syntax_data: {"ast_node_type":"other","tokens":[{"end":21,"kind":"keyword","start":0,"text":"index:"}]}
source_snippet:         index: usize,
context_before:         obligation: &PolyTraitObligation<'tcx>,
context_after:     ) -> Result<ImplSource<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {