Understanding Rust Error E0466: Malformed Macro Import Declaration

Malformed Macro Import Declaration: Rust Error E0466

Rust Error E0466 occurs when there is a syntax error within macro import declarations. It is crucial for macro import declarations to have valid syntax, and incorrect syntax causes the compiler to throw this error.

Erroneous code examples:

#![allow(unused)]
fn main() {
    #[macro_use(a_macro(another_macro))] // error: invalid import declaration
    extern crate core as some_crate;

    #[macro_use(i_want = "some_macros")] // error: invalid import declaration
    extern crate core as another_crate;
}

In the examples above, the macro import declarations are invalid due to incorrect syntax.

Proper syntax for macro imports

To prevent Rust Error E0466, you must adhere to the proper syntax for macro imports. The following example showcases the correct syntax:

// In some_crate:
#[macro_export]
macro_rules! get_tacos {
    ...
}

#[macro_export]
macro_rules! get_pimientos {
    ...
}

// In your crate:
#[macro_use(get_tacos, get_pimientos)] // It imports `get_tacos` and
extern crate some_crate;               // `get_pimientos` macros from some_crate

In the example above, both 'get_tacos' and 'get_pimientos' macros are imported correctly from some_crate.

Importing all exported macros

If you want to import all exported macros, you should use the macro_use attribute without any arguments. The following code demonstrates this approach:

// In your crate:
#[macro_use]
extern crate some_crate;

Using the macro_use attribute without arguments, as shown above, will import all macros exported from some_crate.