如何在同一个 crate 中使用由另一个宏定义的宏？

Posted 2023-02-19

【中文标题】如何在同一个 crate 中使用由另一个宏定义的宏？

【英文标题】：How to use a macro which is defined by another macro in the same crate?

【发布时间】：2019-05-27 02:03:17

【问题描述】：

我正在定义一个像这样定义其他宏的宏：

ma​​cros.rs

#[macro_export]
macro_rules! m1 
    () => 
        #[macro_export]
        macro_rules! m2 
            () => 
        
    


m1!();
m2!(); // no problem;

我可以通过use crate_name::macros::* 在另一个板条箱中使用m2!，我可以在macros.rs 中使用m2!，但我不知道如何在同一个板条箱中的文件中使用m2!。

lib.rs

#[macro_use]
pub mod macros;
pub mod test;
pub mod test2;

test.rs（与 macros.rs 在同一个 crate 中）

use crate::m1; // no problem
use crate::m2; // ERROR: macro-expanded `macro_export` macros from the current crate cannot be referred to by absolute paths

m1!(); // no problem
m2!(); // error, no m2

test2.rs

use crate::*;
m2!(); // this works, but I don't really want to use crate::*

examples/yo.rs

use crate_name::m2;
m2!(); // no problem

在同一个 crate 的其他文件中使用 m2 宏的正确方法是什么？我正在使用 Rust 1.31.1。

【参考方案1】：

阅读并遵循编译器的说明：

error[E0659]: `m2` is ambiguous (macro-expanded name vs less macro-expanded name from outer scope during import/macro resolution)
  --> src/lib.rs:22:5
   |
22 |     m2!();
   |     ^^ ambiguous name
   |
note: `m2` could refer to the macro defined here
  --> src/lib.rs:7:13
   |
7  | /             macro_rules! m2 
8  | |                 () => ;
9  | |             
   | |_____________^
...
21 |       m1!();
   |       ------ in this macro invocation
note: `m2` could also refer to the macro defined here
  --> src/lib.rs:7:13
   |
7  | /             macro_rules! m2 
8  | |                 () => ;
9  | |             
   | |_____________^
...
13 |       m1!();
   |       ------ in this macro invocation

error: a macro named `m2` has already been exported
  --> src/lib.rs:7:13
   |
7  | /             macro_rules! m2 
8  | |                 () => ;
9  | |             
   | |_____________^ `m2` already exported
...
21 |       m1!();
   |       ------ in this macro invocation
   |
   = note: #[deny(duplicate_macro_exports)] on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = note: for more information, see issue #35896 <https://github.com/rust-lang/rust/issues/35896>
note: previous macro export is now shadowed
  --> src/lib.rs:7:13
   |
7  | /             macro_rules! m2 
8  | |                 () => ;
9  | |             
   | |_____________^
...
13 |       m1!();
   |       ------ in this macro invocation

error: macro-expanded `macro_export` macros from the current crate cannot be referred to by absolute paths
  --> src/lib.rs:19:9
   |
19 |     use crate::m2; 
   |         ^^^^^^^^^
   |
   = note: #[deny(macro_expanded_macro_exports_accessed_by_absolute_paths)] on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #52234 <https://github.com/rust-lang/rust/issues/52234>
note: the macro is defined here
  --> src/lib.rs:7:13
   |
7  | /             macro_rules! m2 
8  | |                 () => ;
9  | |             
   | |_____________^
...
21 |       m1!();
   |       ------ in this macro invocation

具体来说：

错误：当前 crate 中的宏扩展 macro_export 宏不能被绝对路径引用

应用：

不要导入宏；没必要。 请勿拨打m1；这样做会创建第二个m2。

test.rs

// use crate::m1;
// use crate::m2; 

// m1!();
m2!();

【讨论】：

谢谢，很清楚，彻底解决了问题。