syntax_data: {"ast_node_type":"variable","tokens":[{"end":62,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:         let pred: PolyTraitPredicate<'tcx> = from.upcast(tcx);
context_before:     fn upcast_from(from: ty::Binder<'tcx, TraitRef<'tcx>>, tcx: TyCtxt<'tcx>) -> Self {
context_after:         pred.upcast(tcx)