Unit testing a compiler for a language like Swift is hard because there are so many features that interact in complex ways. There are unit tests but it's easy t

Unit testing a compiler for a language like Swift is hard because there are so many features that interact in complex ways. There are unit tests but it's easy to miss things that look "obvious" in hindsight. Until recently the focus was on finishing the design of the language itself and not fixing bugs or improving compile time. Now that the language is source stable the focus is shifting to the latter two tasks. Also source stability means we can build up a larger body of code for testing (imagine having to constantly migrate unit tests that exercise corner cases and quirks... not fun).