如何在模块文件中使用宏? [英] How do I use a macro across module files?

问题描述

我在同一板条箱中的单独文件中有两个模块，其中板条箱已启用macro_rules.我想使用在另一个模块中一个模块中定义的宏.

I have two modules in separate files within the same crate, where the crate has macro_rules enabled. I want to use the macros defined in one module in another module.

// macros.rs
#[macro_export] // or not? is ineffectual for this, afaik
macro_rules! my_macro(...)

// something.rs
use macros;
// use macros::my_macro; <-- unresolved import (for obvious reasons)
my_macro!() // <-- how?

我目前遇到了编译器错误"macro undefined: 'my_macro'" ...，这很有意义；宏系统先于模块系统运行.我该如何解决?

I currently hit the compiler error "macro undefined: 'my_macro'"... which makes sense; the macro system runs before the module system. How do I work around that?

推荐答案

同一箱子中的宏

#[macro_use]
mod foo {
    macro_rules! bar {
        () => ()
    }
}

bar!();    // works

如果要在同一包装箱中使用宏，则定义宏的模块需要属性#[macro_use].

If you want to use the macro in the same crate, the module your macro is defined in needs the attribute #[macro_use].

宏只能在定义后 使用.这意味着这行不通:

Macros can only be used after they have been defined. This means that this does not work:

bar!();  // ERROR: cannot find macro `bar!` in this scope

#[macro_use]
mod foo {
    macro_rules! bar {
        () => ()
    }
}

---

板条箱中的宏

要使用其他板条箱中的macro_rules!宏，宏本身需要属性#[macro_export].然后，导入箱可以通过use crate_name::macro_name;导入宏.

---

Macros across crates

To use your macro_rules! macro from other crates, the macro itself needs the attribute #[macro_export]. The importing crate can then import the macro via use crate_name::macro_name;.

// --- Crate `util` ---
#[macro_export]
macro_rules! foo {
    () => ()
}


// --- Crate `user` ---
use util::foo;

foo!();

注意:宏始终位于板条箱的顶层；因此即使foo放在mod bar {}内，user板条箱仍必须写use util::foo;和 not use util::bar::foo;.

Note: macros always live at the top-level of a crate; so even if foo would be inside a mod bar {}, the user crate would still have to write use util::foo; and not use util::bar::foo;.

(在Rust 2018之前，您必须通过在extern crate util;语句中添加属性#[macro_use]从其他包装箱中导入宏.这将从util中导入所有宏.或者，#[macro_use(cat, dog)]仅可用于导入宏cat和dog.不再需要使用此语法.)

(Before Rust 2018, you had to import macro from other crates by adding the attribute #[macro_use] to the extern crate util; statement. That would import all macros from util. Alternatively, #[macro_use(cat, dog)] could be used to only import the macros cat and dog. This syntax should not be necessary anymore.)

更多信息，请参见 Rust编程语言 .

More information is available in The Rust Programming Language.

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