Scoping

The way in which mbe macros are scoped can be somewhat unintuitive. They use two forms of scopes: textual scope, and path-based scope.

When such a macro is invoked by an unqualified identifier(an identifier that isn't part of a multi-segment path), it is first looked up in textual scoping and then in path-based scoping should the first lookup not yield any results. If it is invoked by a qualified identifier it will skip the textual scoping lookup and instead only do a look up in the path-based scoping.

Textual Scope

Firstly, unlike everything else in the language, function-like macros will remain visible in sub-modules.

macro_rules! X { () => {}; }
mod a {
    X!(); // defined
}
mod b {
    X!(); // defined
}
mod c {
    X!(); // defined
}
fn main() {}

Note: In these examples, remember that all of them have the same behavior when the module contents are in separate files.

Secondly, also unlike everything else in the language, macro_rules! macros are only accessible after their definition. Also note that this example demonstrates how macro_rules! macros do not "leak" out of their defining scope:

mod a {
    // X!(); // undefined
}
mod b {
    // X!(); // undefined
    macro_rules! X { () => {}; }
    X!(); // defined
}
mod c {
    // X!(); // undefined
}
fn main() {}

To be clear, this lexical order dependency applies even if you move the macro to an outer scope:

mod a {
    // X!(); // undefined
}
macro_rules! X { () => {}; }
mod b {
    X!(); // defined
}
mod c {
    X!(); // defined
}
fn main() {}

However, this dependency does not apply to macros themselves:

mod a {
    // X!(); // undefined
}
macro_rules! X { () => { Y!(); }; }
mod b {
    // X!(); // defined, but Y! is undefined
}
macro_rules! Y { () => {}; }
mod c {
    X!(); // defined, and so is Y!
}
fn main() {}

Defining macro_rules! macros multiple times is allowed and the most recent declaration will simply shadow previous ones unless it has gone out of scope.


#![allow(unused)]
fn main() {
macro_rules! X { (1) => {}; }
X!(1);
macro_rules! X { (2) => {}; }
// X!(1); // Error: no rule matches `1`
X!(2);

mod a {
    macro_rules! X { (3) => {}; }
    // X!(2); // Error: no rule matches `2`
    X!(3);
}
// X!(3); // Error: no rule matches `3`
X!(2);

}

macro_rules! macros can be exported from a module using the #[macro_use] attribute. Using this on a module is similar to saying that you do not want to have the module's macro's scope end with the module.

mod a {
    // X!(); // undefined
}
#[macro_use]
mod b {
    macro_rules! X { () => {}; }
    X!(); // defined
}
mod c {
    X!(); // defined
}
fn main() {}

Note that this can interact in somewhat bizarre ways due to the fact that identifiers in a macro_rules! macro (including other macros) are only resolved upon expansion:

mod a {
    // X!(); // undefined
}
#[macro_use]
mod b {
    macro_rules! X { () => { Y!(); }; }
    // X!(); // defined, but Y! is undefined
}
macro_rules! Y { () => {}; }
mod c {
    X!(); // defined, and so is Y!
}
fn main() {}

Another complication is that #[macro_use] applied to an extern crate does not behave this way: such declarations are effectively hoisted to the top of the module. Thus, assuming X! is defined in an external crate called mac, the following holds:

mod a {
    // X!(); // defined, but Y! is undefined
}
macro_rules! Y { () => {}; }
mod b {
    X!(); // defined, and so is Y!
}
#[macro_use] extern crate macs;
mod c {
    X!(); // defined, and so is Y!
}
fn main() {}

Finally, note that these scoping behaviors apply to functions as well, with the exception of #[macro_use] (which isn't applicable):

macro_rules! X {
    () => { Y!() };
}

fn a() {
    macro_rules! Y { () => {"Hi!"} }
    assert_eq!(X!(), "Hi!");
    {
        assert_eq!(X!(), "Hi!");
        macro_rules! Y { () => {"Bye!"} }
        assert_eq!(X!(), "Bye!");
    }
    assert_eq!(X!(), "Hi!");
}

fn b() {
    macro_rules! Y { () => {"One more"} }
    assert_eq!(X!(), "One more");
}

fn main() {
    a();
    b();
}

These scoping rules are why a common piece of advice is to place all macro_rules! macros which should be accessible "crate wide" at the very top of your root module, before any other modules. This ensures they are available consistently. This also applies to mod definitions for files, as in:

#[macro_use]
mod some_mod_that_defines_macros;
mod some_mod_that_uses_those_macros;

The order here is important, swap the declaration order and it won't compile.

Path-Based Scope

By default, a macro_rules! macro has no path-based scope. However, if it has the #[macro_export] attribute, then it is declared in the crate root scope and can be referred to similar to how you refer to any other item. The Import and Export chapter goes more in-depth into said attribute.