CastToStructInSwift.swift

//In C:
//==============
 
// The C Function we need to call:
//
// OSStatus someFunctionForData (
//    int *outType,
//    const void **outData
// )

// Will hold the type
UInt32 type = 0;
// The buffer for our important data
const void *data = NULL;
 
// Retrieve the data and its type
OSStatus status = someFunctionForData(&type, &data);
 
// Check for the desired type
if (type == kRealData_myType) {
    // Cast the buffer to the struct for use
    RealDataType *myData = (RealDataType *)data;
}

 
 
//In Swift:
//==============

// The Swift func we need to call
//     
// func someFunctionForData(
//     outType: UnsafePointer<Int>, 
//     outData: UnsafePointer<ConstUnsafePointer<()>>
// ) -> OSStatus
 
// Will hold the type
var type:UInt32 = 0
    
// The buffer for our important data. 
// The ConstUnsafePointer is for data we don't know about ahead of time.
var data = ConstUnsafePointer<()>()

// The status var
var status = OSStatus(noErr)    
 
// Retrieve the data and its type
status = someFunctionForData(&type, &data)
  
// Check for the desired type
if type == kRealData_myType {
    // This is equivalent to the C cast above. It accesses the
    // raw memory at the pointer address. The var does not need
    // a specified type due to type inference, but can have one.
    var myData = UnsafePointer<RealDataType>(data).memory
}

This no longer works in Xcode 6 beta 3. I'm working on the fix.

This gist has been updated for Xcode 6 beta 3.