[swift-evolution] Code comments that check code validity.

[Amir Michail]

The code in a “code comment" must compile (not just be syntactically correct) yet must not have any effect on the resulting executable.

For example, commented entries in an array would be checked for compilability but would not be included in the executable.

Such “code comments" would allow you to have code/data that is currently unused but is constantly checked to be valid just in case you want to use it in the future.