swc-project/swc · error

invalid syntax

Error message

invalid syntax

What it means

The React display_name pass scans object literals (e.g. options passed to forwardRef/memo/createReactClass-like calls) for a `displayName` key. While matching property keys it treats Prop::Assign as impossible: `{a = 1}` assignment shorthand is cover grammar that is only valid inside destructuring patterns, so an object literal expression containing it is invalid syntax, hence unreachable!("invalid syntax").

Solutions

1. Normalize the AST before the React transform: convert Prop::Assign to Prop::KeyValue (and bare shorthand to key:value).
2. Fix the generator/codemod that produced `{a = b}` in expression position — it is invalid JavaScript.
3. Report to the producer tool, not swc, when the AST provably contains Prop::Assign in an expression.

Example fix

// before: cover-grammar node left in an object expression
// ast: ObjectLit { props: [Prop::Assign { key: displayName, value }) }] }

// after
// ast: ObjectLit { props: [Prop::KeyValue { key: displayName, value }) }] }
// source equivalent: ({ displayName: 'Foo' })

use swc_ecma_ast::{ObjectLit, Prop, PropOrSpread};
fn no_assign_props(o: &ObjectLit) -> bool {
    o.props.iter().all(|p| !matches!(p, PropOrSpread::Prop(p) if matches!(**p, Prop::Assign(_))))
}

fn is_plain_object_expr(o: &ObjectLit) -> bool {
    o.props.iter().all(|p| match p {
        PropOrSpread::Prop(p) => !matches!(**p, Prop::Assign(_)),
        PropOrSpread::Spread(_) => true,
    })
}

When it happens

Trigger: Feeding the React display_name transform an object expression containing Prop::Assign nodes — produced by custom transforms that turn object/array patterns into object literals, by lenient parsers, or by hand-built ASTs. Not reproducible from normal parsed source because the parser rejects it.

Common situations: Codemods migrating destructuring patterns into object expressions; ESLint/AST tooling feeding modified ASTs back into swc; tests constructing ObjectLit props manually.

Related errors

• assignment property be removed before generator pass
• attr_to_prop(JSXEmptyExpr)
• Duplicate __self is found
• Duplicate __source is found
• empty expression container

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/84baa88ce628a3fd. Report an issue: GitHub.

Appendix: source

Thrown at crates/swc_ecma_transforms_react/src/display_name/mod.rs:215

fn is_key_display_name(prop: &PropOrSpread) -> bool {
    match *prop {
        PropOrSpread::Prop(ref prop) => match **prop {
            Prop::Shorthand(ref i) => i.sym == "displayName",
            Prop::Method(MethodProp { ref key, .. })
            | Prop::Getter(GetterProp { ref key, .. })
            | Prop::Setter(SetterProp { ref key, .. })
            | Prop::KeyValue(KeyValueProp { ref key, .. }) => match *key {
                PropName::Ident(ref i) => i.sym == "displayName",
                PropName::Str(ref s) => {
                    matches!(s.value.as_str(), Some(value) if value == "displayName")
                }
                PropName::Num(..) => false,
                PropName::BigInt(..) => false,
                PropName::Computed(..) => false,
                #[cfg(swc_ast_unknown)]
                _ => panic!("unable to access unknown nodes"),
            },
            Prop::Assign(..) => unreachable!("invalid syntax"),
            #[cfg(swc_ast_unknown)]
            _ => panic!("unable to access unknown nodes"),
        },
        _ => false,
        // TODO(kdy1): maybe.. handle spread
    }
}

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