diesel-rs/diesel · error

Only enums are supported as multiconnection type

Error message

Only enums are supported as multiconnection type

What it means

Compile-time error in the #[multi_connection] attribute macro. The macro expects the annotated item to be an enum whose variants are the supported backend connections; it generates the MultiBackend/MultiRow plumbing only from that enum. It fires when the user applies #[diesel::multi_connection] to a struct, union, or other non-enum item, where no backend dispatch can be generated. Fix: annotate an enum listing one variant per backend connection.

Solutions

1. Convert the type into an enum with one backend per variant
2. Apply the derive only to enum types
3. Use a single concrete connection type instead of a struct wrapper

When it happens

Trigger: Thrown at diesel_derives/src/multiconnection.rs:65 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/3d6c055b24859f4b. Report an issue: GitHub.

Appendix: source

Thrown at diesel_derives/src/multiconnection.rs:65

                mod row {
                    use super::*;
                    #row
                }

                mod connection {
                    use super::*;
                    #connection
                }

                pub use self::backend::{MultiBackend, MultiRawValue};
                pub use self::row::{MultiRow, MultiField};
            }

            pub use self::multi_connection_impl::{MultiBackend, MultiRow, MultiRawValue, MultiField};
        }
    } else {
        panic!("Only enums are supported as multiconnection type");
    }
}

/// A helper struct containing common tokens used in the sync and async in similar.
struct MultiHelper {
    is_async: bool,
    /// Type of Connection trait.
    conn: TokenStream,
    /// Trait with the Backend Type.
    conn_backend: TokenStream,
    /// Trait implementing the load function.
    conn_load: TokenStream,
    /// The multi connection helper trait, used to cast to Any.
    multi_conn_helper: TokenStream,
    /// Type of the SimpleConnection trait.
    simple_connection: TokenStream,
    /// Type of the TransactionManager trait.
    transaction_manager: TokenStream,

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