root@bacccc9eabda:/test-package# PATH=/swift-DEVELOPMENT-SNAPSHOT-2019-02-26-a-ubuntu16.04/usr/bin:$PATH swift test -Xcc -fcf-runtime-abi=swift-5.0 -Xcc -I/swift-DEVELOPMENT-SNAPSHOT-2019-02-26-a-ubuntu16.04/usr/lib/swift
[5/5] Linking ./.build/x86_64-unknown-linux/debug/test-packagePackageTests.xctest
Test Suite 'All tests' started at 2019-02-28 17:20:56.427
Test Suite 'debug.xctest' started at 2019-02-28 17:20:56.428
Test Suite 'test_packageTests' started at 2019-02-28 17:20:56.429
Test Case 'test_packageTests.testExample' started at 2019-02-28 17:20:56.429
test a cfstr
Test Case 'test_packageTests.testExample' passed (0.0 seconds)
Test Suite 'test_packageTests' passed at 2019-02-28 17:20:56.429
     Executed 1 test, with 0 failures (0 unexpected) in 0.0 (0.0) seconds
Test Suite 'debug.xctest' passed at 2019-02-28 17:20:56.429
     Executed 1 test, with 0 failures (0 unexpected) in 0.0 (0.0) seconds
Test Suite 'All tests' passed at 2019-02-28 17:20:56.429

root@bacccc9eabda:/test-package# cat Sources/CLibTest/include/CLibTest.h
#include <CoreFoundation/CoreFoundation.h>

extern const _Nonnull CFStringRef kTestACFString;
root@bacccc9eabda:/test-package# cat Sources/CLibTest/CLibTest.c
#include <CLibTest.h>

#include <CoreFoundation/CoreFoundation.h>

const CFStringRef kTestACFString = CFSTR("test a cfstr");
root@bacccc9eabda:/test-package# cat Sources/test-package/test_package.swift
import Foundation
import CLibTest

struct test_package {
    var text = "Hello, World!"

    func test(string: String) {
        print(string)
    }

    func test() {
        test(string: kTestACFString as! String)
    }
}