Visibility is restricted to a module which isn't an ancestor of the current
item.
Erroneous code example:
```compile_fail,E0742,edition2018
pub mod Sea {}
pub (in crate::Sea) struct Shark; // error!
fn main() {}
```
To fix this error, we need to move the `Shark` struct inside the `Sea` module:
```edition2018
pub mod Sea {
pub (in crate::Sea) struct Shark; // ok!
}
fn main() {}
```
Of course, you can do it as long as the module you're referring to is an
ancestor:
```edition2018
pub mod Earth {
pub mod Sea {
pub (in crate::Earth) struct Shark; // ok!
}
}
fn main() {}
```