databendlabs/databend · error

unions are not supported

Error message

unions are not supported

What it means

A compile-time guard in the derive macro's emit_walk_body: derive(Walk)/derive(WalkMut) can only be applied to structs and enums. This error fires when the macro is invoked on a syn::Data::Union, because there is no defined traversal order for union fields; the derive expansion fails at compile time.

Solutions

1. Refactor the AST node to avoid union types in positions processed by the Walk/WalkMut derive, since emit_walk_body only handles structs and enums.
2. Implement Walk/WalkMut manually for the affected type instead of using the derive.
3. Restructure the union as an enum with tagged variants so the derive can enumerate cases.

When it happens

Trigger: Thrown at src/query/ast/visit_derive/src/lib.rs:85 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of databendlabs/databend@288d84d76e (2026-09-11). Data as JSON: /api/errors/59acf73c5d559123. Report an issue: GitHub.

Appendix: source

Thrown at src/query/ast/visit_derive/src/lib.rs:85

            #[automatically_derived]
            impl #impl_generics crate::visit::Walk for #ident #ty_generics #where_clause {
                fn walk<V: crate::visit::Visitor + ?Sized>(
                    &self,
                    visitor: &mut V,
                ) -> std::result::Result<crate::visit::VisitControl<V::Break>, V::Error> {
                    #walk_body
                    Ok(crate::visit::VisitControl::Continue)
                }
            }
        })
    }
}

fn emit_walk_body(data: &Data, mutable: bool) -> syn::Result<TokenStream2> {
    match data {
        Data::Struct(data) => emit_struct_body(data, mutable),
        Data::Enum(data) => emit_enum_body(data, mutable),
        Data::Union(data) => Err(Error::new(
            data.union_token.span(),
            "unions are not supported",
        )),
    }
}

fn emit_struct_body(data: &DataStruct, mutable: bool) -> syn::Result<TokenStream2> {
    let access = |member: TokenStream2| {
        if mutable {
            quote!(&mut self.#member)
        } else {
            quote!(&self.#member)
        }
    };

    let stmts = match &data.fields {
        Fields::Named(fields) => fields
            .named

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