databendlabs/databend · error

unsupported type

Error message

unsupported type

What it means

A generic compile-time guard in emit_walk_for_type of the visit_derive macro: the field or variant type is not a Type::Path (e.g. it is a reference, slice, tuple, function pointer, or other exotic type), so the macro cannot resolve it to a Walk/WalkMut impl. It fires when derive(Walk)/derive(WalkMut) is applied to a type containing such a field, aborting the derive expansion.

Solutions

1. Use a supported type path in the derived field: primitives, Box, Option, Vec, or types implementing Walk/WalkMut.
2. If the field is a custom non-Walk type, wrap it in a newtype and implement Walk manually, or skip the field via #[visit(skip)] if the derive supports it.
3. For type paths with no last segment (e.g. exotic syn::Type paths), restructure the field declaration to a plain named type.

When it happens

Trigger: Thrown at src/query/ast/visit_derive/src/lib.rs:197 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/c0f137805bf3ec19. Report an issue: GitHub.

Appendix: source

Thrown at src/query/ast/visit_derive/src/lib.rs:197

    })
}

fn emit_field_stmt(
    field: &Field,
    access: TokenStream2,
    mutable: bool,
) -> syn::Result<TokenStream2> {
    emit_walk_for_type(&field.ty, access, mutable)
}

fn emit_walk_for_type(ty: &Type, access: TokenStream2, mutable: bool) -> syn::Result<TokenStream2> {
    match ty {
        Type::Path(type_path) => {
            let segment = type_path
                .path
                .segments
                .last()
                .ok_or_else(|| Error::new(type_path.span(), "unsupported type"))?;
            let name = segment.ident.to_string();

            if is_primitive_noop_leaf(&name) {
                return Ok(TokenStream2::new());
            }

            let trait_name = if mutable {
                quote!(crate::visit::WalkMut)
            } else {
                quote!(crate::visit::Walk)
            };
            let method = if mutable {
                quote!(walk_mut)
            } else {
                quote!(walk)
            };
            Ok(quote! {
                match #trait_name::#method(#access, visitor)? {

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