How do I use a macro across module files?

by

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.

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