Function-like Macro

  运行: fib!(30) = 832040   fib!(10) = 55
  展开后的 main:
    fn main() {
        const F30: u64 = 832040u64;
        {
            ::std::io::_print(format_args!("fib!(30) = {0}   fib!(10) = {1}\n",
                    F30, 55u64));
        };
    }

  编译退出码 0
  运行:
    [("a", 1), ("b", 5), ("c", 4)]
    空: 0

  build 后运行: 版本 = "1.0"
  把 version.txt 改成 2.0 后 cargo build 输出: ['Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s']
  运行: 版本 = "1.0" ← 仍是旧值(宏没有重新运行)
  改动 main.rs(加一个换行)再 build 后运行: 版本 = "2.0"

  error: 读取 不存在.txt 失败: No such file or directory (os error 2)
   --> src/main.rs:3:28
    |
  3 | fn main() { let _ = embed!("不存在.txt"); let _ = fib!(99999999999); let _ = fib!("x"); }
    |                            ^^^^^^^^^^^^
  error: number too large to fit in target type
   --> src/main.rs:3:53
    |
  3 | fn main() { let _ = embed!("不存在.txt"); let _ = fib!(99999999999); let _ = fib!("x"); }
    |                                                        ^^^^^^^^^^^
  error: expected integer literal
   --> src/main.rs:3:80
    |
  3 | fn main() { let _ = embed!("不存在.txt"); let _ = fib!(99999999999); let _ = fib!("x"); }
    |                                                                                   ^^^
  error: could not compile `app` (bin "app") due to 3 previous errors

  build 后运行: include_str 版本 = "3.0"
  改文件为 4.0 后 cargo build: ['Compiling app v0.1.0 (<实验目录>/proj/u82/app)', 'Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s']
  运行: include_str 版本 = "4.0" ← 新值,cargo 追踪了 include_str! 读的文件