Anything can be Sliceable

SubSliceView.swift

public struct SubSliceView<C: CollectionType>: Sliceable {
    let collection: C
    let bounds: Range<C.Index>

    public var startIndex: C.Index { return bounds.startIndex }
    public var endIndex: C.Index { return bounds.endIndex }
    
    public subscript(idx: C.Index) -> C.Generator.Element
    { return collection[idx] }
    
    typealias SubSlice = SubSliceView<C>
    public subscript(bounds: Range<C.Index>) -> SubSliceView<C> {
        return SubSliceView(collection: collection, bounds: bounds)
    }
}

extension AnyRandomAccessCollection: Sliceable {
    public subscript(bounds: Range<Index>) -> SubSliceView<AnyRandomAccessCollection> {
        return SubSliceView(collection: self, bounds: bounds)
    }
}

let a = [1,2,3]
let r = AnyRandomAccessCollection(a)
print("")
print(dropFirst(dropFirst(r)).first)

// now in theory this ought to work? but compiler is segfaulting...
//extension CollectionType {
//    public subscript(bounds: Range<Index>) -> SubSliceView<Self> {
//        return SubSliceView(collection: self, bounds: bounds)
//    }
//}
//
// then all you'd need to do is tag each collection
//extension UnsafeBufferPointer: Sliceable { }
//extension Dictionary: Sliceable { }
//extension AnyRandomAccessIndex: Sliceable { }

// or maybe some day extension CollectionType: Sliceable { }