swc-project/swc · error

failed to parse as `InputFieldAttr`

Error message

failed to parse as `InputFieldAttr`

What it means

Inside MyField::from_field, a #[span(...)] list attribute on a struct field failed to parse into InputFieldAttr. This fires at macro-expansion time when the span attribute's inner tokens are not the expected list of span kinds — e.g. #[span(foo)] with an unrecognized keyword or a malformed argument — while processing fields for the Spanned derive.

Source

Thrown at crates/ast_node/src/spanned.rs:59

            is_unknown
        })
}

impl MyField {
    fn from_field(f: &Field) -> Self {
        let mut lo = false;
        let mut hi = false;

        for attr in &f.attrs {
            if !is_attr_name(attr, "span") {
                continue;
            }

            match &attr.meta {
                Meta::Path(..) => {}
                Meta::List(list) => {
                    let input = parse2::<InputFieldAttr>(list.tokens.clone())
                        .expect("failed to parse as `InputFieldAttr`");

                    for kind in input.kinds {
                        if kind == "lo" {
                            lo = true
                        } else if kind == "hi" {
                            hi = true
                        } else {
                            panic!("Unknown span attribute: {kind:?}")
                        }
                    }
                }
                _ => panic!("Unknown span attribute"),
            }
        }

        Self {
            ident: f.ident.clone(),
            ty: f.ty.clone(),

View on GitHub (pinned to 5176682b65)

Solutions

1. Use only the span kinds supported by the Spanned derive inside #[span(...)]
2. Remove or fix malformed span attributes on the offending field
3. Compare with existing fields in swc AST types for correct #[span] usage examples

When it happens

Trigger: Thrown at crates/ast_node/src/spanned.rs:59 when the library encounters an invalid state.

Common situations: See trigger scenarios.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/7ce9268916090dad. Report an issue: GitHub.