What causes "derive(Merge) does not support an enum" in swc-project/swc?

Convert the enum into a struct whose fields are each merge-able (structs with named/unnamed fields are supported).

derive(Merge) does not support an enum

Error message

derive(Merge) does not support an enum

What it means

The #[derive(Merge)] macro from swc_config_macro generates merge code by matching on syn::Data; only structs with named or unnamed fields are supported. Deriving it on an enum reaches unimplemented!("derive(Merge) does not support an enum") at merge.rs:22, which panics during macro expansion — i.e. at compile time of the crate using the derive.

Source

Thrown at crates/swc_config_macro/src/merge.rs:22

use syn::{parse_quote, DeriveInput, Expr, Field, Fields, Stmt, Token};

pub fn expand(input: DeriveInput) -> TokenStream {
    match &input.data {
        syn::Data::Struct(s) => {
            let body = call_merge_for_fields(&quote!(self), &s.fields);
            let body = join_stmts(&body);

            let ident = &input.ident;
            parse_quote!(
                #[automatically_derived]
                impl swc_config::merge::Merge for #ident {
                    fn merge(&mut self, _other: Self) {
                        #body
                    }
                }
            )
        }
        syn::Data::Enum(_) => unimplemented!("derive(Merge) does not support an enum"),
        syn::Data::Union(_) => unimplemented!("derive(Merge) does not support a union"),
    }
}

fn call_merge_for_fields(obj: &dyn ToTokens, fields: &Fields) -> Vec<Stmt> {
    fn call_merge(obj: &dyn ToTokens, idx: usize, f: &Field) -> Expr {
        let r = quote!(_other);

        let l = access_field(obj, idx, f);
        let r = access_field(&r, idx, f);
        parse_quote!(swc_config::merge::Merge::merge(&mut #l, #r))
    }

    match fields {
        Fields::Named(fs) => fs
            .named
            .iter()
            .enumerate()

View on GitHub (pinned to d7d7434666)

Solutions

Convert the enum into a struct whose fields are each merge-able (structs with named/unnamed fields are supported).
Implement swc_config::merge::Merge manually for the enum with your chosen precedence (e.g. other wins when non-default).
Wrap the enum in a struct and derive Merge on the wrapper, implementing Merge by hand for the inner enum.
If Unit-like variants are involved, also see the unit-struct restriction.

Example fix

// before
#[derive(Merge)]
enum Mode { Fast, Slow }

// after
enum Mode { Fast, Slow }
impl swc_config::merge::Merge for Mode {
    fn merge(&mut self, other: Self) {
        // define precedence explicitly
        if matches!(self, Mode::Fast) { *self = other; }
    }
}

Defensive patterns

Strategy: validation

Validate before calling

// The panic is at macro-expansion time; the only 'validation' is reviewing
// the derive input shape before compiling:
// - #[derive(Merge)] requires a struct (named or unnamed fields, at least one field).
// - Enums and unit structs are rejected.
// Fail fast in review/CI with a grep:
//   rg -U '#\[derive\([^)]*Merge[^)]*\)\s*(pub\s+)?enum' src/

Prevention

Derive Merge only on structs with fields; for enums write a manual Merge impl with explicit precedence.
If you need enum config options, model them as newtype structs over an inner enum and implement Merge for the inner type by hand.
Add a CI grep/regex gate for `#[derive(...Merge...)]` immediately followed by `enum` or a unit struct body.
Read the derive's docs (swc_config::merge) before applying it to new type shapes.

When it happens

Trigger: Annotating an enum with #[derive(Merge)] in a crate that depends on swc_config / swc_config_macro, e.g. trying to make a config enum overridable. rustc aborts with the macro's panic message during compilation.

Common situations: Defining SWC config option types (transforms, plugin options) as enums for ergonomics and assuming derive(Merge) behaves like serde derives; upgrading config types from structs to tagged enums; error appears as 'proc-macro derive panicked' in build output.

Related errors

AI-assisted analysis of swc-project/swc@d7d7434666 (2026-08-16). Data as JSON: /api/errors/db4d59354e59197f. Report an issue: GitHub.