syntax_data: {"ast_node_type":"other","tokens":[{"end":73,"kind":"keyword","start":0,"text":"for"}]}
source_snippet:                 for (f_index, f_def) in v_def.fields.iter().enumerate() {
context_before:                 let variant = if def.is_struct() { None } else { Some(v_index) };
context_after:                     let field_ty = f_def.ty(tcx, args);