Visibility is restricted to a module which isn't an ancestor of the current item.

Erroneous code example:

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:

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:

pub mod Earth {
    pub mod Sea {
        pub (in crate::Earth) struct Shark; // ok!
    }
}

fn main() {}