Understanding Rust Error E0468: Macro Import Issue

Introduction to Rust Error E0468

Rust Error E0468 occurs when a non-root module attempts to import macros from another crate. Macros in Rust should only be imported at the crate root level. This error aims to maintain the modularity and organization of Rust code.

Erroneous Code Example

#![allow(unused)]
fn main() {
mod foo {
    #[macro_use(debug_assert)]  // error: must be at crate root to import
    extern crate core;          //        macros from another crate
    fn run_macro() { debug_assert!(true); }
}
}

In the code above, an attempt is made to import the `debug_assert!` macro from the `core` crate at the module level within `mod foo`. This will result in Rust Error E0468.

How to Resolve Rust Error E0468

To resolve Error E0468, you must follow one of the two options: 1. Move the macro import to the crate root level. 2. Remove the macro import and rewrite the code without the use of the foreign macro.

Working Code Example

#[macro_use(debug_assert)] // ok!
extern crate core;

mod foo {
    fn run_macro() { debug_assert!(true); }
}

fn main() {}

In the code above, the macro import was moved to the crate root level, thus allowing the use of the `debug_assert!` macro within the `mod foo` code block without encountering Rust Error E0468.

Conclusion

To avoid Rust Error E0468, always remember to import macros at the crate root level rather than inside submodules. This will help maintain modularity and organization in your Rust code, while also ensuring compatibility with different scopes and imports.