denoland/deno · error · syn::Error

missing top-level #[webidl] attribute

Error message

missing top-level #[webidl] attribute

What it means

The `#[webidl(...)]`-driven derive in libs/ops/webidl/mod.rs scans the item's top-level attributes for one that yields a `ConverterType` (`webidl(dictionary)` or `webidl(enum)`). If none is found it raises this error on the whole item's span — the derive cannot know which converter to generate without the top-level attribute.

Source

Thrown at libs/ops/webidl/mod.rs:29

use syn::DeriveInput;
use syn::Error;
use syn::Token;
use syn::parse::Parse;
use syn::parse::ParseStream;
use syn::parse2;
use syn::spanned::Spanned;

pub fn webidl(item: TokenStream) -> Result<TokenStream, Error> {
  let input = parse2::<DeriveInput>(item)?;
  let span = input.span();
  let ident = input.ident;
  let ident_string = ident.to_string();
  let converter = input
    .attrs
    .into_iter()
    .find_map(|attr| ConverterType::from_attribute(attr).transpose())
    .ok_or_else(|| {
      Error::new(span, "missing top-level #[webidl] attribute")
    })??;

  let out = match input.data {
    Data::Struct(data) => match converter {
      ConverterType::Dictionary => {
        create_impl(ident, dictionary::get_body(ident_string, span, data)?)
      }
      ConverterType::Enum => {
        return Err(Error::new(span, "Structs do not support enum converters"));
      }
    },
    Data::Enum(data) => match converter {
      ConverterType::Dictionary => {
        return Err(Error::new(
          span,
          "Enums currently do not support dictionary converters",
        ));
      }

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Add a top-level attribute naming the converter: `#[webidl(dictionary)]` for structs, `#[webidl(enum)]` for enums.
2. Make sure the attribute sits directly on the type (above `#[derive(...)]` or next to it), not on a field or variant.
3. Check the spelling and argument — the attribute must be parsed by `ConverterType::from_attribute`, so `webidl` alone is not enough.

Example fix

// before
#[derive(WebIDL)]
struct ColorOptions { pub alpha: bool }

// after
#[derive(WebIDL)]
#[webidl(dictionary)]
struct ColorOptions { pub alpha: bool }

// Compile-time: `cargo check`. Every `#[derive(WebIDL)]` item needs exactly one
// top-level `#[webidl(dictionary)]` or `#[webidl(enum)]` attribute on the item itself.

When it happens

Trigger: `#[derive(WebIDL)]` (or the equivalent derive import) on a struct/enum without a top-level `#[webidl(dictionary)]` or `#[webidl(enum)]` attribute; also when the attribute is placed on a field instead of the item, or is misspelled.

Common situations: Adding the derive via IDE quick-fix without the helper attribute; moving the `#[webidl(...)]` attribute below the `#[derive(...)]` line so tooling strips it; typos such as `#[webidl]` (no converter argument) — note the bare form has no converter type either.

Related errors

• derive(CppgcInherits) requires #[cppgc_inherits_from(BaseTyp
• Unnamed fields are currently not supported
• Unit fields are currently not supported
• variants with fields are not allowed for enum converters
• Structs do not support enum converters

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/3a18453802e6e062. Report an issue: GitHub.