astral-sh/ruff · error · syn::Error

Mandatory `default` field is missing in `#[option]` attribut

Error message

Mandatory `default` field is missing in `#[option]` attribute. Specify the default using `#[option(default="..")]`.

What it means

A syn compile error from the #[option(...)] macro: the attribute does not include a `default` key. A default is mandatory because the generated documentation and schema must state the option's default value.

Source

Thrown at crates/ruff_macros/src/config.rs:274

            scope = Some(get_string_literal(&meta, "scope", "option")?.value());
        } else if meta.path.is_ident("example") {
            let example_text = get_string_literal(&meta, "value_type", "option")?.value();
            example = Some(dedent(&example_text).trim_matches('\n').to_string());
        } else {
            return Err(syn::Error::new(
                meta.path.span(),
                format!(
                    "Deprecated meta {:?} is not supported by ruff's option macro.",
                    meta.path.get_ident()
                ),
            ));
        }

        Ok(())
    })?;

    let Some(default) = default else {
        return Err(syn::Error::new(
            attribute.span(),
            "Mandatory `default` field is missing in `#[option]` attribute. Specify the default using `#[option(default=\"..\")]`.",
        ));
    };

    let Some(value_type) = value_type else {
        return Err(syn::Error::new(
            attribute.span(),
            "Mandatory `value_type` field is missing in `#[option]` attribute. Specify the value type using `#[option(value_type=\"..\")]`.",
        ));
    };

    let Some(example) = example else {
        return Err(syn::Error::new(
            attribute.span(),
            "Mandatory `example` field is missing in `#[option]` attribute. Add an example using `#[option(example=\"..\")]`.",
        ));
    };

View on GitHub (pinned to 26f38c119c)

Solutions

1. Add #[option(default = "...")] to the field
2. Ensure the key is spelled exactly `default`

When it happens

Trigger: Thrown at crates/ruff_macros/src/config.rs:274 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of astral-sh/ruff@26f38c119c (2026-09-05). Data as JSON: /api/errors/3e38f7b32bca0c27. Report an issue: GitHub.