One thing that it would break is that strings read from files would be treated differently from those in source code, even those read from files that logically “belong” to the application (say config file)
I don’t think that’s an issue, though.
Also, in Swift "\(foo)" does string interpolation. I haven’t seen people complain it leaks data or makes Swift slow (but then, it’s not fast at compiling at all because of its rather complicated type inference)