Are there examples where borrow checker stands on your way? Like the mentioned (in comments) way of building a tree. I'm sure there should be really weird edge cases.
Strictly speaking, [string UTF8String] is equivalent to '((const char <star>(<star>)(id, SEL))objc_msgSend)(string, selector)'. Since objc_msgSend is vararg function, it will promote it's arguments, so for example method taking float will be called with double. Casting to a proper function type works around that.