A non-root module attempts to import macros from another crate.
Example of erroneous code:
```compile_fail,E0468
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); }
}
```
Only `extern crate` imports at the crate root level are allowed to import
macros.
Either move the macro import to crate root or do without the foreign macros.
This will work:
```
#[macro_use(debug_assert)]
extern crate core;
mod foo {
fn run_macro() { debug_assert!(true); }
}
# fn main() {}
```