Here is a fleshed out argument:

Iterators are very important for performance since we use them for for loops. By default collections if they do not provide a custom iterator use IndexingIterator as an iterator. IndexingIterator, while only supporting forward movement, is based on indices which possess the ability to go backwards and forwards. For some indices backwards iteration adds unnecessary cost/complexity.

In order to preserve future flexibility, we want to change certain of the types to use custom types that are essentially just IndexingIterator<T>. Then later on when we want to implement these optimizations, we can do it on the custom type underneath the hood without changing the API.

Has this been looked into? In the Xcode 9.4.1 toolchain, `String.Iterator` is still `IndexingIterator`

This was done for everything except ClosedRange in #13489

The ClosedRange change was in #13342

@airspeedswift Can you comment on the String.Iterator question?