Import/Export

There are two ways to expose a macro to a wider scope. The first is the #[macro_use] attribute. This can be applied to either modules or external crates. For example:

#[macro_use]
mod macros {
    macro_rules! X { () => { Y!(); } }
    macro_rules! Y { () => {} }
}

X!();

fn main() {}

Macros can be exported from the current crate using #[macro_export]. Note that this ignores all visibility.

Given the following definition for a library package macs:

mod macros {
    #[macro_export] macro_rules! X { () => { Y!(); } }
    #[macro_export] macro_rules! Y { () => {} }
}

// X! and Y! are *not* defined here, but *are* exported,
// despite `macros` being private.

The following code will work as expected:

X!(); // X is defined
#[macro_use] extern crate macs;
X!();
# 
# fn main() {}

Note that you can only #[macro_use] an external crate from the root module.

Finally, when importing macros from an external crate, you can control which macros you import. You can use this to limit namespace pollution, or to override specific macros, like so:

// Import *only* the `X!` macro.
#[macro_use(X)] extern crate macs;

// X!(); // X is defined, but Y! is undefined

macro_rules! Y { () => {} }

X!(); // X is defined, and so is Y!

fn main() {}

When exporting macros, it is often useful to refer to non-macro symbols in the defining crate. Because crates can be renamed, there is a special substitution variable available: $crate. This will always expand to an absolute path prefix to the containing crate (e.g. :: macs).

Note that this does not work for macros, since macros do not interact with regular name resolution in any way. That is, you cannot use something like $crate::Y! to refer to a particular macro within your crate. The implication, combined with selective imports via #[macro_use] is that there is currently no way to guarantee any given macro will be available when imported by another crate.

It is recommended that you always use absolute paths to non-macro names, to avoid conflicts, including names in the standard library.