Visibility

Modules are a privacy boundary:

• Module items are private by default (hides implementation details).
• Parent and sibling items are always visible.
• In other words, if an item is visible in module foo, it’s visible in all the descendants of foo.

mod outer {
    fn private() {
        println!("outer::private");
    }
    pub fn public() {
        println!("outer::public");
    }
    mod inner {
        fn private() {
            println!("outer::inner::private");
        }
        pub fn public() {
            println!("outer::inner::public");
            super::private();
        }
    }
}
fn main() {
    outer::public();
}

• Use the pub keyword to make modules public.

Additionally, there are advanced pub(...) specifiers to restrict the scope of public visibility.

• See the Rust Reference).
• Configuring pub(crate) visibility is a common pattern.
• Less commonly, you can give visibility to a specific path.
• In any case, visibility must be granted to an ancestor module (and all of its descendants).