denoland/deno · error · syn::Error

`content` requires `tag` to be specified

Error message

`content` requires `tag` to be specified

What it means

In ToV8's enum mode parser, the (tag, content) pair maps to: neither = externally tagged, tag only = internally tagged, tag+content = adjacently tagged, and content without tag is impossible (adjacent tagging needs a tag key to work), so it returns this compile error. The message points you at the missing half of the pair.

Source

Thrown at libs/ops/conversion/to_v8/enum.rs:281

      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(),
        "`content` requires `tag` to be specified",
      )),
    }
  }
}

#[allow(dead_code, reason = "unused properties")]
enum EnumModeArgument {
  Tag {
    name_token: shared_kw::tag,
    eq_token: Token![=],
    value: LitStr,
  },
  Content {
    name_token: shared_kw::content,
    eq_token: Token![=],
    value: LitStr,

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Add the tag argument: #[to_v8(tag = "kind", content = "data")] — content is only valid as the second half of adjacent tagging.
2. If you did not want a tag at all, remove the content argument to fall back to plain externally-tagged output.

Example fix

// before
#[derive(ToV8)]
#[to_v8(content = "data")] // error: `content` requires `tag` to be specified
enum Event {
  Click { x: i32 },
}

// after
#[derive(ToV8)]
#[to_v8(tag = "type", content = "data")]
enum Event {
  Click { x: i32 },
}

When it happens

Trigger: Writing #[to_v8(content = "data")] on a #[derive(ToV8)] enum without also specifying #[to_v8(tag = "...")].

Common situations: Copying just the content argument from an adjacently-tagged serde/ToV8 example; renaming or deleting the tag argument during a refactor and missing that content depends on it.

Related errors

• Cannot use serde on unit variant
• Cannot combine `untagged` with `tag` or `content`
• 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/e9bc86171a978820. Report an issue: GitHub.