If you got this message try __unsafe_unretained. It is only safe, if the objects in the struct are unretained.
Example: If you use OpenFeint with ARC the Class OFBragDelegateStrings says this error in a struct.
typedef struct OFBragDelegateStrings
{
NSString* prepopulatedText;
NSString* originalMessage;
} OFBragDelegateStrings;
to
typedef struct OFBragDelegateStrings
{
__unsafe_unretained NSString* prepopulatedText;
__unsafe_unretained NSString* originalMessage;
} OFBragDelegateStrings;