I thought it would be useful to allow some uses of a module to be
'@_implementationOnly' and others to not be in case someone wanted to
change from one to the other gradually, but it turns out that if
you're trying to /make/ an import implementation-only, you want to
know everywhere you used it.
rdar://problem/50748157
This is an attribute that gets put on an import in library FooKit to
keep it from being a requirement to import FooKit. It's not checked at
all, meaning that in this form it is up to the author of FooKit to
make sure nothing in its API or ABI depends on the implementation-only
dependency. There's also no debugging support here (debugging FooKit
/should/ import the implementation-only dependency if it's present).
The goal is to get to a point where it /can/ be checked, i.e. FooKit
developers are prevented from writing code that would rely on FooKit's
implementation-only dependency being present when compiling clients of
FooKit. But right now it's not.
rdar://problem/48985979
We already have something called "module interfaces" -- it's the
generated interface view that you can see in Xcode, the interface
that's meant for developers using a library. Of course, that's also a
textual format. To reduce confusion, rename the new module stability
feature to "parseable [module] interfaces".
