[ardel95]

This is a good example of the type of challenges you face as an author of widely used library. I can see a lot of scenarios where an optimization like this would bring benefits. But there are also many where it would hurt performance (not to mention memory usage), including most "collect once read many times" use-cases.

But I think the real thing for me is that this violates the principle of least surprise. If I wanted the type of memory reuse / lazy transformation behavior this optimization introduces, I would be looking at working with an iterator with a bunch of functinoal transforms. And if I'm calling .collect() it's because I want to convert the iterator into a data structure optimized for reads.

But I can also see how others would land on the other end, and hence the challenges for the library authors.