Associated types lets you expose private types in public signatures (?)

[japaric]

STR

#![crate_type = "lib"]

pub struct Public;
struct Private;

impl Iterator for Public {
    type Item = Private;

    fn next(&mut self) -> Option<Private> {  // OK(???) `Private` type in public method 
        unimplemented!();
    }
}

// an inherent method rejects the same pattern
impl Public {
    pub fn my_next(&mut self) -> Option<Private> {  //~ error: private type in exported type signature
        unimplemented!();
    }
}

pub trait MyIterator<Item> {
    fn next(&mut self) -> Option<Item>;
}

// the old version of `Iterator` doesn't allow returning `Private` from `next()` either
impl MyIterator<Private> for Public {  //~ error: private type in exported type signature
    fn next(&mut self) -> Option<Private> {
        unimplemented!();
    }
}

AFAIK, you can't put private types in "public" items (public as in marked with pub). Iterator::next is not explicitly marked with pub, but for all intent and purposes all the methods in a trait are public if the trait is public. So, I think the impl Iterator for Public { type Item = Private; .. } should be rejected by the privacy checker. (Or, I could be wrong and this is "correct" behavior - and this wouldn't be the first time that the privacy/module system allows things that I find unreasonable)

Version

rustc 1.0.0-nightly (e233987ce 2015-02-27) (built 2015-02-28)

cc @alexcrichton @nikomatsakis

[alexcrichton]

I agree, I think this may have just been accidentally overlooked when implementing associated types.

[nikomatsakis]

On Sat, Feb 28, 2015 at 10:59:14PM -0800, Alex Crichton wrote:

I agree, I think this may have just been accidentally overlooked when implementing associated types.

Yes, that. There was language in the RFC describing trait item signatures and how they should
be treated, and I think we just failed to extend that to associated types.
Definitely a bug.

[nikomatsakis]

triage: P-backcompat-lang (1.0 beta)

[pnkfelix]

Note also that #22261 is still open.