Does rust really need higher kinded types?

[BurntSushi]

That is not the right definition. You removed the lifetime annotation. Here's the trait:

pub trait Streamer<'a> {
    type Item: 'a;
    fn next(&'a mut self) -> Option<Self::Item>;
}

In some world where HKT exists, the trait definition might look like:

pub trait Streamer {
    type Item;
    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>>;
}

But this is polymorphic code on type constructors, where Self::Item is a type constructor.

@keean

Regarding the borrowing, it is an additional requirement because it is not necessary in the definition of streaming filters, as you can see from the example posted, I can define a streaming filter without this.

You asked for examples where HKT was needed. I gave you a link to a use case. Your code does not apply to my use case.

@keean

To me it seems you are not talking about a general streaming-iterator, but a specific one that has particular requirements regarding buffering.

I want an abstraction that lets me write iterators whose elements can be borrowed from the iterator.