missing_doc should not deny on invisible internal items

[dcrewi]

I expect missing_doc lint to only check that publicly-visible code is documented. The following gets caught by the lint checker when I think that it shouldn't:

#[deny(missing_doc)]

mod internal_impl {
    pub fn secret_sauce() {
        // unstable, messy, or unsafe code
    }
}

pub mod public_interface {
    use super::internal_impl;

    /// I do something.
    pub fn do_something() {
        // [do stuff]
        internal_impl::secret_sauce();
        // [do more stuff]
    }
}

[alexcrichton]

I agree!

On an implementation note, I would recommend using the results of the analysis from the privacy pass of the compiler instead of trying to re-implement it in a lint pass.

One tricky case to keep in mind is that you may pub use an overall private item, and the item should probably still get documented. i.e.

pub use imp::foo;

mod imp {
    // this should require documentation even though impl::foo isn't a public path
    pub fn foo() {}
}