bevyengine/bevy · error · syn::Error

EntityEvent derive does not work on unit structs. Your type

Error message

EntityEvent derive does not work on unit structs. Your type must have a field to store the `Entity` target, such as `Attack(Entity)` or `Attack { entity: Entity }`.

What it means

Compile error from get_event_target_field: the struct is a unit struct with zero fields, so there is nowhere to store the Entity target required by EntityEvent. This is the final match arm guarding Fields::Unit, with a hint message suggesting field shapes that work.

Source

Thrown at crates/bevy_ecs/macros/src/event.rs:204

            fields.span(),
            "EntityEvent derive expected a field name 'entity' or a field annotated with #[event_target]."
        )),
        Fields::Unnamed(fields) if fields.unnamed.len() == 1 => Ok(Member::Unnamed(Index::from(0))),
        Fields::Unnamed(fields) => fields.unnamed.iter().enumerate().find_map(|(index, field)| {
                if field
                    .attrs
                    .iter()
                    .any(|attr| attr.path().is_ident(EVENT_TARGET)) {
                        Some(Member::Unnamed(Index::from(index)))
                    } else {
                        None
                    }
            })
            .ok_or(syn::Error::new(
                fields.span(),
                "EntityEvent derive expected unnamed structs with one field or with a field annotated with #[event_target].",
            )),
        Fields::Unit => Err(syn::Error::new(
            fields.span(),
            "EntityEvent derive does not work on unit structs. Your type must have a field to store the `Entity` target, such as `Attack(Entity)` or `Attack { entity: Entity }`.",
        )),
    }
}

View on GitHub (pinned to 396ca72708)

Solutions

1. Add a field to store the target, e.g. struct Attack(Entity); or struct Attack { entity: Entity }
2. Use a marker component separately instead of deriving EntityEvent on a unit struct

When it happens

Trigger: Thrown at crates/bevy_ecs/macros/src/event.rs:204 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/34b124a1356155d1. Report an issue: GitHub.