rust-lang/rust · error · syn::Error

expected one attribute

Error message

expected one attribute

What it means

Thrown by the `#[function_enum]` attribute macro (enums.rs:21) when it is invoked with no attribute argument. The macro requires exactly one attribute — the identifier of the companion `BaseName` enum — so an empty attribute list is rejected.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/enums.rs:21

use proc_macro2::{Ident, Span};
use quote::quote;
use syn::spanned::Spanned;
use syn::{Fields, ItemEnum, Variant};

use crate::{ALL_OPERATIONS, base_name};

/// Implement `#[function_enum]`, see documentation in `lib.rs`.
pub fn function_enum(
    mut item: ItemEnum,
    attributes: pm2::TokenStream,
) -> syn::Result<pm2::TokenStream> {
    expect_empty_enum(&item)?;
    let attr_span = attributes.span();
    let mut attr = attributes.into_iter();

    // Attribute should be the identifier of the `BaseName` enum.
    let Some(tt) = attr.next() else {
        return Err(syn::Error::new(attr_span, "expected one attribute"));
    };

    let pm2::TokenTree::Ident(base_enum) = tt else {
        return Err(syn::Error::new(tt.span(), "expected an identifier"));
    };

    if let Some(tt) = attr.next() {
        return Err(syn::Error::new(
            tt.span(),
            "unexpected token after identifier",
        ));
    }

    let enum_name = &item.ident;
    let mut as_str_arms = Vec::new();
    let mut from_str_arms = Vec::new();
    let mut base_arms = Vec::new();

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Add the BaseName enum identifier as the single attribute: `#[function_enum(BaseName)]`.
2. Ensure the named enum is itself annotated with `#[base_name_enum]`.

Example fix

// before
#[function_enum]
enum Func {}

// after
#[function_enum(BaseName)]
enum Func {}

When it happens

Trigger: Writing `#[function_enum]` or `#[function_enum()]` on an enum without providing the `BaseName` enum identifier as the attribute.

Common situations: Copy-pasting the macro from docs and dropping the attribute; renaming the macro invocation while refactoring.

Related errors

• expected an identifier
• unexpected token after identifier
• no attributes expected
• expected an empty enum
• unrecognized function name `{mentioned}`

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/8f903157f4bfaabb. Report an issue: GitHub.