tracel-ai/burn · error

Path segment {} has no generic

Error message

Path segment {} has no generic

What it means

Segment-level guard in the same helper: `path_generic_argument` takes the last path segment and panics with the segment name when that segment has no generic arguments (e.g. field type `Tensor` without `<B, D>`, or a segment like `Vec` accessed mid-path). It reports derive-codegen expectations versus the actual written type; the input at fault is the generic-less path segment.

Source

Thrown at crates/burn-derive/src/shared/field.rs:39

            syn::Type::Path(path) => {
                let name = Self::path_name(path);
                paths.contains(&name.as_str())
            }
            _ => false,
        }
    }

    #[allow(dead_code)]
    pub fn first_generic_field(&self) -> TypePath {
        let err = || panic!("Field {} as no generic", self.field.ident.clone().unwrap());
        match &self.field.ty {
            syn::Type::Path(path) => Self::path_generic_argument(path),
            _ => err(),
        }
    }
    pub fn path_generic_argument(path: &TypePath) -> TypePath {
        let segment = path.path.segments.last().unwrap();
        let err = || panic!("Path segment {} has no generic", segment.ident.clone(),);
        match &segment.arguments {
            syn::PathArguments::None => err(),
            syn::PathArguments::AngleBracketed(param) => {
                let first_param = param.args.first().unwrap();

                if let syn::GenericArgument::Type(Type::Path(path)) = first_param {
                    path.clone()
                } else {
                    err()
                }
            }
            syn::PathArguments::Parenthesized(_) => err(),
        }
    }

    fn path_name(path: &TypePath) -> String {
        let length = path.path.segments.len();
        let mut name = String::new();

View on GitHub (pinned to d16f7ba2ed)

Solutions

1. Add the expected generic parameters to the field type (e.g. `MyType<B>` rather than `MyType`).
2. Verify the codegen path targets the correct segment — fully qualify the type so the generic segment is analyzed.
3. Use the appropriate module attribute to skip fields that legitimately have no generics.

When it happens

Trigger: Thrown at crates/burn-derive/src/shared/field.rs:39 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/a1332c2b67dbcbd0. Report an issue: GitHub.