone(), field_type));
            }
        }
        syn::Data::Enum(_) => panic!("Only struct can be derived"),
        syn::Data::Union(_) => panic!("Only struct can be derived"),
    };

    for ident in module_generics.intersection(&skip_generics) {
        if let Some(param) = ast.generics.params.iter().find_map(|p| match p {
            syn::GenericParam::Type(tp) if tp.ident == *ident => Some(tp),
            _ => None,
        }) {
            return Err(syn::Error::new(
                param.ident.span(),
                format!(
                    "Generic type `{ident}` should not be used on both a module field and a skipped field. \
                     Consider removing `#[module(skip)]` or using a different type for one of the fields.",
                ),
            ));
        }
    }

View on GitHub (pinned to d16f7ba2ed)

Solutions

1. Apply the Module derive only to structs; model alternatives with nested module structs or an enum of separate module types.
2. Remove the derive from the enum and implement `Module` manually if enum-like behavior is needed.
3. Wrap each variant's data in its own struct and dispatch outside the derive system.

When it happens

Trigger: Thrown at crates/burn-derive/src/module/codegen_struct.rs:340 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/0b1fc399f52c306c. Report an issue: GitHub.