bevyengine/bevy · error · syn::Error

Not supported in this position, please use one of the follow

Error message

Not supported in this position, please use one of the following:
- path to function
- nothing to default to MapEntities implementation

What it means

Compile error from MapEntitiesAttributeKind::from_expr: the #[map_entities] attribute value was parsed as an expression that is not a plain path (e.g. a literal, call, or struct literal), so the macro rejected it with this sentinel listing the accepted forms — a function path or nothing at all.

Source

Thrown at crates/bevy_ecs/macro_logic/src/map_entities.rs:120

}

/// The type of `MapEntities` attribute.
#[derive(Debug)]
pub enum MapEntitiesAttributeKind {
    /// expressions like function or struct names
    ///
    /// structs will throw compile errors on the code generation so this is safe
    Path(ExprPath),
    /// When no value is specified
    Default,
}

impl MapEntitiesAttributeKind {
    fn from_expr(value: Expr) -> syn::Result<Self> {
        match value {
            Expr::Path(path) => Ok(Self::Path(path)),
            // throw meaningful error on all other expressions
            _ => Err(syn::Error::new(
                value.span(),
                [
                    "Not supported in this position, please use one of the following:",
                    "- path to function",
                    "- nothing to default to MapEntities implementation",
                ]
                .join("\n"),
            )),
        }
    }

    fn to_token_stream(&self, bevy_ecs_path: &Path) -> TokenStream {
        match self {
            MapEntitiesAttributeKind::Path(path) => path.to_token_stream(),
            MapEntitiesAttributeKind::Default => {
                quote!(
                   <Self as #bevy_ecs_path::entity::MapEntities>::map_entities
                )

View on GitHub (pinned to 396ca72708)

Solutions

1. Pass a plain function path: #[map_entities(my_map_fn)]
2. Omit the value entirely to use the type's MapEntities impl: #[map_entities]
3. Wrap the logic in a free function and reference it by name instead of an inline expression

When it happens

Trigger: Thrown at crates/bevy_ecs/macro_logic/src/map_entities.rs:120 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/e3cbdb0c37cb15ef. Report an issue: GitHub.