swc-project/swc · error

#[derive(Spanned)] requires a field to get span from

Error message

#[derive(Spanned)] requires a field to get span from

What it means

Compile-time panic from `#[derive(Spanned)]` (crates/ast_node/src/spanned.rs). The derive computes a span from a struct field; if the type has zero bindings (a unit struct `struct X;`, or `struct X {}` / `struct X()` with no fields) there is nothing to take a span from and the macro panics. Types flagged as `swc_ast_unknown` return DUMMY_SP before this check.

Source

Thrown at crates/ast_node/src/spanned.rs:146

    item.with_generics(input.generics)
}

fn make_body_for_variant(v: &VariantBinder<'_>, bindings: Vec<BindedField<'_>>) -> Box<Expr> {
    /// `swc_common::Spanned::span(#field)`
    fn simple_field(field: &dyn ToTokens) -> Box<Expr> {
        Box::new(parse_quote_spanned! (def_site() => {
            swc_common::Spanned::span(#field)
        }))
    }

    if is_unknown(v.attrs()) {
        return Box::new(parse_quote_spanned! { v.data().span() => {
            swc_common::DUMMY_SP
        }});
    }

    if bindings.is_empty() {
        panic!("#[derive(Spanned)] requires a field to get span from")
    }

    if bindings.len() == 1 {
        if let Fields::Unnamed(..) = *v.data() {
            // Call self.0.span()
            return simple_field(&bindings[0]);
        }
    }

    //  Handle #[span] attribute.
    if let Some(f) = bindings
        .iter()
        .find(|b| has_empty_span_attr(&b.field().attrs))
    {
        //TODO: Verify that there's no more #[span]
        return simple_field(f);
    }

View on GitHub (pinned to 5176682b65)

Solutions

1. Give the struct a field to derive the span from (e.g. a `span: Span` field)
2. Implement `swc_common::Spanned` manually and return `DUMMY_SP` if the type genuinely has no location

Example fix

// before
#[derive(Spanned)]
struct Empty;

// after
struct Empty;

impl swc_common::Spanned for Empty {
    fn span(&self) -> swc_common::Span {
        swc_common::DUMMY_SP
    }
}

When it happens

Trigger: Deriving Spanned on `struct Marker;` or any empty struct.

Common situations: Adding Spanned to marker/error sentinel types while building custom AST nodes on top of swc_common.

Related errors

• Unknown span attribute: {kind:?}
• Unknown span attribute
• #[derive(Spanned)]: cannot determine span field to use for {
• #[derive(Spanned)]: #[span(lo)] and #[span(hi)] is required
• An element descriptor's .kind property must be either "metho

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/713176e69ce22820. Report an issue: GitHub.