rust-lang/rust · error · syn::Error

no attributes expected

Error message

no attributes expected

What it means

Thrown by the `#[base_name_enum]` attribute macro (enums.rs:110) when any attribute argument is provided. Unlike `function_enum`, this macro takes no arguments because it derives its variants from the global operation list.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/enums.rs:110

                match self {
                    #( #base_arms, )*
                }
            }
        }
    };

    Ok(res)
}

/// Implement `#[base_name_enum]`, see documentation in `lib.rs`.
pub fn base_name_enum(
    mut item: ItemEnum,
    attributes: pm2::TokenStream,
) -> syn::Result<pm2::TokenStream> {
    expect_empty_enum(&item)?;
    if !attributes.is_empty() {
        let sp = attributes.span();
        return Err(syn::Error::new(sp.span(), "no attributes expected"));
    }

    let mut base_names: Vec<_> = ALL_OPERATIONS
        .iter()
        .map(|func| base_name(func.name))
        .collect();
    base_names.sort_unstable();
    base_names.dedup();

    let item_name = &item.ident;
    let mut as_str_arms = Vec::new();

    for base_name in base_names {
        let ident = Ident::new(&base_name.to_upper_camel_case(), Span::call_site());

        // Match arm for `fn as_str(self)` matcher
        as_str_arms.push(quote! { Self::#ident => #base_name });

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Use `#[base_name_enum]` with empty parentheses (or none).
2. Remember: base_name_enum takes no attributes; function_enum takes one.

Example fix

// before
#[base_name_enum(Some)]
enum Bn {}

// after
#[base_name_enum]
enum Bn {}

When it happens

Trigger: Writing `#[base_name_enum(SomeArg)]` or any form with tokens between the parentheses.

Common situations: Confusing `base_name_enum` with `function_enum` (which does take an argument); copy-pasting an attribute shape from the other macro.

Related errors

• expected one attribute
• expected an identifier
• unexpected token after identifier
• expected an empty enum
• unrecognized function name `{mentioned}`

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/590801661536703c. Report an issue: GitHub.