syntax_data: {"ast_node_type":"other","tokens":[{"end":73,"kind":"keyword","start":0,"text":"for"}]}
source_snippet:         for arg in tcx.type_of(field.did).instantiate_identity().walk() {
context_before:     for field in prefix_fields {
context_after:             if let Some(i) = maybe_unsizing_param_idx(arg) {