$ mkdir cf-swift
$ cd cf-swift
$ swift package init --type library
$ cat <<EOF >> Sources/cf-swift/cf_swift.swift
> import CoreFoundation
> import Foundation
> struct Dummy {
>   static let string: String? = {
>     let cfEncoding : CFStringEncoding = CFStringConvertNSStringEncodingToEncoding(String.Encoding.utf8.rawValue)
>     guard let _ = CFStringConvertEncodingToIANACharSetName(cfEncoding) else { return nil }
>     return nil
>   }()
> }
> EOF
$ swift build
$ swift test

Compile Swift Module 'cf_swiftTests' (1 sources)
Compile Swift Module 'cf_swiftPackageTests' (1 sources)
Linking ./.build/x86_64-unknown-linux/debug/cf-swiftPackageTests.xctest
/*/*/cf-swift/Sources/cf-swift/cf_swift.swift:9: error: undefined reference to 'objc_retainAutoreleasedReturnValue'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: link command failed with exit code 1 (use -v to see invocation)
error: terminated(1): /opt/Swift-4.0/bin/swift-build-tool -f /*/*/cf-swift/.build/debug.yaml test