Yesterday a new Swift source compatibility test suite was made available by those fine folks from the fruit. The suite is a community project that can be found here.
This interesting list of projects in the suite are periodically tested against development versions of Swift to hopefully find and fix incompatibility issues.
The goal is to have a strong source compatibility test suite containing thousands of projects. We look forward to project owners helping to achieve this goal by including their open source Swift projects in the test suite.