Here is a fairly quick thought.
Would it be possible to allow to escape comments at end of the multiline string literal's line?
let str = """ \(foo)\n\ // comment \(bar) """
I have a tiny workaround for this, but it's not ideal and on top of that it seems to confused syntax highlighting really badly.
let str = """ \(foo /* comment */) \(bar) """
Maybe we can generalize this and even allow inlined comments, but that's not the main priority for me.
let str = """ \(foo) \/* comment still a comment */ \(bar) """
If this something reasonable and someone would like to pick up and tackle formal proposal + implementation, feel free to do so.