tracel-ai/burn · error

Path meta unsupported

Error message

Path meta unsupported

What it means

Attribute-syntax guard in the shared derive helper: `AttributeAnalyzer::item` accepts only list-style (`#[attr(key = value)]`) and name-value (`#[attr = "x"]`) metadata; a bare path attribute like `#[attr]` with no value reaches the `Meta::Path` arm and panics during macro expansion. The failing input is an attribute written without an `= value` part.

Source

Thrown at crates/burn-derive/src/shared/attribute.rs:21

pub struct AttributeAnalyzer {
    attr: Attribute,
}

#[derive(Clone)]
pub struct AttributeItem {
    pub value: syn::Lit,
}

impl AttributeAnalyzer {
    pub fn new(attr: Attribute) -> Self {
        Self { attr }
    }

    pub fn item(&self) -> AttributeItem {
        let value = match &self.attr.meta {
            Meta::List(val) => val.parse_args::<syn::MetaNameValue>().unwrap(),
            Meta::NameValue(meta) => meta.clone(),
            Meta::Path(_) => panic!("Path meta unsupported"),
        };

        let lit = match value.value {
            syn::Expr::Lit(lit) => lit.lit,
            _ => panic!("Only literal is supported"),
        };

        AttributeItem { value: lit }
    }

    pub fn has_name(&self, name: &str) -> bool {
        Self::path_syn_name(self.attr.path()) == name
    }

    fn path_syn_name(path: &syn::Path) -> String {
        let length = path.segments.len();
        let mut name = String::new();
        for (i, segment) in path.segments.iter().enumerate() {

View on GitHub (pinned to d16f7ba2ed)

Solutions

1. Use a plain literal (string, integer, bool) as the attribute value instead of an expression.
2. Precompute the value in source (e.g. `1_024` not `1024 * 1024` is still a literal; function calls are not).
3. Extend `AttributeAnalyzer` to evaluate constant expressions if dynamic values are genuinely required.

When it happens

Trigger: Thrown at crates/burn-derive/src/shared/attribute.rs:21 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/5ad69864ebddf2d9. Report an issue: GitHub.