For some things like NSMutableString (and by extension, NSString), calling Id::retain on &T cannot be safe, since there could be a mutable reference to it somewhere else in the program (and the lifetime is erased by calling Id::retain, that's kinda the whole point).

The solution there is to use NSCopying::copy to instead store a copy of your string (which is cheap when given &NSString, and correct when given &NSMutableString).

But for types like NSView and NSWindow, this is overly restrictive, since we know there can never be a mutable reference to these (they always use interior mutability); so perhaps we can make a trait for expressing this? (This comes up especially often in user-defined init methods).