How do I use a macro across module files?

ModuleRustRust Macros

Module Problem Overview

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?

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?

Module Solutions

Solution 1 - Module

Macros within the same crate

New method (since Rust 1.32, 2019-01-17)

foo::bar!();  // works

mod foo {
    macro_rules! bar {
        () => ()
    }

    pub(crate) use bar;    // <-- the trick
}

foo::bar!();  // works

With the pub use, the macro can be used and imported like any other item. And unlike the older method, this does not rely on source code order, so you can use the macro before (source code order) it has been defined.

Old method

bar!();   // Does not work! Relies on source code order!

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

bar!();    // works

If you want to use the macro in the same crate, the module your macro is defined in needs the attribute #[macro_use]. Note that macros can only be used after they have been defined!


Macros across crates

Crate util

#[macro_export]
macro_rules! foo {
    () => ()
}

Crate user

use util::foo;

foo!();

Note that with this method, 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;. By using pub use, you can export a macro from a module of your crate (in addition to it being exported at the root).

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. This syntax should not be necessary anymore.

Solution 2 - Module

Alternative approach as of 1.32.0 (2018 edition)

Note that while the instructions from @lukas-kalbertodt are still up to date and work well, the idea of having to remember special namespacing rules for macros can be annoying for some people.

On the 2018 edition and onwards, since the version 1.32.0 of Rust, there is another approach which works as well, and which has the benefit, imho, of making it easier to teach (e.g., it renders #[macro_use] obsolete). The key idea is the following:

> ### A re-exported macro behaves as any other item (function, type, constant, etc.): it is namespaced within the module where the re-export occurs.

  • It can then be referred to with a fully qualified path.

  • It can also be locally used / brought into scope so as to refer to it in an unqualified fashion.

Example

macro_rules! macro_name { ... }
pub(crate) use macro_name; // Now classic paths Just Work™

And that's it. Quite simple, huh?

Feel free to keep reading, but only if you are not scared of information overload ;) I'll try to detail why, how and when exactly does this work.

More detailed explanation

In order to re-export (pub(...) use ...) a macro, we need to refer to it! That's where the rules from the original answer are useful: a macro can always be named within the very module where the macro definition occurs, but only after that definition.

macro_rules! my_macro { ... }
my_macro!(...); // OK

// Not OK
my_macro!(...); /* Error, no `my_macro` in scope! */
macro_rules! my_macro { ... }

Based on that, we can re-export a macro after the definition; the re-exported name, then, in and of itself, is location agnostic, as all the other global items in Rust 

  • In the same fashion that we can do:

    struct Foo {}

fn main() {
    let _: Foo;
}

  • We can also do:

    fn main() {
    let _: A;
}

struct Foo {}
use Foo as A;

  • The same applies to other items, such as functions, but also to macros!

    fn main() {
    a!();
}

macro_rules! foo { ... } // foo is only nameable *from now on*
use foo as a;            // but `a` is now visible all around the module scope!

    And it turns out that we can write use foo as foo;, or the common use foo; shorthand, and it still works.

The only question remaining is: pub(crate) or pub?

  • For #[macro_export]-ed macros, you can use whatever privacy you want; usually pub.

  • For the other macro_rules! macros, you cannot go above pub(crate).

Detailed examples

  • For a non-#[macro_export]ed macro
    mod foo {
    use super::example::my_macro;

    my_macro!(...); // OK
}

mod example {
    macro_rules! my_macro { ... }
    pub(crate) use my_macro;
}

example::my_macro!(...); // OK
  • For a #[macro_export]-ed macro

    Applying #[macro_export] on a macro definition makes it visible after the very module where it is defined (so as to be consistent with the behavior of non-#[macro_export]ed macros), but it also puts the macro at the root of the crate (where the macro is defined), in an absolute path fashion.

    This means that a pub use macro_name; right after the macro definition, or a pub use crate::macro_name; in any module of that crate will work.

    • Note: in order for the re-export not to collide with the "exported at the root of the crate" mechanic, it cannot be done at the root of the crate itself.
    pub mod example {
    #[macro_export] // macro nameable at `crate::my_macro`
    macro_rules! my_macro { ... }
    pub use my_macro; // macro nameable at `crate::example::my_macro`
}

pub mod foo {
    pub use crate::my_macro; // macro nameable at `crate::foo::my_macro`
}

When using the pub / pub(crate) use macro_name;, be aware that given how namespaces work in Rust, you may also be re-exporting constants / functions or types / modules. This also causes problems with globally available macros such as #[test], #[allow(...)], #[warn(...)], etc.

In order to solve these issues, remember you can rename an item when re-exporting it:

macro_rules! __test__ { ... }
pub(crate) use __test__ as test; // OK

macro_rules! __warn__ { ... }
pub(crate) use __warn__ as warn; // OK

Also, some false positive lints may fire:

Solution 3 - Module

This answer is outdated as of Rust 1.1.0-stable.

You need to add #![macro_escape] at the top of macros.rs and include it using mod macros; as mentioned in the Macros Guide.

$ cat macros.rs
#![macro_escape]

#[macro_export]
macro_rules! my_macro {
    () => { println!("hi"); }
}

$ cat something.rs
#![feature(macro_rules)]
mod macros;

fn main() {
    my_macro!();
}

$ rustc something.rs
$ ./something
hi

For future reference,

$ rustc -v
rustc 0.13.0-dev (2790505c1 2014-11-03 14:17:26 +0000)

Solution 4 - Module

Adding #![macro_use] to the top of your file containing macros will cause all macros to be pulled into main.rs.

For example, let's assume this file is called node.rs:

#![macro_use]

macro_rules! test {
    () => { println!("Nuts"); }
}

macro_rules! best {
    () => { println!("Run"); }
}

pub fn fun_times() {
    println!("Is it really?");
}

Your main.rs would look sometime like the following:

mod node;  //We're using node.rs
mod toad;  //Also using toad.rs

fn main() {
    test!();
    best!();
    toad::a_thing();
}

Finally let's say you have a file called toad.rs that also requires these macros:

use node; //Notice this is 'use' not 'mod'

pub fn a_thing() {
  test!();

  node::fun_times();
}

Notice that once files are pulled into main.rs with mod, the rest of your files have access to them through the use keyword.

Solution 5 - Module

I have came across the same problem in Rust 1.44.1, and this solution works for later versions (known working for Rust 1.7).

Say you have a new project as:

src/
    main.rs
    memory.rs
    chunk.rs

In main.rs, you need to annotate that you are importing macros from the source, otherwise, it will not do for you.

#[macro_use]
mod memory;
mod chunk;

fn main() {
    println!("Hello, world!");
}

So in memory.rs you can define the macros, and you don't need annotations:

macro_rules! grow_capacity {
	( $x:expr ) => {
		{
			if $x < 8 { 8 } else { $x * 2 }
		}
	};
}

Finally you can use it in chunk.rs, and you don't need to include the macro here, because it's done in main.rs:

grow_capacity!(8);

The upvoted answer caused confusion for me, with this doc by example, it would be helpful too.

Note: This solution does work, but do note as @ineiti highlighted in the comments, the order u declare the mods in the main.rs/lib.rs matters, all mods declared after the macros mod declaration try to invoke the macro will fail.

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Content TypeOriginal AuthorOriginal Content on Stackoverflow
QuestionuserView Question on Stackoverflow
Solution 1 - ModuleLukas KalbertodtView Answer on Stackoverflow
Solution 2 - ModuleDaniel H-MView Answer on Stackoverflow
Solution 3 - ModuleDogbertView Answer on Stackoverflow
Solution 4 - ModuleLuke DupinView Answer on Stackoverflow
Solution 5 - Moduleknh190View Answer on Stackoverflow