[sciolizer]

Good question. Technically, ToOwned counts:

  pub trait ToOwned {
    type Owned: Borrow<Self>;
    fn clone_into(&self, target: &mut Self::Owned) { ... }
  }

Here the `Owned` is technically an input to `clone_into`, but of course semantically it's still an output.

A more subtle one is `StreamExt`:

  pub trait StreamExt: Stream {
    type Item;
    fn map<T, F>(self, f: F) -> Map<Self, F>
       where F: FnMut(Self::Item) -> T,
             Self: Sized { ... }
  }

Here, the associated type `Item` is the input to the mapping function, and since the mapping function is an input to the map function, it is an input-to-an-input - which basically makes it an output. i.e. the stream "outputs" the items into the mapping function. (aka two contravariants make a covariant).

I couldn't find any more direct examples.