syntax_data: {"ast_node_type":"variable","tokens":[{"end":71,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:                     let arg_ty = self.typeck_results.expr_ty(arg_expr);
context_before:                     let ty = args.type_at(0);
context_after:                     let mut arg = self.mirror_expr(arg_expr);