What code coverage tool would be useful in Xcode projects for Swift?

As of Xcode 7, code coverage is part of the testing capabilities:

Code coverage : Visualize the completeness of your test suite by enabling code coverage for your schema. The code coverage tab in the test report shows which files, functions, and lines of code were executed and, more importantly, which were not. The source editor can also display code coverage information on a line, allowing you to see at a glance which lines and parts of the line — tests were running.