Macro face_fields 

face_fields!() { /* proc-macro */ }

The face field front end, re-exported so a host does not have to depend on the proc-macro crate itself. 注册面字段前端；再导出后宿主无需自己依赖 proc-macro crate。 Rewrite one face declaration’s field list into its accepted order, with tolerant separators and spanned diagnostics. 把一条注册面声明的字段列表改写成它接受的顺序，容忍分隔符差异并给出带 span 的诊断。

The macro is the compile-time front end of the kernel’s face grammar: it parses the same tokens the authoring parser does, so the compiler and the tooling reject exactly the same declarations. 本宏是内核注册面语法的编译期前端：它解析与创作解析器相同的 token，因此编译器与工具 拒绝的是完全相同的声明。