We warn like this:
t.swift:3:12: warning: 'let' pattern has no effect; sub-pattern didn't bind any variables
case let .Bar: println("bar")
^~~ ~~~~
Swift SVN r27747
Most tests were using %swift or similar substitutions, which did not
include the target triple and SDK. The driver was defaulting to the
host OS. Thus, we could not run the tests when the standard library was
not built for OS X.
Swift SVN r24504
