swc-project/swc · error

failed to parse input as a DeriveInput

Error message

failed to parse input as a DeriveInput

What it means

The #[ast_serde] proc-macro attribute received a token stream that syn could not parse as a DeriveInput. It fires at macro-expansion time when the attribute is applied to something that is not a struct, enum, or union definition (e.g. a fn, mod, or free statement), or when the item's tokens are otherwise malformed, so the macro cannot inspect the type's data to emit serde impls.

Source

Thrown at crates/ast_node/src/lib.rs:99

/// enum Expr {
///   #[tag("StringLiteral")]
///   #[tag("NumericLiteral")]
///   #[tag("BooleanLiteral")]
///   Lit(Lit),
/// }
/// ```
///
/// so the deserializer can decide which variant to use.
///
///
/// `#[tag]` also supports wildcard like `#[tag("*")]`. You can use this if
/// there are two many variants.
#[proc_macro_attribute]
pub fn ast_serde(
    args: proc_macro::TokenStream,
    input: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let input: DeriveInput = parse(input).expect("failed to parse input as a DeriveInput");

    // we should use call_site
    let mut item = TokenStream::new();
    match input.data {
        Data::Enum(..) => {
            if !args.is_empty() {
                panic!("#[ast_serde] on enum does not accept any argument")
            }

            item.extend(quote!(
                #[derive(::serde::Serialize, ::swc_common::DeserializeEnum)]
                #[serde(untagged)]
                #input
            ));
        }
        _ => {
            let args: Option<ast_node_macro::Args> = if args.is_empty() {
                None

View on GitHub (pinned to 5176682b65)

Solutions

1. Apply #[ast_serde] only to struct, enum, or union definitions
2. Check that the annotated item is complete and syntactically valid
3. If the item is generated by another macro, verify the generated tokens form a valid DeriveInput

When it happens

Trigger: Thrown at crates/ast_node/src/lib.rs:99 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/ed277823fbfe7bd4. Report an issue: GitHub.