no-escape.swift

parallelMap.swift

extension Array {
    func parallelMap<U>(transform: Element -> U) -> [U] {
        var result: [U?] = Array<U?>(count: count, repeatedValue: nil)
        dispatch_apply(count, dispatch_get_global_queue(0, 0)) { 
            result[$0] = transform(self[$0])
        }
        return result.map { $0! }
    }
}

parallelMapUnsafeBitcast.swift

extension Array {
    func parallelMap<U>(@noescape transform: Element -> U) -> [U] {
        typealias Transform = Element -> U
        let transform1 = unsafeBitCast(transform, Transform.self)
        var result: [U?] = Array<U?>(count: count, repeatedValue: nil)
        dispatch_apply(count, dispatch_get_global_queue(0, 0)) { x in
            result[x] = transform1(self[x])
        }
        return result.map { $0! }
    }
}

You could use return result.flatMap { $0 } to unwrap optionals and avoid using the bang.