title: trouble with NSFastEnumeration protocol and ARC
body: <p>I'm having trouble implementing the NSFastEnumeration protocol while migrating Objective-C code to ARC.
  Could someone tell me, how to get rid of the following warnig (see code snippet)? Thanks in advance.</p>
  
  <pre><code>// I changed it due to ARC, was before
  // - (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*) state objects: (id*) stackbuf count: (NSUInteger) len
  - (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*) state objects: (__unsafe_unretained id *) stackbuf count: (NSUInteger) len
  {
      ... 
      *stackbuf = [[ZBarSymbol alloc] initWithSymbol: sym]; //Warning: Assigning retained object to unsafe_unretained variable; object will be released after assignment
      ... 
  }
  
  - (id) initWithSymbol: (const zbar_symbol_t*) sym
  {
      if(self = [super init]) {
          ... 
      }
      return(self);
  }
  </code></pre>