{"ast_node_type":"impl","tokens":[{"end":59,"kind":"keyword","start":0,"text":"///"}]}
    /// `async gen`, which returns an `impl AsyncIterator`.
    Gen { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
    AsyncGen { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },