{"ast_node_type":"variable","tokens":[{"end":84,"kind":"keyword","start":0,"text":"let"}]}
        let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
    ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
        context[from_region] = Trace::StartRegion;