linebender/druid · error

Unknown attribute

Error message

Unknown attribute

What it means

This error comes from druid-derive's parsing of helper attributes (e.g. #[data(...)]) during derive macro expansion. The derive expects only recognized nested attributes inside the helper attribute list; any other attribute path, name-value pair, or literal is rejected with "Unknown attribute". It is a compile-time proc-macro error pointing at the offending token's span.

Solutions

1. Replace the unknown attribute with one supported by the derive (for #[data]: data(eq); for #[lens]: lens(name = "...") or lens(ignore)).
2. Check the druid-derive documentation/version for the exact accepted attribute names — names changed between druid versions.
3. Remove the stray attribute if it was intended for another derive macro, and place it on the correct item.
4. If the attribute belongs to another derive on the same struct, ensure you are not putting it inside the wrong helper attribute list.

Example fix

// before
#[derive(Data)]
enum Msg {
    #[data(ignore)]
    A(i32),
}
// after
#[derive(Data)]
enum Msg {
    #[data(eq)]
    A(i32),
}

// Before expanding the derive, audit each helper attribute:
const DATA_ALLOWED: &[&str] = &["eq"];
fn validate_data_attr(meta: &str) -> Result<(), String> {
    let inner = meta.strip_prefix("data(")?.strip_suffix(')')?;
    inner.split(',').map(str::trim).try_for_each(|opt| {
        if DATA_ALLOWED.contains(&opt) { Ok(()) } else { Err(format!("unknown data option: {}", opt)) }
    })
}

When it happens

Trigger: Writing an unrecognized entry inside #[data(...)] or #[lens(...)], such as #[data(foo)] or a typo like #[data(eq)] (correct form is data(eq)), or passing a name-value item where only path attributes are accepted.

Common situations: Typos in attribute names, copy-pasting attributes from other derive crates (serde-style attributes), misremembering the helper attribute syntax after a version change in druid-derive.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Expected attribute list (the form #[data(one, two)])
• Duplicate attribute
• Expected attribute list (the form #[lens(one, two)])
• expected str, found... something else
• Data implementations cannot be derived from unions

AI-assisted analysis of linebender/druid@0f8b1195e4 (2026-09-10). Data as JSON: /api/errors/077d352e8b797760. Report an issue: GitHub.

Appendix: source

Thrown at druid-derive/src/attr.rs:160

                        if let Some(nested) = meta.nested.first() {
                            match nested {
                                NestedMeta::Meta(Meta::Path(path))
                                    if path.is_ident(IGNORE_ATTR_PATH) =>
                                {
                                    data_attr = DataAttr::Ignore;
                                }
                                NestedMeta::Meta(Meta::NameValue(meta))
                                    if meta.path.is_ident(DATA_SAME_FN_ATTR_PATH) =>
                                {
                                    let path = parse_lit_into_expr_path(&meta.lit)?;
                                    data_attr = DataAttr::SameFn(path);
                                }
                                NestedMeta::Meta(Meta::Path(path))
                                    if path.is_ident(DATA_EQ_ATTR_PATH) =>
                                {
                                    data_attr = DataAttr::Eq;
                                }
                                other => return Err(Error::new(other.span(), "Unknown attribute")),
                            }
                        }
                    }
                    other => {
                        return Err(Error::new(
                            other.span(),
                            "Expected attribute list (the form #[data(one, two)])",
                        ));
                    }
                }
            }
        }
        Ok(Field {
            ident,
            ty,
            attrs: data_attr,
        })
    }

View on GitHub (pinned to 0f8b1195e4)