call_callback.swift

// Given an imported C function, that uses a context pointer and a C callback function:
//
//     func for_each(_ x: X, context: UnsafeMutableRawPointer!, callback: @convention(c) (UnsafeMutableRawPointer?, Element) -> Void)
//
// When is it safe to use this pattern?

// Example with two captured variables.
typealias Context = (Type1, Type2)
var context = (capture1, capture2)
for_each(x, context: &context) { context, element in
    let (capture1, capture2) = context!.assumingMemoryBound(to: Context.self).pointee
    // Perform operations with captures and `element`…
}