In Xcode I can put the caret on Darwin in "import Darwin” and use Jump to Definition to see a Swift source file for the Darwin module. But the file has no name, and clicking on the Show In Finder contextual menu item (which is enabled) does nothing but activate the Finder. I have looked every which way and haven’t found a Swift interface source file for Darwin, Swift, Foundation, anything.
If I follow the Package Manager's instructions for wrapping a C library for Swift import <https://github.com/apple/swift-package-manager/blob/master/Documentation/Usage.md#require-system-libraries> and then try to use Jump to Definition to see the built module’s interface source file, nothing happens. Again, the module’s Swift interface source code is nowhere to be found in the built files.
Is there some trick haven’t found?
How does anyone know what the interface to a wrapped library looks like?