tt元变量类型在Rust宏中意味着什么? [英] What does the tt metavariable type mean in Rust macros?

问题描述

我正在读一本有关Rust的书，并开始使用 Rust宏.除了最后一个-tt外，所有元变量类型都在此处进行了说明并提供了示例.根据这本书，这是一棵单一令牌树".我很好奇，它是做什么用的?您能举个例子吗?

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

引入此概念是为了确保宏调用中的内容正确匹配()，[]和{}对. tt将匹配任何单个标记或括号/括号/大括号及其内容.

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
• main
• ()
  • ∅
  • fn
  • main
  • ()
    • ∅
    • println
    • !
    • ("Hello world!")
      • "Hello world!"
      • println
      • !
      • ("Hello world!")
        • "Hello world!"

        每一个都形成一棵树，简单的标记(fn，main等)是叶子，而被()，[]或{}包围的任何事物都有一个子树.请注意，(不会单独出现在令牌树中:如果不匹配相应的)，则不可能匹配(.

        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) => { /* … */ }
        }
        

        将上述功能与$name → main，$params → ()，$body → { println!("Hello world!"); }匹配.

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

        令牌树是要求最少的元变量类型:它匹配任何内容.它通常用在根本不在乎"部分的宏中，尤其是在具有头"和尾部"部分的宏中.例如，println!宏具有与($fmt:expr, $($arg:tt)*)匹配的分支，其中$fmt是格式字符串，而$($arg:tt)*表示其余所有"，并被转发到format_args!.这意味着println!不需要知道实际的格式并对其进行复杂的匹配.

        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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