https://developer.apple.com/documentation/swift/imported_c_and_objective-c_apis/working_with_core_foundation_types
If we define a C function that returns a CFType, Swift can (sometimes) bridge the memory management semantics. If Swift cannot bridge the memory management semantics, we can end up with an Unmanaged type:
CFStringRef CreateStringByAddingTwoStrings(CFStringRef s1, CFStringRef s2);
bridges to:
public func CreateStringByAddingTwoStrings(_ s1: CFString!, _ s2: CFString!) -> Unmanaged<CFString>!
We can improve the bridging with the CF_RETURNS_RETAINED annotation:
CFStringRef CreateStringByAddingTwoStrings(CFStringRef s1, CFStringRef s2) CF_RETURNS_RETAINED;
now bridges to:
public func CreateStringByAddingTwoStrings(_ s1: CFString!, _ s2: CFString!) -> CFString!
Alternatively, we can use the CF_IMPLICIT_BRIDGING_ENABLED macro on this function:
CF_IMPLICIT_BRIDGING_ENABLED
CFStringRef CreateStringByAddingTwoStrings(CFStringRef s1, CFStringRef s2);
CF_IMPLICIT_BRIDGING_DISABLED
What does not seem to be documented is how to improve bridging when function pointer return values should be annotated. For example:
typedef CFStringRef (*CreateStringByAddingTwoStrings)(CFStringRef, CFStringRef);
bridges to:
public typealias CreateStringByAddingTwoStrings = @convention(c) (CFString?, CFString?) -> Unmanaged<CFString>?
Is there any way to annotate the function pointer to properly communicate the memory management semantics? Adding the CF_RETURNS_RETAINED and CF_IMPLICIT_BRIDGING_ENABLED macros do not seem to help.
