bevyengine/bevy · error · syn::Error

Currently, caching is only supported for scene assets. Pleas

Error message

Currently, caching is only supported for scene assets. Please remove the ':' prefix for now.

What it means

In BsnScene::parse, the ':' cache prefix is consumed first; if the next token is NOT a string literal (the scene-asset form), the parser immediately rejects it with this message. Caching is currently implemented only for scene assets (":\"path.glb\""); the source comment notes broader caching is planned and this guard block exists to be removed once real caching lands.

Source

Thrown at crates/bevy_scene/macros/src/bsn/parse.rs:230

        } else {
            None
        };

        let err_if_cached = |msg: &str| {
            if let Some(colon) = cached {
                Err(syn::Error::new(colon.span(), msg))
            } else {
                Ok(())
            }
        };

        // It may seem odd how this is checking LitStr again
        // and how there doesn't seem to be a need for all the specific `err_if_cached`
        // in later code. But since caching is planned, and will very likely
        // have the limitations which are ensured by the other errors below,
        // this is its own block so its very simple to remove once caching is implemented.
        if !input.peek(LitStr) {
            err_if_cached("Currently, caching is only supported for scene assets. Please remove the ':' prefix for now.")?;
        }

        Ok(if input.peek(LitStr) {
            let path = input.parse::<LitStr>()?;
            if cached.is_none() {
                return Err(syn::Error::new(
                    path.span(),
                    "Cannot use scene assets without caching, please add the ':' prefix.",
                ));
            }
            BsnScene::Asset(path)
        } else if input.peek(Brace) {
            err_if_cached("Cannot cache scene expressions")?;
            BsnScene::Expression(braced_tokens(input)?)
        } else if input.peek(At) {
            input.parse::<At>()?;
            let sc = input.parse::<BsnType>()?;
            if sc.fields.len() > 0 {

View on GitHub (pinned to 396ca72708)

Solutions

1. Remove the ':' prefix — only string-literal scene assets can be cached today
2. If you wanted a cached asset, make the entry a string literal: :"scene.glb#Scene0"

Example fix

// before
bsn!(:{ my_scene_expr });

// after
bsn!({ my_scene_expr });

// Rule to enforce before writing BSN: ':' may only precede a string literal
// scene asset path. Anything else must be written without the prefix.

When it happens

Trigger: Prefixing any non-asset scene form with ':' — e.g. :{ some_expr }, :@MySceneComponent, or :some_function() — inside bsn!/bsn_list!.

Common situations: Assuming BSN caching works for inline expressions or scene components because it works for asset paths; migrating macros where ':' was used as a namespace separator; early adoption of BSN syntax while caching support is partial.

Related errors

• Caching entries after the first is not supported, remove the
• Consts are not currently supported in this position
• Cannot use scene assets without caching, please add the ':'
• Cannot cache scene expressions
• Cannot cache Scene Components with props/fields

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/941863a1b35936a6. Report an issue: GitHub.