有没有办法将参数与 Rust 宏匹配？

Question

考虑以下代码：

trait Trait {
    fn x(&self) -> u32;
}

struct A {}
impl Trait for A {
    fn x(&self) -> u32 {
        10
    }
}

struct B {}
impl Trait for B {
    fn x(&self) -> u32 {
        20
    }
}

struct C {
    created_time: u64,
}

impl Trait for C {
    fn x(&self) -> u32 {
        30
    }
}

impl C {
    pub fn new() -> C {
        C { created_time: 1000 } // for simplicity
    }
}

macro_rules! create {
    ($type:ident) => {
        match stringify!($type) {
            "C" => Box::new(C::new()) as Box<dyn Trait>,
            _ => Box::new($type {}) as Box<dyn Trait>,
        }
    };
}

fn main() {
    let a: Box<dyn Trait> = create!(A);
    let b: Box<dyn Trait> = create!(B);
    let c: Box<dyn Trait> = create!(C);

    assert_eq!(a.x(), 10);
    assert_eq!(b.x(), 20);
    assert_eq!(c.x(), 30);
}

如果您要求编译器扩展宏，则解析为：

let a: Box<dyn T> =
    match "A" {
        "C" => Box::new(C::new()) as Box<dyn T>,
        _ => Box::new(A{}) as Box<dyn T>,
    };
let b: Box<dyn T> =
    match "B" {
        "C" => Box::new(C::new()) as Box<dyn T>,
        _ => Box::new(B{}) as Box<dyn T>,
    };
let c: Box<dyn T> =
    match "C" {
        "C" => Box::new(C::new()) as Box<dyn T>,
        _ => Box::new(C{}) as Box<dyn T>,
    };

这很好地解释了为什么编译器在尝试编译它时会给出以下错误：

error[E0063]: missing field `created_time` in initializer of `C`
  --> mwe.rs:29:27
   |
29 |             _ => Box::new($type { }) as Box<dyn T>,
   |                           ^^^^^ missing `created_time`
...
37 |     let c: Box<dyn T> = create!(C);
   |                         ---------- in this macro invocation

error: aborting due to previous error

但是，我希望编译器会注意到 match "C" { "C" => ..., _ => ... } 的情况并删除第二个子句，因为它无论如何都无法运行。遗憾的是它没有，而是抱怨第二个（不可能的）子句无法编译。

我还尝试将宏中的match 替换为if，如下所示，但无济于事：

macro_rules! create {
    ($type:ident) => {
        if stringify!($type) == "C" {
            Box::new(C::new()) as Box<dyn T>
        } else {
            Box::new($type { }) as Box<dyn T>
        }
    }
}

导致

let c: Box<dyn T> =
    if "C" == "C" { Box::new(C::new()) as Box<dyn T> }
    else { Box::new(C{}) as Box<dyn T> };

与match 尝试的错误相同。

希望 Haskell 的保护管道语法能在 Rust 中以某种方式工作，我最后还尝试了以下方法：

macro_rules! create {
    ($type:ident) | (stringify!($type) == "C") => {
        Box::new(C::new()) as Box<dyn T>
    },
    ($type:ident) | (stringify!($type) != "C") => {
        Box::new($type { }) as Box<dyn T>
    },
}

但这给了error: no rules expected the token '|'

---

这最终让我回到标题中的问题：

有没有办法在宏规则中添加“守卫”来告诉编译器“只有在传递了这个参数时才运行 A，或者在其他东西上运行 B”？

Answer 1

虽然您的问题看起来确实是一个 X/Y 问题，并且可以使用诸如 Default 之类的特征来更优雅地解决，但可以在某种程度上匹配宏参数。

你的宏可以重写为

macro_rules! create {
    (C) => {
        Box::new(C::new()) as Box<dyn Trait>
    };

    ($type:ident) => {
        Box::new($type {}) as Box<dyn Trait>
    };
}

编译器在第一次成功匹配时停止。

请注意，这有一些限制：如您所料，编译器会对标记进行字面比较，例如以下内容将失败：

type D = C;

let really_just_another_c: Box<dyn Trait> = create!(D);

与

error[E0063]: missing field `created_time` in initializer of `C`
  --> src/main.rs:41:18
   |
41 |         Box::new($type {}) as Box<dyn Trait>
   |                  ^^^^^ missing `created_time`
...
51 |     let c: Box<dyn Trait> = create!(D);
   |                             ---------- in this macro invocation