{"ast_node_type":"other","tokens":[{"end":45,"kind":"keyword","start":0,"text":"used_mut_upvars:"}]}
    used_mut_upvars: SmallVec<[FieldIdx; 8]>,
    /// mutable upvars that have been used. This field keeps track of them.
    /// Region inference context. This contains the results from region inference and lets us e.g.