学习经验
过程宏是 Rust 中进阶的内容，它是声明宏的拓展。但它的学习资料比较少，我整理了一些：你可以在 此链接 网页中搜索【过程宏】找到。
如果你想编写过程宏，那么以下内容是必须掌握的：
- Rust 几乎所有的语法：因为 Rust 的宏就是在操作 AST （或者说 CST？），所以掌握源代码的语法结构是第一步。当你能自如地查阅 Reference 一书，那么这一步就成功了。
- Rust 的声明宏：声明宏可以单独学习，它完全不涉及过程宏；但过程宏涉及声明宏，而且它们之间有许多相似的地方。 你可以不需要掌握声明宏高阶模式部分，但你至少要掌握声明宏最通用的部分。 当你对 The Little Book of Rust Macros 一书中的大部分内容熟悉时，你可以准备迎接过程宏了。
对我来说，学习过程宏的过程：
- 从 Rust Book: ch19-06-macros 中知道过程宏的分类。
- 从 Reference: procedural-macros 中知道过程宏真正的编写框架。
- 从 syn/examples 中学习如何在特定任务下真实地编写过程宏。你对这四个例子理解地越仔细，那么你就能越快地上手过程宏。
- 最重要的资料是文档：quote 和 syn 。过程宏不像声明宏那样开箱即用，你需要引入别的库，所以你需要掌握这两个库。
谨记 dtolnay 在 proc-macro-workshop 教程中的这些话：
There is only one profound insight about Rust macro development, and this test case begins to touch on it: what makes someone an "expert at macros" mostly has nothing to do with how good they are "at macros".
95% of what enables people to write powerful and user-friendly macro libraries is in their mastery of everything else about Rust outside of macros, and their creativity to put together ordinary language features in interesting ways that may not occur in handwritten code.
You may occasionally come across procedural macros that you feel are really advanced or magical. If you ever feel this way, I encourage you to take a closer look and you'll discover that as far as the macro implementation itself is concerned, none of those libraries are doing anything remotely interesting. They always just parse some input in a boring way, crawl some syntax trees in a boring way to find out about the input, and paste together some output code in a boring way exactly like what you've been doing so far. In fact once you've made it this far in the workshop, it's okay to assume you basically know everything there is to know about the mechanics of writing procedural macros.
To the extent that there are any tricks to macro development, all of them revolve around what code the macros emit, not how the macros emit the code. This realization can be surprising to people who entered into macro development with a vague notion of procedural macros as a "compiler plugin" which they imagine must imply all sorts of complicated APIs for how to integrate with the rest of the compiler. That's not how it works. The only thing macros do is emit code that could have been written by hand. If you couldn't have come up with some piece of tricky code from one of those magical macros, learning more "about macros" won't change that; but learning more about every other part of Rust will. Inversely, once you come up with what code you want to generate, writing the macro to generate it is generally the easy part.