CF_RETURNS_RETAINED on function pointer return values?

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 StringByAddingTwoStrings(CFStringRef s1, CFStringRef s2);

bridges to:

public func StringByAddingTwoStrings(_ s1: CFString!, _ s2: CFString!) -> Unmanaged<CFString>!

We can improve the bridging with the CF_RETURNS_RETAINED annotation:

CFStringRef StringByAddingTwoStrings(CFStringRef s1, CFStringRef s2) CF_RETURNS_RETAINED;

now bridges to:

public func StringByAddingTwoStrings(_ s1: CFString!, _ s2: CFString!) -> CFString!

What does not seem to be documented is how to improve bridging when function pointer return values should be annotated. For example:

typedef CFStringRef (*StringByAddingTwoStrings)(CFStringRef, CFStringRef);

bridges to:

public typealias StringByAddingTwoStrings = @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 to the function pointer does not seem to help.