Ok, now I've read a couple of those discussions and I have a one suggestion to whoever writes the documentation, call it what it is:
For this reason, Swift strings can’t be indexed by integer values.
The String type is not optimized for random access and we want to discourage developers of using it that way.
The reason why the API doesn't cover the use case from above is because Swift developers are worried about people shooting themselves in the foot with it. One thread says this issue comes up every so often. I would, therefore, have them rethink the position. I would gladly favor ease of use over performance in this case.
The other thing I would suggest is to be explicit, from what I've read with Swift, there's so much misleading stuff in the standard library that unless the documentation is abundant and explicit, any efforts to prevent people misusing the language are going to be futile.
In any case, thanks for pointing me to those posts. I did some searching but those didn't come up.