I'm a fan of test-driven development.
I use it myself, and teach it to students and colleagues.
One of the nice things about Swift 1.0 was that it was easy to write module tests.
When access controls were introduced into Swift, I found that many functions and objects that had no genuine need to be public, suddenly had to become public in order to be tested. That just seems wrong to me.
I'd like to see a way for tests, which are not normally part of the module, to have access to a module's contents (I'm talking source only - not packages that do not include source). That might simply be a feature of the IDE, rather than a language feature.
On another note, I see "file private" (whatever it's called) as a legacy of C. I have trouble seeing it as being truly useful, but can see that others might have genuine uses for it (actually, I'd like to hear what they might be)
Dr. J. Heerema