denoland/deno · error · syn::Error

Cannot combine `untagged` with `tag` or `content`

Error message

Cannot combine `untagged` with `tag` or `content`

What it means

#[derive(ToV8)] supports several tagging modes set at the container level: #[to_v8(tag = "...")] (internally tagged), adding #[to_v8(content = "...")] (adjacently tagged), and #[to_v8(untagged)]. When untagged is combined with tag or content, the modes contradict (untagged emits no tag at all), so parse_enum_mode returns this compile error at the call site.

Source

Thrown at libs/ops/conversion/to_v8/enum.rs:264

        let args = list.parse_args_with(
          Punctuated::<EnumModeArgument, Token![,]>::parse_terminated,
        )?;

        for arg in args {
          match arg {
            EnumModeArgument::Tag { value, .. } => tag = Some(value.value()),
            EnumModeArgument::Content { value, .. } => {
              content = Some(value.value())
            }
            EnumModeArgument::Untagged { .. } => untagged = true,
          }
        }
      }
    }

    if untagged {
      if tag.is_some() || content.is_some() {
        return Err(Error::new(
          Span::call_site(),
          "Cannot combine `untagged` with `tag` or `content`",
        ));
      }
      return Ok(EnumMode::Untagged);
    }

    match (tag, content) {
      (None, None) => Ok(EnumMode::ExternallyTagged),
      (Some(tag), None) => Ok(EnumMode::InternallyTagged {
        tag: crate::get_internalized_string(Ident::new(&tag, span))?,
      }),
      (Some(tag), Some(content)) => Ok(EnumMode::AdjacentlyTagged {
        tag: crate::get_internalized_string(Ident::new(&tag, span))?,
        content: crate::get_internalized_string(Ident::new(&content, span))?,
      }),
      (None, Some(_)) => Err(Error::new(
        Span::call_site(),

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Decide the mode: keep #[to_v8(untagged)] alone, or remove untagged and keep tag (optionally with content).
2. If you need per-variant control, use rename/serde attributes on variants instead of mixing container modes.

Example fix

// before
#[derive(ToV8)]
#[to_v8(untagged, tag = "kind")] // error: Cannot combine `untagged` with `tag` or `content`
enum Value {
  Num(u32),
  Str(String),
}

// after
#[derive(ToV8)]
#[to_v8(untagged)]
enum Value {
  Num(u32),
  Str(String),
}

When it happens

Trigger: Writing #[to_v8(untagged, tag = "kind")] or #[to_v8(untagged, content = "data")] on an enum derived with ToV8.

Common situations: Migrating a serde enum that used both untagged and tag-like attributes; merging attributes from two examples; editing a tagged enum to untagged and forgetting to delete the old tag/content arguments.

Related errors

• Cannot use serde on unit variant
• `content` requires `tag` to be specified
• FromV8 enum derive currently supports only unit and single-e
• Unions are not supported
• Unit structs cannot be destructured

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/04e7adf8584a5dca. Report an issue: GitHub.