宏可以与常量参数而不是文字匹配吗? [英] Can macros match against constant arguments instead of literals?

问题描述

给出宏匹配示例，该示例显示了宏如何匹配一个论点.

我在这里做了一些小的改动以使用数字:

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

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

作品，印刷:mode Y: 3

但是我想使用常量作为参数，这可以使它起作用吗?

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);
}

在Rust中有可能吗?

---

请注意，对我而言，不能使用常规的match语句，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. 因此，我特别想知道是否可以将常量传递给宏.

解决方案

否.

宏在抽象语法树上运行，因此它们在语法层次上进行推理:他们对令牌及其拼写进行推理. >

例如:

fn main() {
    let v = 3;
}

在这种情况下，AST将类似于:

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

如果您问一个宏v是否是3，它将使您看起来很滑稽，并且想知道为什么您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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