{"record":{"id":"518bc2550b75c868","repo":"diesel-rs/diesel","slug":"foreign-derive-requires-at-least-one-field","errorCode":null,"errorMessage":"foreign_derive requires at least one field","messagePattern":"foreign_derive requires at least one field","errorType":"exception","errorClass":"syn::Error","httpStatus":null,"severity":"error","filePath":"diesel_derives/src/util.rs","lineNumber":66,"sourceCode":"                AngleBracketed(ref args) if last_segment.ident == \"Option\" => {\n                    match args.args.iter().next_back() {\n                        Some(GenericArgument::Type(ty)) => Some(ty),\n                        _ => None,\n                    }\n                }\n                _ => None,\n            }\n        }\n        _ => None,\n    }\n}\n\npub fn ty_for_foreign_derive(item: &DeriveInput, model: &Model) -> Result<Type> {\n    if model.foreign_derive {\n        match item.data {\n            Data::Struct(ref body) => match body.fields.iter().next() {\n                Some(field) => Ok(field.ty.clone()),\n                None => Err(syn::Error::new(\n                    proc_macro2::Span::mixed_site(),\n                    \"foreign_derive requires at least one field\",\n                )),\n            },\n            _ => Err(syn::Error::new(\n                proc_macro2::Span::mixed_site(),\n                \"foreign_derive can only be used with structs\",\n            )),\n        }\n    } else {\n        let ident = &item.ident;\n        let (_, ty_generics, ..) = item.generics.split_for_impl();\n        Ok(parse_quote!(#ident #ty_generics))\n    }\n}\n\npub fn camel_to_snake(name: &str) -> String {\n    let mut result = String::with_capacity(name.len());","sourceCodeStart":48,"sourceCodeEnd":84,"githubUrl":"https://github.com/diesel-rs/diesel/blob/6fa6ed01b24b24248ab2a611698d0a7c6a2e9120/diesel_derives/src/util.rs#L48-L84","documentation":"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.","triggerScenarios":"Thrown at diesel_derives/src/util.rs:66 when the library encounters an invalid state.","commonSituations":"See trigger scenarios.","solutions":["Add a field whose type is the foreign model type","Stop using foreign_derive on an empty struct"],"exampleFix":null,"handlingStrategy":"validation","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"6fa6ed01b24b24248ab2a611698d0a7c6a2e9120","analyzedAt":"2026-09-07T01:50:13.074Z","contentChangedAt":"2026-09-07T01:50:13.074Z","schemaVersion":2},"datasetVersion":"2026-09-16T09:17:16.951Z"}