[swift-users] Using withUnsafePointer on char arrays within C structs

[Russell Finn]

Thanks to Joe and Quinn for their answers. I have a related followup — a
co-worker learning Swift wrote the following function:

    func allZerosUUID() -> String {
        var allZeros = [UInt8](repeating: 0, count: 32)
        return withUnsafePointer(to: &allZeros) { zerosPtr in
            return NSUUID(uuidBytes: zerosPtr).uuidString
        }
    }

but was puzzled that Xcode 8.2.1 gave an error "Cannot convert value of
type 'UnsafePointer<_>' to expected argument type 'UnsafePointer<UInt8>!'"
on the line with the NSUUID initializer.  Their expectation was that
`zerosPtr` would be of type `UnsafePointer<UInt8>` because `allZeros` is of
type `[UInt8]`.

They discovered that they could work around this by adding a call to
`withMemoryRebound`:

    func allZerosUUID() -> String {
        var allZeros = [UInt8](repeating: 0, count: 32)
        return withUnsafePointer(to: &allZeros) { zerosPtr in
            zerosPtr.withMemoryRebound(to: UInt8.self, capacity:
allZeros.count) { zerosPtr in
                return NSUUID(uuidBytes: zerosPtr).uuidString
            }
        }
    }

but felt that this should be unnecessary. Perhaps I'm missing something
simple, but I was unable to explain this compiler behavior; can anyone on
the list do so?

(Yes, I did point out that they could pass `&allZeros` directly to
`NSUUID(uuidBytes:)`.)

Thanks — Russell
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