denoland/deno · error · syn::Error

unknown webidl argument, expected `default` or `options`

Error message

unknown webidl argument, expected `default` or `options`

What it means

`WebIDLArgs::parse` accepts exactly two keys inside `webidl(...)`: `default` and `options`. Any other identifier key raises this error on that key's span, listing the two valid alternatives.

Source

Thrown at libs/ops/op2/signature.rs:321

            "duplicate `default` argument",
          ));
        }
        input.parse::<Token![=]>()?;
        default = Some(WebIDLDefault(input.parse::<syn::Expr>()?));
      } else if key == "options" {
        if !options.is_empty() {
          return Err(syn::Error::new(
            key.span(),
            "duplicate `options` argument",
          ));
        }
        let content;
        syn::parenthesized!(content in input);
        let parsed_options =
          content.parse_terminated(WebIDLPair::parse, Token![,])?;
        options = parsed_options.into_iter().collect();
      } else {
        return Err(syn::Error::new(
          key.span(),
          "unknown webidl argument, expected `default` or `options`",
        ));
      }

      if !input.is_empty() {
        input.parse::<Token![,]>()?;
      }
    }

    Ok(WebIDLArgs { default, options })
  }
}

/// Args are not a 1:1 mapping with Rust types, rather they represent broad classes of types that
/// tend to have similar argument handling characteristics. This may need one more level of indirection
/// given how many of these types have option variants, however.
#[derive(Clone, Debug, Eq, PartialEq)]

View on GitHub (pinned to 9ad36f7a2c)

Solutions

Use only `default = <expr>` (fallback value) and `options(...)` (allowed-value pairs) inside `webidl(...)`.
For optional parameters, type the parameter as `Option<T>` (the dictionary generator auto-defaults Options to None) instead of inventing an `optional` key.
Check the spelling — `defaults`/`Default` will not match.

Example fix

// before
#[op2(webidl(optional = true))]
color: Color,

// after
#[op2(webidl(default = Color::Red))]
color: Color,

Defensive patterns

Strategy: validation

Validate before calling

// Compile-time: `cargo check`. Remember the closed set:
//   webidl(default = <expr>)  and  webidl(options(...))
// — there is no `optional`, `required`, or `values` key.

Prevention

For optional parameters use `Option<T>` in the signature instead of an invented key.
Double-check key spelling (`default`, `options`) before compiling.

When it happens

Trigger: `#[op2(webidl(value = 3))]` or any `webidl(<key> ...)` where `<key>` is not `default` or `options`, e.g. typos like `defaults`, `optional`, or `values`.

Common situations: Guessing argument names based on general WebIDL knowledge (`optional`, `required`, `values`) instead of the two this macro supports; typos after refactoring.

Related errors

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/9faa5304b95d3a96. Report an issue: GitHub.