Problem
I recently encountered a warning in a third party utility (WEPopover) in this piece of code:
_effectivePopoverContentSize = _con
As of version 2.0 (currently in beta as of this writing), Swift still does not include a preprocessor and that doesn't look to change anytime soon (if ever). The beneficial abilities that preprocessors allow are built into the language itself in various ways (I won't cover all of them here, see the docs) and are compile-time level features.
There are two features I do want to mention (one old, one new) that may be what you're looking for:
These tools (along with many others built into the language) should allow a developer to replace the Objective-C and C preprocessor macros. Personally, I think they are a huge improvement.