swc-project/swc · error

#[derive(Spanned)]: cannot determine span field to use for {

Error message

#[derive(Spanned)]: cannot determine span field to use for {}

What it means

Compile-time panic from `#[derive(Spanned)]` (crates/ast_node/src/spanned.rs). When no field carries any `#[span]`/`#[span(..)]` attribute, the macro falls back to using a field literally named `span`; if the struct has multiple fields and none is named `span`, it cannot pick one and panics, naming the offending type in the message.

Source

Thrown at crates/ast_node/src/spanned.rs:183

    // If all fields do not have `#[span(..)]`, check for field named `span`.
    let has_any_span_attr = bindings.iter().any(|b| {
        b.field()
            .attrs
            .iter()
            .any(|attr| is_attr_name(attr, "span"))
    });
    if !has_any_span_attr {
        let span_field = bindings
            .iter()
            .find(|b| {
                b.field()
                    .ident
                    .as_ref()
                    .map(|ident| ident == "span")
                    .unwrap_or(false)
            })
            .unwrap_or_else(|| {
                panic!(
                    "#[derive(Spanned)]: cannot determine span field to use for {}",
                    v.qual_path().into_token_stream()
                )
            });

        return simple_field(span_field);
    }

    let fields: Vec<_> = bindings
        .iter()
        .map(|b| (b, MyField::from_field(b.field())))
        .collect();

    // TODO: Only one field should be `#[span(lo)]`.
    let lo = fields.iter().find(|&(_, f)| f.lo);
    let hi = fields.iter().find(|&(_, f)| f.hi);

    match (lo, hi) {

View on GitHub (pinned to 5176682b65)

Solutions

1. Add a `pub span: Span` field (the swc AST convention)
2. Or mark exactly one field with a bare `#[span]` attribute to select the span source

Example fix

// before
#[derive(Spanned)]
struct Bin {
    left: Expr,
    right: Expr,
}

// after
#[derive(Spanned)]
struct Bin {
    left: Expr,
    #[span]
    right: Expr,
}

When it happens

Trigger: `#[derive(Spanned)] struct Bin { left: Expr, right: Expr }` — multiple fields, no `#[span]` attribute, no `span` field.

Common situations: Defining custom AST node structs with several fields and forgetting the span field or marker attribute, unlike swc_ecma_ast nodes which all carry `span: Span` first.

Related errors

• Unknown span attribute: {kind:?}
• Unknown span attribute
• #[derive(Spanned)] requires a field to get span from
• #[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/4acca02c8d15369d. Report an issue: GitHub.