DioxusLabs/dioxus · error · syn::Error

Store macro does not support unions

Error message

Store macro does not support unions

What it means

The #[derive(Store)] macro from dioxus-signals (stores-macro) only implements Store for structs and enums. Rust unions have no field-access discipline the field-tracking store machinery can hook, so the derive refuses unions outright.

Source

Thrown at packages/stores-macro/src/derive.rs:64

    let mut extension_generics = generics.clone();
    extension_generics.params.insert(0, parse_quote!(__Lens));

    match &input.data {
        syn::Data::Struct(data_struct) => derive_store_struct(
            &input,
            data_struct,
            extension_trait_name,
            transposed_name,
            extension_generics,
        ),
        syn::Data::Enum(data_enum) => derive_store_enum(
            &input,
            data_enum,
            extension_trait_name,
            transposed_name,
            extension_generics,
        ),
        syn::Data::Union(_) => Err(syn::Error::new(
            input.span(),
            "Store macro does not support unions",
        )),
    }
}

/// Build a constructor expression: `prefix { a, b }`, `prefix(a, b)`, or just `prefix`.
fn construct_from_fields(prefix: TokenStream2, fields: &Fields, names: &[Ident]) -> TokenStream2 {
    match fields {
        Fields::Named(_) => quote! { #prefix { #(#names),* } },
        Fields::Unnamed(_) => quote! { #prefix(#(#names),*) },
        Fields::Unit => quote! { #prefix },
    }
}

/// Zip original fields with their transposed store types, preserving visibility and names.
fn zip_transposed_fields(fields: &Fields, types: &[TokenStream2]) -> Vec<TokenStream2> {
    match fields {

View on GitHub (pinned to 393d190a80)

Solutions

1. Convert the union to an enum or struct (e.g. a tagged enum) and derive Store on that
2. Wrap the union in a struct, derive Store on the wrapper, and expose union fields through methods
3. Drop the derive from the union and manage that state without Store

Example fix

// before
#[derive(Store)]
union Packet { raw: u32, fields: Parts }

// after
struct Packet { raw: PacketRepr }
#[derive(Store)]
struct AppState { packet: Packet }

When it happens

Trigger: Annotating any `union U { a: u32, b: f32 }` declaration with #[derive(Store)].

Common situations: FFI/bit-pattern types declared as unions accidentally receiving the derive via a blanket copy-paste; bringing dioxus into an existing crate that models raw memory with unions.

Related errors

• Use `api_route` instead of `route` to use OpenAPI options
• path parameter `{}` not found in function arguments
• query parameter `{}` not found in function arguments
• Cannot have multiple query parameters when one is a catch-al
• HTTP method specified both in macro and in attribute

AI-assisted analysis of DioxusLabs/dioxus@393d190a80 (2026-08-16). Data as JSON: /api/errors/478a064c2ad79781. Report an issue: GitHub.