{"ast_node_type":"struct","tokens":[{"end":102,"kind":"keyword","start":0,"text":"self.field_idents(def_id)?.iter().map(|&f|"}]}
        self.field_idents(def_id)?.iter().map(|&f| f.span).reduce(Span::to) // None for `struct Foo()`
        let def_id = self.tcx.parent(ctor_def_id);
    }