宏可以匹配常量参数而不是文字吗? [英] Can macros match against constant arguments instead of literals?

问题描述

给定宏匹配示例，这显示了宏如何匹配一个论点.

为了使用数字，我在这里做了很小的改动:

macro_rules！富{(0 => $e:expr) =>(println!("模式 X: {}", $e));(1 => $e:expr) =>(println!("mode Y: {}", $e));}fn 主(){foo!(1 => 3);}

作品，打印:mode Y: 3

但是我想使用一个常量作为参数，这是否可行:

const CONST: usize = 1;宏规则！富{(0 => $e:expr) =>(println!("模式 X: {}", $e));(1 => $e:expr) =>(println!("mode Y: {}", $e));}fn 主(){foo!(CONST => 3);}

这在 Rust 中可行吗?

<小时>

注意，使用常规的 match 语句对我来说不可用，因为在我的代码中，每个分支都解析为不同的类型，从而产生错误.所以我特别想知道是否可以将常量传递给宏.

解决方案

没有

宏在抽象语法树上运行，因此它们在句法级别进行推理:它们对标记和它们的拼写进行推理.>

例如:

fn main() {让 v = 3;}

在这种情况下，AST 将类似于:

fn main\_ 让绑定 v\_ 文字 3

如果你问一个宏 v 是否是 3，它会看着你很有趣，并想知道你为什么要尝试比较变量名和文字.>

Given the macro matching example, this shows how macros can match an argument.

I've made very minor changes here to use numbers:

macro_rules! foo {
    (0 => $e:expr) => (println!("mode X: {}", $e));
    (1 => $e:expr) => (println!("mode Y: {}", $e));
}

fn main() {
    foo!(1 => 3);
}

Works, printing: mode Y: 3

However I would like to use a constant as an argument, can this be made to work:

const CONST: usize = 1;

macro_rules! foo {
    (0 => $e:expr) => (println!("mode X: {}", $e));
    (1 => $e:expr) => (println!("mode Y: {}", $e));
}

fn main() {
    foo!(CONST => 3);
}

Is this possible in Rust?

---

Note, using a regular match statement isn't usable for me, since in my code each branch resolves to different types, giving an error. So I'm specifically interested to know if a constant can be passed to a macro.

解决方案

No.

Macros operate on the Abstract Syntax Tree, so they reason at the syntactic level: they reason about tokens and their spelling.

For example:

fn main() {
    let v = 3;
}

In this case, the AST will look something like:

fn main
    \_ let-binding v
        \_ literal 3

If you ask a macro whether v is 3, it will look at you funny, and wonder why you would try comparing a variable name and a literal.

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