BoundaryML/baml · error · syn::Error

Rust unions are not supported for this derive macro.

Error message

Rust unions are not supported for this derive macro.

What it means

Compile-time error from the `BamlSerde` derive macro: Rust `union` declarations are not supported at all. Rust unions lack safe field access and do not fit the serde data model the macro targets. Emitted by `derive_serde` for `Data::Union`.

Solutions

1. Remove `BamlSerde` from the union
2. Wrap the union in a struct and manually implement serialization over its bytes
3. Convert the union to a tagged enum if the discriminant is known

Example fix

// before
#[derive(BamlSerde)]
union U { int: i32, float: f32 }
// after
#[derive(BamlSerde)]
#[serde(untagged)]
#[baml(union = true)]
enum U { Int(i32), Float(f32) }

When it happens

Trigger: Applying `#[derive(BamlSerde)]` to a `union` declaration, e.g. `#[derive(BamlSerde)] union U { a: i32, b: f32 }`.

Common situations: FFI code that defines raw unions and mistakenly applies BamlSerde expecting serialization support.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• BamlSerde cannot be derived for enums, use…
• BamlSerde cannot be derived for structs, use…
• Failed to deserialize value
• Failed to find in union
• Failed to serialize request result

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/4012289ffbe147ce. Report an issue: GitHub.

Appendix: source

Thrown at languages/rust/baml-macros/src/baml_serde.rs:22

use crate::shared::{ContainerAttrs, VariantAttrs};

pub(crate) fn derive_serde(input: &DeriveInput) -> Result<TokenStream> {
    let container_attrs = ContainerAttrs::from_attrs(&input.attrs)?;

    match &input.data {
        Data::Struct(..) => Err(syn::Error::new(
            Span::call_site(),
            "BamlSerde cannot be derived for structs, use `#[serde(flatten)]` on dynamic instead.",
        )),
        Data::Enum(data_enum) if container_attrs.union => {
            derive_serde_union(data_enum, &input.ident)
        }
        Data::Enum(..) => Err(syn::Error::new(
            Span::call_site(),
            "BamlSerde cannot be derived for enums, use `#[serde(untagged)]` on dynamic instead.",
        )),
        Data::Union(..) => Err(syn::Error::new(
            Span::call_site(),
            "Rust unions are not supported for this derive macro.",
        )),
    }
}

fn derive_serde_union(data: &DataEnum, ident: &Ident) -> Result<TokenStream> {
    let name = ident.to_string();

    let mut literal_serde = Vec::new();

    for variant in &data.variants {
        let variant_attrs = VariantAttrs::from_attrs(&variant.attrs)?;
        let variant_ident = &variant.ident;

        if let Some(ref literal_string) = variant_attrs.literal_string {
            literal_serde.push(quote! {
                #[allow(non_snake_case)]

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