There are a lot mismatches.

For example, I reported a bug about CFRunLoopMode, where the original C-based CF used a comparison of pointer address to determine the equality of two run mode names. The original C version actually compared using the identity rather than equality as the commonModes was defined as a static CFStringRef (CONST_STRING_DECL(kCFRunLoopCommonModes, "kCFRunLoopCommonModes")). Of course this is an optimization for speed on Mac but this is, unfortunately, not supported in Swift and its replication of CF, because in Swift it has to be a public static String. As for today you cannot use commonModes in Linux.

Unfortunately Apple still considers (Core)Foundation to be outside the auspices of Swift Evolution, which makes getting issues like these fixed much harder.

Yeah, but a good chunk of it throws Not Implemented exceptions at runtime too.

That’s why I qualified my statement with “substantial portion”.