diesel-rs/diesel · error · syn::Error

foreign_derive requires at least one field

Error message

foreign_derive requires at least one field

What it means

Compile-time error from ty_for_foreign_derive, used by derives on models marked #[foreign_derive] (for mapping foreign-schema types). For a struct input the derive needs at least one field to extract the target type; when the struct has zero fields this guard fires. Fix: declare at least one field in the struct used with foreign_derive.

Solutions

1. Add a field whose type is the foreign model type
2. Stop using foreign_derive on an empty struct

When it happens

Trigger: Thrown at diesel_derives/src/util.rs:66 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/518bc2550b75c868. Report an issue: GitHub.

Appendix: source

Thrown at diesel_derives/src/util.rs:66

                AngleBracketed(ref args) if last_segment.ident == "Option" => {
                    match args.args.iter().next_back() {
                        Some(GenericArgument::Type(ty)) => Some(ty),
                        _ => None,
                    }
                }
                _ => None,
            }
        }
        _ => None,
    }
}

pub fn ty_for_foreign_derive(item: &DeriveInput, model: &Model) -> Result<Type> {
    if model.foreign_derive {
        match item.data {
            Data::Struct(ref body) => match body.fields.iter().next() {
                Some(field) => Ok(field.ty.clone()),
                None => Err(syn::Error::new(
                    proc_macro2::Span::mixed_site(),
                    "foreign_derive requires at least one field",
                )),
            },
            _ => Err(syn::Error::new(
                proc_macro2::Span::mixed_site(),
                "foreign_derive can only be used with structs",
            )),
        }
    } else {
        let ident = &item.ident;
        let (_, ty_generics, ..) = item.generics.split_for_impl();
        Ok(parse_quote!(#ident #ty_generics))
    }
}

pub fn camel_to_snake(name: &str) -> String {
    let mut result = String::with_capacity(name.len());

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