astral-sh/ruff · error · syn::Error

CacheKey does not support unions. Only structs and enums are

Error message

CacheKey does not support unions. Only structs and enums are supported

What it means

A syn compile error from the CacheKey derive macro: the derive was applied to a union. The generated cache-key logic only knows how to walk structs and enums field by field, so unions are rejected at compile time.

Source

Thrown at crates/ruff_macros/src/cache_key.rs:92

                        continue;
                    }
                }

                let field_attr = if let Some(ident) = &field.ident {
                    quote!(self.#ident)
                } else {
                    let index = syn::Index::from(i);
                    quote!(self.#index)
                };

                fields.push(quote!(#field_attr.cache_key(key);));
            }

            quote! {#(#fields)*}
        }

        Data::Union(_) => {
            return Err(Error::new(
                item.span(),
                "CacheKey does not support unions. Only structs and enums are supported",
            ));
        }
    };

    let name = &item.ident;
    let (impl_generics, ty_generics, where_clause) = &item.generics.split_for_impl();

    Ok(quote!(
        #[automatically_derived]
        impl #impl_generics ruff_cache::CacheKey for #name #ty_generics #where_clause {
            fn cache_key(&self, key: &mut ruff_cache::CacheKeyHasher) {
                use std::hash::Hasher;
                use ruff_cache::CacheKey;
                #fields
            }
        }

View on GitHub (pinned to 26f38c119c)

Solutions

1. Implement CacheKey manually for the union
2. Change the type to a struct or enum so the derive applies

When it happens

Trigger: Thrown at crates/ruff_macros/src/cache_key.rs:92 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/fcbb994373ed6732. Report an issue: GitHub.