Exporting declarative macro in shared module in `benches`

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1

Hello all, I have a crate that looks something like:

├── Cargo.toml
├── src
│   └── lib.rs
├── benches
     └── common.rs
     └── my_bench.rs

Inside common.rs, I have a macro intended for use in my other benchmark files, e.g. my_bench.rs:

// common.rs

#[macro_export]
macro_rules! my_macro {
    ($key:expr) => { 
        // does something
    }
}

// some other functions
pub fn helper_func() {}

Here, I used #[macro_export] to export my_macro. From my understanding, this means that my_macro will be declared at the crate root scope.

But when I try to do this in my_bench.rs:

mod common;
use crate::my_macro;

// benchmark code
// ...

I get the error:

error[E0255]: the name `my_macro` is defined multiple times
 --> my-crate/benches/common.rs:4:1
  |
4 | macro_rules! my_macro {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `my_macro` redefined here
  |
 ::: my-crate/benches/my_bench.rs:7:5
  |
7 | use crate::my_macro;
  |     ------------------------ previous import of the macro `my_macro` here
  |
  = note: `my_macro` must be defined only once in the macro namespace of this module

The solution to get around this, it turns out, is simply to omit use crate::my_macro, and everything works fine! But why is this the case? Does this have something to do with how benchmarks are compiled? In case it matters, my_bench has harness = false as well in Cargo.toml.

Follow-up

Another thing I tried, which works, is to replace #[macro_export] in common.rs with pub(crate) use my_macro (got the idea from here):

// common.rs

macro_rules! my_macro {
	// ...
}

pub(crate) use my_macro;

Then in my_bench.rs:

mod common;
use common::{helper_func, my_macro};

However, now I get the warning:

warning: unused macro definition: `my_macro`
 --> my-crate/benches/common.rs:3:14
  |
3 | macro_rules! my_macro {
  |              ^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_macros)]` on by default

warning: unused import: `my_macro`
  --> my-crate/benches/common.rs:27:16
   |
27 | pub(crate) use my_macro;
   |                ^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

Once again, I don't quite get why is this the case. Any pointers, explanations, or links to detailed explanations would be greatly appreciated!

2

It indeed incurs two macros with same name which is exactly reported from the error

  • #[macro_export] defines the macro in the root
  • use crate::my_macro; brings the macro agian into the root: see the fine result if brings it via an alias Rust Playground

Update: to have the path-based scope for macros, you can use this trick ( play )

use crate::common::m;

mod common {
    #[macro_export]
    macro_rules! my_macro {
        () => { };
    }
    pub use my_macro as m; // alias + reexport for path-based scope
}

fn main() {
    my_macro!();
    m!(); // works
    crate::common::m!(); // works
}
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