All those iterator types are exactly the same as C++ having std::vector::iterator, std::vector::const_iterator, std::deque::iterator, ..., and exist for similar

All those iterator types are exactly the same as C++ having std::vector::iterator, std::vector::const_iterator, std::deque::iterator, ..., and exist for similar reasons: mostly zero-overhead abstractions. (This is even more visible in the range proposals for C++.) Using concrete types for every adaptor means all the steps in a iterator transformation pipeline are statically known and (importantly) obvious to the compiler, and thus is highly amenable to inlining/optimisation to get it down to something close to a plain 'for' loop. If you're willing to forgo that performance, it's possible to just put everything behind a pointer/virtual call with a Box<Iterator<Item = ...>>, which ends up more similar to sequences in F# etc. It does seem as if the motivation for this design isn't discussed in the documentation.