bevyengine/bevy · error · syn::Error

Invalid key override. Must be either `default` or a valid Ru

Error message

Invalid key override. Must be either `default` or a valid Rust expression of the correct key type

What it means

Bevy's `Specializer` derive lets fields override how they contribute to a specialization key with `#[key(...)]`. The payload must be either the single lowercase identifier `default` or a syntactically valid Rust expression of the field's key type. When the payload parses as an identifier that is not `default`, or fails to parse as an expression, the derive emits this KEY_ERROR_MSG.

Source

Thrown at crates/bevy_render/macros/src/specializer.rs:80

}

const KEY_ERROR_MSG: &str = "Invalid key override. Must be either `default` or a valid Rust expression of the correct key type";

impl Parse for Key {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        if let Ok(ident) = input.parse::<Ident>() {
            if ident == KEY_DEFAULT_IDENT {
                Ok(Key::Default)
            } else {
                Err(syn::Error::new_spanned(ident, KEY_ERROR_MSG))
            }
        } else {
            input
                .parse::<Expr>()
                .map(Box::new)
                .map(Key::Custom)
                .map_err(|mut err| {
                    err.extend(syn::Error::new(err.span(), KEY_ERROR_MSG));
                    err
                })
        }
    }
}

#[derive(Clone)]
struct FieldInfo {
    ty: Type,
    member: Member,
    key: Key,
}

impl FieldInfo {
    fn key_ty(&self, specialize_path: &Path, target_path: &Path) -> Option<Type> {
        let ty = &self.ty;
        matches!(self.key, Key::Whole | Key::Index(_))
            .then_some(parse_quote!(<#ty as #specialize_path::Specializer<#target_path>>::Key))

View on GitHub (pinned to 396ca72708)

Solutions

1. Use the exact lowercase identifier: `#[key(default)]`
2. Otherwise supply one complete, valid expression of the field's key type, e.g. `#[key(Mode::Alpha as u8)]`
3. Check for stray characters, unmatched parens or duplicated tokens inside the attribute

Example fix

// before
#[key(Default)]
mode: Mode,

// after
#[key(default)]
mode: Mode,

When it happens

Trigger: `#[key(Default)]` (capital D parses as an identifier that is not `default`), `#[key()]` or any leftover fragment where the expression parser also fails.

Common situations: Typing `Default` out of habit instead of `default`; leaving a half-edited expression like `#[key(Mode::A +)]` after refactoring a key enum.

Related errors

• Invalid key override attribute
• #[key(default)] is the only key override type allowed with #
• #[derive({derive_name})] must be accompanied by #[specialize
• Union types are not supported yet.
• Expected a Template type path

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/d2b32b1907983d76. Report an issue: GitHub.