What causes "Expected '${name}' to be a function" in swc-project/swc?

Confirm #[derive(Spanned)] from swc is placed on a plain struct/enum/union definition with valid syntax (let rustc's own parse errors surface first - fix any preceding syntax errors in the item).

swc-project/swc · error · TypeError

Expected '${name}' to be a function

Error message

Expected '${name}' to be a function

What it means

The #[proc_macro_derive(Spanned)] entry point parses the incoming TokenStream as a syn::DeriveInput and unwraps with .expect("failed to parse input as DeriveInput"). rustc only feeds derive macros with the AST of the annotated item, so in normal builds this never fails; if it does, the token stream handed to the macro was not a valid derive target (struct/enum/union item), and the proc macro panics with 'proc-macro panicked' plus this message.

Source

Thrown at crates/swc_ecma_transforms_base/src/helpers/generated/_decorate.rs:257

        throw new TypeError("A class descriptor's .kind property must be \"class\", but a decorator" + " created a class descriptor with .kind \"" + kind + "\"");
    }
    _disallowProperty(obj, "key", "A class descriptor");
    _disallowProperty(obj, "placement", "A class descriptor");
    _disallowProperty(obj, "descriptor", "A class descriptor");
    _disallowProperty(obj, "initializer", "A class descriptor");
    _disallowProperty(obj, "extras", "A class descriptor");
    var finisher = _optionalCallableProperty(obj, "finisher");
    var elements = _toElementDescriptors(obj.elements);

    return { elements: elements, finisher: finisher };
}
function _disallowProperty(obj, name, objectType) {
    if (obj[name] !== undefined) throw new TypeError(objectType + " can't have a ." + name + " property.");
}
function _optionalCallableProperty(obj, name) {
    var value = obj[name];
    if (value !== undefined && typeof value !== "function") {
        throw new TypeError("Expected '" + name + "' to be a function");
    }

    return value;
}
function _runClassFinishers(constructor, finishers) {
    for (var i = 0; i < finishers.length; i++) {
        var newConstructor = (0, finishers[i])(constructor);
        if (newConstructor !== undefined) {
            if (typeof newConstructor !== "function") throw new TypeError("Finishers must return a constructor.");
            constructor = newConstructor;
        }
    }

    return constructor;
}
"#,
    #[cfg(feature = "inline-helpers")]
    deps: super::HelperBitmap::from_bits(0x0000000601a000044000080000000018),

View on GitHub (pinned to 5176682b65)

Solutions

Confirm #[derive(Spanned)] from swc is placed on a plain struct/enum/union definition with valid syntax (let rustc's own parse errors surface first - fix any preceding syntax errors in the item).
If the item is macro-generated, expand the outer macro (cargo expand) and check the emitted item is a well-formed struct/enum/union.
Check for duplicate/conflicting derives or attributes that rewrite the item before derivation.
If it persists on valid code, report it to the swc repo with the item source and rustc/cargo versions.

Defensive patterns

Strategy: validation

Prevention

Place #[derive(Spanned)] only on ordinary struct/enum/union items that already compile without the derive.
When the item comes from another macro, expand it (cargo expand) to confirm it is a plain item before adding derives.

When it happens

Trigger: Using #[derive(Spanned)] on something that is not a plain item (e.g. inside macro-generated code whose expansion is not a valid DeriveInput), invoking the macro directly with raw tokens (macro playgrounds, quote-and-invoke tooling), or a syn version mismatch between the macro crate and what produced the tokens.

Common situations: Almost always a tooling/implementation accident rather than user code: nightly vs stable parser differences, third-party crates re-exporting the derive with incompatible syn versions, or test harnesses that call proc macros with hand-built token streams.

Related errors

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/dded5689bbdd91ac. Report an issue: GitHub.