指示符

宏的参数前缀是一个美元符号 $，并且使用一个 指示符 进行类型标注

macro_rules! create_function {
    // This macro takes an argument of designator `ident` and
    // creates a function named `$func_name`.
    // The `ident` designator is used for variable/function names.
    ($func_name:ident) => {
        fn $func_name() {
            // The `stringify!` macro converts an `ident` into a string.
            println!("You called {:?}()",
                     stringify!($func_name));
        }
    };
}

// Create functions named `foo` and `bar` with the above macro.
create_function!(foo);
create_function!(bar);

macro_rules! print_result {
    // This macro takes an expression of type `expr` and prints
    // it as a string along with its result.
    // The `expr` designator is used for expressions.
    ($expression:expr) => {
        // `stringify!` will convert the expression *as it is* into a string.
        println!("{:?} = {:?}",
                 stringify!($expression),
                 $expression);
    };
}

fn main() {
    foo();
    bar();

    print_result!(1u32 + 1);

    // Recall that blocks are expressions too!
    print_result!({
        let x = 1u32;

        x * x + 2 * x - 1
    });
}

以下是一些可用的指示符

• 块
• expr 用于表达式
• ident 用于变量/函数名
• 项
• literal 用于字面量常量
• pat (模式)
• 路径
• stmt (语句)
• tt (Token 树)
• ty (类型)
• vis (可见性限定符)

有关完整列表，请参阅 Rust 参考手册。