Extension traits (blanket impls) not showing up in docs

[Manishearth]

IteratorExt is now impl'd on all Iterators:

impl<A, I> IteratorExt<A> for I where I: Iterator<A>

However, it doesn't show up on any given Iterator: eg Map

This makes it even more confusing. For any newbie who doesn't know of the existence of this extension trait, all the useful methods will be hidden.

[tomjakubowski]

I think it's tough, or maybe intractable, for rustdoc in its current form to do this the "right way". As best as I can tell, if rustdoc would need to take every trait implementation which has a type parameter (impl Trait for Vec<T> where T: SomeOtherTrait is similar to a blanket implementation), and walk all documented types to check which ones fall under the impl. It gets worse when you consider generic types like Vec<T> (Vec<i32> might fall under a blanket impl, but not Vec<u32>, for example).

A workable solution might be some kind of rustdoc-specific attribute that indicates an impl is a "blanket impl", with a (Rust) path indicating where the docs for that trait's methods should be inlined. That way at least the common extension traits in the standard library would be documented on the types they extend.

Something like:

// Not totally relevant since `IteratorExt` is gone now, but...

pub trait Iterator { /* ... */ }
pub trait IteratorExt { /* ... */ }

// vvv problematic if this path is re-exported eventually
#[rustdoc_blanket_impl("$crate::iter::Iterator")]
impl IteratorExt for T where T: Iterator {
    /* ... */
}

[Manishearth]

That works, IMO

[tomjakubowski]

I do wonder - what are the remaining blanket impls in std? With the relaxed object safety rules it's harder to find them these days. In fact, all I can find are deprecated ones in old_io and the AsPath blanket impl.

[alexcrichton]

I agree with @tomjakubowski's analysis in that this is probably not possible to solve in the general case, but some specialization of attributes could perhaps help us here. I also agree with @Manishearth that this is all working as intended today.

We don't have too too many more blanket impls in the standard library beyond conversion traits, so I don't think the need for this is so great any more. I'm going to close this for now due to it working as intended, and we can reopen in the future if the need is great enough with a specialized attribute like @tomjakubowski showed perhaps.

[troplin]

Sorry to resurrect such an old and closed issue, but shouldn't blanket impls at least be documented on the doc page of the extension trait?