https://twitter.com/mattt/status/1055582012801613824

BoundedSequence.swift

public struct BoundedSequence<Base>: Sequence, IteratorProtocol where Base: Sequence {
    public struct Boundary: Equatable {
        public let isStart: Bool
        public let isEnd: Bool
    }
    
    private var _iterator: Base.Iterator
    private var _previous: Base.Element?
    private var _current: Base.Element?
    private var _next: Base.Element?

    public init(_ base: Base) {
        _iterator = base.makeIterator()
        _next = _iterator.next()
    }
    
    public mutating func next() -> (Boundary, Base.Element)? {
        (_previous, _current, _next) = (_current, _next, _iterator.next())
        
        switch (_previous, _current, _next) {
        case let (nil, element?, nil):
            return (Boundary(isStart: true, isEnd: true), element)
        case let (nil, element?, _):
            return (Boundary(isStart: true, isEnd: false), element)
        case let (_, element?, nil):
            return (Boundary(isStart: false, isEnd: true), element)
        case let (_, element?, _):
            return (Boundary(isStart: false, isEnd: false), element)
        default:
            return nil
        }
    }
}

extension Sequence {
    func bounded() -> BoundedSequence<Self> {
        return BoundedSequence(self)
    }
}

main.swift

let array = [1, 2, 3, 4, 5]
for (boundary, element) in array.bounded() {
    if boundary.isStart {
        print("START")
    }
    
    print(element)
    
    if boundary.isEnd {
        print("END")
    }
}

/*
Prints:
START
1
2
3
4
5
END
*/