denoland/deno · error · syn::Error

Unions are not supported

Error message

Unions are not supported

What it means

#[derive(ToV8)] routes Data::Struct and Data::Enum inputs to generated ToV8 impls, but Data::Union is rejected with this error on the item's span. A union's active-field semantics cannot be represented when converting to a V8 value, so the derive fails fast rather than guessing a layout.

Source

Thrown at libs/ops/conversion/to_v8/mod.rs:41

  let out = match input.data {
    Data::Struct(data) => {
      let fields = destruct_fields(&data.fields)?;
      let body = r#struct::get_body(span, data)?;

      create_impl(
        ident,
        &generics,
        quote! {
          let Self #fields = self;
          #body
        },
      )
    }
    Data::Enum(data) => {
      create_impl(ident, &generics, r#enum::get_body(span, input.attrs, data)?)
    }
    Data::Union(_) => return Err(Error::new(span, "Unions are not supported")),
  };

  Ok(out)
}

fn destruct_fields(fields: &Fields) -> Result<TokenStream, Error> {
  match fields {
    Fields::Named { 0: named } => {
      let fields = named
        .named
        .iter()
        .map(|field| field.ident.as_ref().unwrap());

      Ok(quote! {
        {
          #(#fields),*
        }
      })

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Model the 'one of several layouts' as an enum with a variant per alternative, which serializes naturally to V8.
2. Keep unions behind manual conversion: write an op that reads the union and returns a normal struct/array to JS.

Example fix

// before
#[derive(ToV8)]
union Raw {
  a: u32,
  b: f32,
} // error: Unions are not supported

// after
#[derive(ToV8)]
enum Raw {
  A(u32),
  B(f32),
}

When it happens

Trigger: Annotating a union with #[derive(ToV8)], e.g. #[derive(ToV8)] union Raw { a: u32, b: f32 } returned or accepted by an op.

Common situations: Shared derive preludes applied to every type in a module including FFI unions; refactoring a struct into a union while keeping its derives.

Related errors

• Unions are not supported
• Cannot use serde on unit variant
• Cannot combine `untagged` with `tag` or `content`
• `content` requires `tag` to be specified
• Unit structs cannot be destructured

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/e46ffb66d8da00c6. Report an issue: GitHub.