Private structure can escape from its module through impl for another private structure

[petrochenkov]

struct PrivOuter;

mod m {
    struct PrivInner;
    impl PrivInner {
        pub fn secret(&self) { println!("Hello!"); }
    }

    // This impl is considered private because its type `super::PrivOuter` is private
    // Probably, impls should be considered public if their type is defined outside of the current module even if this type is private
    impl super::PrivOuter {
        pub fn reveal(&self) -> PrivInner { PrivInner }
    }

}

fn main() {
    PrivOuter.reveal().secret();
}

This is not fixed in #29973.
It shouldn't be a common scenario, so it can be fixed later.

[petrochenkov]

This problem exists for trait impls as well:

struct PrivOuter;
trait PrivOuterTrait {
    type Output;
    fn reveal(&self) -> Self::Output;
}

mod m {
    struct PrivInner;
    impl PrivInner {
        pub fn secret(&self) { println!("Hello!"); }
    }

    impl super::PrivOuterTrait for super::PrivOuter {
        type Output = PrivInner;
        fn reveal(&self) -> PrivInner { PrivInner }
    }

}

fn main() {
    PrivOuter.reveal().secret();
}