foo.rs

#[derive(Clone, Copy, Debug)]
enum Foo {
    One,
    Two,
}

impl Foo {
    pub fn get_it(&self) -> &str {
        match self {
            Foo::One => "Doodoo",
            Foo::Two => "Kaka",
        }
    }
}

trait Bar {
    // This is an implementation detail.
    fn get_foo(&self) -> Foo;
    // This is user-facing API that uses the internal `foo` member.
    fn do_something(&self) {
        println!("{}", self.get_foo().get_it())
    }
}

struct Quux {
    foo: Foo,
}

impl Bar for Quux {
    fn get_foo(&self) -> Foo { self.foo }
}

fn main() {
    let quux = Quux { foo: Foo::One };
    // So far so good.
    quux.do_something();
    // WTF? We're not meant to have access to the internal foo variable.
    println!("{:?}", quux.get_foo());
}