swc-project/swc · error

unknown attribute: {attr:?}

Error message

unknown attribute: {attr:?}

What it means

Compile-time panic from the `#[ast_node]` proc-macro attribute. The attribute list is parsed as comma-separated idents and only two flags are recognized: `no_clone` (skip deriving Clone) and `no_unknown` (skip the `#[cfg_attr(swc_ast_unknown, non_exhaustive)]` emission). Any other ident panics with `unknown attribute`.

Source

Thrown at crates/ast_node/src/lib.rs:184

    // we should use call_site
    let mut item = TokenStream::new();
    match &input.data {
        Data::Enum(data) => {
            use syn::parse::Parser;

            let attrs = <syn::punctuated::Punctuated<syn::Ident, syn::Token![,]>>::parse_terminated
                .parse(args)
                .expect("failed to parse #[ast_node]");

            let mut has_no_clone = false;
            let mut has_no_unknown = false;
            for attr in &attrs {
                if attr == "no_clone" {
                    has_no_clone = true;
                } else if attr == "no_unknown" {
                    has_no_unknown = true;
                } else {
                    panic!("unknown attribute: {attr:?}")
                }
            }

            let clone = if !has_no_clone {
                Some(quote!(#[derive(Clone)]))
            } else {
                None
            };
            let non_exhaustive = if !has_no_unknown {
                Some(quote!(#[cfg_attr(swc_ast_unknown, non_exhaustive)]))
            } else {
                None
            };

            let mut data = data.clone();
            if !has_no_unknown {
                let unknown: syn::Variant = if data
                    .variants

View on GitHub (pinned to 5176682b65)

Solutions

1. Remove the unknown ident from the attribute list
2. Use exactly `no_clone` or `no_unknown` (or both, comma-separated)
3. Check the attribute parsing code in crates/ast_node/src/lib.rs and its tests for the currently accepted flag set

Example fix

// before
#[ast_node(no_visit)]
pub struct Node { pub span: Span }

// after
#[ast_node(no_clone)]
pub struct Node { pub span: Span }

When it happens

Trigger: Using `#[ast_node(no_visit)]`, `#[ast_node(clone)]`, or a typo like `#[ast_node(no_clne)]`; anything not exactly `no_clone` or `no_unknown` reaches the else arm.

Common situations: Assuming ast_node supports visitor or serde flags it does not; copying flag names from other SWC macros or older versions; IDE autocomplete suggesting nonexistent flags.

Related errors

• An element descriptor's .kind property must be either "metho
• An element descriptor's .placement property must be one of "
• A class descriptor's .kind property must be "class", but a d
• ${objectType} can't have a .${name} property.
• Expected '${name}' to be a function

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/239db0dc9204072e. Report an issue: GitHub.