What causes "expected an empty enum" in rust-lang/rust?

Declare the enum with no variants: `enum Func {}`.

rust-lang/rust · error · syn::Error

expected an empty enum

Error message

expected an empty enum

What it means

Thrown by `expect_empty_enum` (enums.rs:159), called by both `#[function_enum]` and `#[base_name_enum]`, when the annotated enum already has variants. Both macros populate the enum themselves from the global operation list, so the source enum must be empty.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/enums.rs:159

        #item

        impl #item_name {
            /// The stringified version of this base name.
            pub const fn as_str(self) -> &'static str {
                match self {
                    #( #as_str_arms ),*
                }
            }
        }
    };

    Ok(res)
}

/// Verify that an enum is empty, otherwise return an error
fn expect_empty_enum(item: &ItemEnum) -> syn::Result<()> {
    if !item.variants.is_empty() {
        Err(syn::Error::new(
            item.variants.span(),
            "expected an empty enum",
        ))
    } else {
        Ok(())
    }
}

View on GitHub (pinned to 7088e4b63a)

Solutions

Declare the enum with no variants: `enum Func {}`.
Remove any manually-added variants; the macro generates them.

Example fix

// before
#[function_enum(BaseName)]
enum Func {
    Foo,
}

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

Defensive patterns

Strategy: validation

Prevention

Both function_enum and base_name_enum require an empty enum; variants are generated.
Never hand-write variants into a macro-annotated enum.

When it happens

Trigger: Annotating an enum that already declares variants, e.g. `enum Func { Foo, Bar }`.

Common situations: Adding a placeholder variant during development; merging an existing enum under the macro; misunderstanding that the macro generates the variants.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/c31663e3bd83b9c4. Report an issue: GitHub.