You can look at existing adapters in itertools or the standard library.
Peekable is a simple stateful one. Flatten is one that alters state between driving a source iterator and then getting items from the inner iterator.
However, if it is not
sumandcount, but some complex iterator adaptors/consumers provided by other crates
The issue here is that summing and counting are reducers/folders, not consumers or iterators. Those crates are simply providing the wrong API for what you want to do. Composable reduction could be done when provided as a simple binary operation.
Previous discussion: Adding Iterator::extrema, argmin, argmax - #4 by dlight