syntax_data: {"ast_node_type":"other","tokens":[{"end":74,"kind":"keyword","start":0,"text":"for"}]}
source_snippet:     for arg in tcx.type_of(tail_field.did).instantiate_identity().walk() {
context_before:     let mut unsizing_params = DenseBitSet::new_empty(num_params);
context_after:         if let Some(i) = maybe_unsizing_param_idx(arg) {