swc-project/swc · error

union unsupported

Error message

union unsupported

What it means

The final match of the Decode derive only handles Data::Struct and Data::Enum; deriving it on a union hits `Data::Union(_) => panic!("union unsupported")`. Unions have no field order or tag semantics, so there is no CBOR mapping the macro could generate correctly. The sibling Encode derive has the same restriction.

Source

Thrown at crates/ast_node/src/encoding/decode.rs:282

            };

            syn::parse_quote! {
                impl<'de> cbor4ii::core::dec::Decode<'de> for #ident {
                    #[inline]
                    fn decode<R: cbor4ii::core::dec::Read<'de>>(reader: &mut R)
                        -> Result<Self, cbor4ii::core::error::DecodeError<R::Error>>
                    {
                        #tag
                        let value = match tag {
                            #(#fields)*
                            #unknown_arm
                        };
                        Ok(value)
                    }
                }
            }
        }
        Data::Union(_) => panic!("union unsupported"),
    }
}

View on GitHub (pinned to 5176682b65)

Solutions

1. Replace the union with a struct or enum that makes the alternative explicit.
2. Or hand-write the cbor4ii Decode/Encode impls, encoding a manual tag for the active alternative.

Example fix

// before
#[derive(Encode, Decode)]
union Value {
    u: u32,
    f: f32,
}

// after — model the alternatives explicitly
#[derive(Encode, Decode)]
enum Value {
    U(u32),
    F(f32),
}

When it happens

Trigger: #[derive(Decode)] (or swc_common::Decode via #[ast_node]) applied to a `union U { a: u32, b: f32 }`.

Common situations: Trying to give FFI/interop unions wire codecs; converting low-level memory-layout types into AST-node-like types.

Related errors

• Cannot use both #[encoding(with)] and #[encoding(ignore)] at
• more than 1 unnamed member field are not allowed
• enum member types must be consistent: {:?}
• unknown member must be a tag and a value
• named enum unsupported

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/b5a1ae8ed315c2ed. Report an issue: GitHub.