Rust 宏中的 tt 元变量类型是什么意思? [英] What does the tt metavariable type mean in Rust macros?

问题描述

我正在阅读一本关于 Rust 的书，并开始使用 Rust 宏.除了最后一个 - tt 之外，所有元变量类型都在此处进行了解释并提供了示例.根据这本书，它是一个单一的令牌树".我很好奇，它是什么，它是用来做什么的?可以举个例子吗?

I'm reading a book about Rust, and start playing with Rust macros. All metavariable types are explained there and have examples, except the last one – tt. According to the book, it is a "a single token tree". I'm curious, what is it and what is it used for? Can you please provide an example?

推荐答案

引入这个概念是为了确保宏调用中的任何内容都正确匹配 (), [] 和 {} 对.tt 将匹配任何单个标记或任何一对括号/括号/大括号与其内容.

That's a notion introduced to ensure that whatever is in a macro invocation correctly matches (), [] and {} pairs. tt will match any single token or any pair of parenthesis/brackets/braces with their content.

例如，对于以下程序:

fn main() {
    println!("Hello world!");
}

令牌树将是:

• fn
• 主要
• ()
  • ∅
  • println
  • !
  • ("Hello world!")
    • 世界你好！"

    每个组成一棵树，其中简单的标记(fn、main 等)是叶子，而任何被 ()、[] 或 {} 有一个子树.请注意，( 不会单独出现在令牌树中:如果不匹配相应的 )，则无法匹配 (.

    Each one forms a tree where simple tokens (fn, main etc.) are leaves, and anything surrounded by (), [] or {} has a subtree. Note that ( does not appear alone in the token tree: it's not possible to match ( without matching the corresponding ).

    例如:

    macro_rules! {
        (fn $name:ident $params:tt $body:tt) => { /* … */ }
    }
    

    将与上述函数匹配 $name → main, $params → (), $body → { println!("Hello world!");}.

    would match the above function with $name → main, $params → (), $body → { println!("Hello world!"); }.

    令牌树是要求最低的元变量类型:它匹配任何东西.它通常用于具有不关心"部分的宏中，尤其是在具有头"和尾"部分的宏中.例如，println! 宏有一个与 ($fmt:expr, $($arg:tt)*) 匹配的分支，其中 $fmt是格式字符串，$($arg:tt)* 表示所有其余部分"，只是转发到 format_args!.这意味着 println! 不需要知道实际的格式并进行复杂的匹配.

    Token tree is the least demanding metavariable type: it matches anything. It's often used in macros which have a "don't really care" part, and especially in macros which have a "head" and a "tail" part. For example, the println! macros have a branch matching ($fmt:expr, $($arg:tt)*) where $fmt is the format string, and $($arg:tt)* means "all the rest" and is just forwarded to format_args!. Which means that println! does not need to know the actual format and do complicated matching with it.

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