This is a spinoff of a discussion from:
In my opinion the current strategy used to chose what iterables to add to the Rust standard library isn’t good enough. In my opinion adding iterators is not free, the more there are the higher cognitive load for the programmer there is and the time to find the right function grows. So functions should be added on the base of data. One way to look for such data is to measure in some way what itertools functions are more used. Instead of flatten() there are more commonly useful iterators to add to the std library.