syntax_data: {"ast_node_type":"other","tokens":[{"end":50,"kind":"keyword","start":0,"text":"mut"}]}
source_snippet:         mut obligation: PolyTraitObligation<'tcx>,
context_before:         previous_stack: TraitObligationStackList<'o, 'tcx>,
context_after:     ) -> Result<EvaluationResult, OverflowError> {