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I'm attempting to call SecItemCopyMatching in my keychain utility class in order to get data out of the keychain, yet I'm running into a problem with getting the result argument, UnsafeMutablePointer<CFTypeRef?> .

The original statement (in Swift 2, before migrating to Swift 3) was

// query is a dictionary of [String : AnyObject]
var result: Data?
let status = withUnsafeMutablePointer(to: &result) {
    SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
But in Swift 3, you are now required to call .withMemoryRebound in order to view memory. Based on what Xcode tells you to do, I tried this
var result: Data?
let status = withUnsafeMutablePointer(to: &result){
    $0.withMemoryRebound(to: Data.self, capacity: 1){
        SecItemCopyMatching(query as CFDictionary, UnsafePointer($0))
Yet doing this, I get an error
  
Cannot convert value of type 'UnsafePointer<_>' to expected argument type 'UnsafeMutablePointer<CFTypeRef?>?'
So, I tried using CFTypeRef instead of Data
var result: CFTypeRef?
let status = withUnsafeMutablePointer(to: &result){
    $0.withMemoryRebound(to: CFTypeRef.self, capacity: 1){
        SecItemCopyMatching(query as CFDictionary, UnsafePointer($0))
Replacing UnsafePointer($0) with simply $0 results in the same error message.
How can I get an UnsafeMutablePointer<CFTypeRef?> for getting data from keychain?
                Please would you post how you're creating your query. I was previously doing just this and casting from a swift Dictionary to CFDictionary, but that no longer works for me in Swift 3. If I could see just a little more of the code I can apply this solution.
– Tim Spears
                Sep 19, 2016 at 22:37
                @TimSpears the problem with your code would probably be that the dictionary has to be a [String : AnyObject] - the auto-migrator (for me) changed this to [String : Any]. I updated my answer to the question to include the code I use
– Jojodmo
                Sep 19, 2016 at 22:53
In your case, you do not need to use withMemoryRebound or withUnsafeMutablePointer(to:).
Instead, you can just use
var result: AnyObject?
let status = SecItemCopyMatching(query as CFDictionary, &result)
if status == noErr, let data = result as? Data {
    //use data...
Generally, when you need to pass an UnsafeMutablePointer<T> to a function, declare a variable of type T and pass it as an inout argument &variable. In your case, T is CFTypeRef?, and in Swift 3, CFTypeRef is just a typealias of AnyObject.
Even in Swift 2.2, you did not need to use withUnsafeMutablePointer
var result: AnyObject?  
let status = SecItemCopyMatching(query, &result)  
                Your solution is working for me. My result variable (defined using var, not let) is a SecKey, which is easily cast to CFTypeRef. Using just the ampersand results in the error Cannot pass immutable value of type 'CFTypeRef?' as inout argument
– Matthew Barker
                Jun 16, 2017 at 15:27
The error message is somewhat misleading - the actual problem is that result has to be an AnyObject? - withMemoryRebound doesn't need to be used.
var result: AnyObject?
let status = withUnsafeMutablePointer(to: &result){
    SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
Works as expected and gets the correct result from keychain - it just needs to be casted to Data. In fact, withUnsafeMutablePointer doesn't even have to be used.
So, my new code is 
var query: [String : AnyObject] = [:]
//set up the query
var result: AnyObject?
let status = SecItemCopyMatching(query as CFDictionary, &result)
var data: Data?
if status == noErr{
    data = result as? Data
                I tried your solution and the one by @OOPer, and neither seem to work for me, see my comment there. Any idea what's up?
– Matthew Barker
                Jun 16, 2017 at 15:30
        
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