visible_private_types lint sometimes should explain why type is private

[pnkfelix]

Here is some code:

#![crate_type="lib"]

pub mod hello {
    pub use self::inner::foo;

    mod inner {
        pub fn foo() -> Bar { Bar }

        pub struct Bar;
    }
}

when I try to compile it, I get:

% rustc /tmp/p.rsb_
/tmp/p.rs:7:25: 7:28 warning: private type in exported type signature, #[warn(visible_private_types)] on by default
/tmp/p.rs:7         pub fn foo() -> Bar { Bar }
                                    ^~~

Now, maybe in the above code the problem is obvious, but in the original context where this arose, the inner was in a separate file from hello, and I did a lot of head scratching, saying "but I said pub struct Bar..."

I think that if the lint is complaining about a type that is itself declared public, then the lint should have a note indicating at least one module along the chain that is private. And maybe a note saying "you can fix this by either doing pub use self::inner::Bar or by making the mod inner public."

[reem]

Triage: it appears that our privacy-tracking has become a lot more simplistic, since this no longer errors at all. Nominating for close.