ArrayScan.swift

extension Array {
    
    func scan(forEachNeighbors action: (Element, Element) -> Void) {
        guard count >= 2 else {
            return
        }
        var indexA = 0
        var indexB = 1
        
        while indexB <= count - 1 {
            let elementA = self[indexA]
            let elementB = self[indexB]
            action(elementA, elementB)
            indexA += 1
            indexB += 1
        }
    }
    
}

And of course this is more in the spirit of your scan() function:

[1,2,3,4,5,6,7].adjacent_pairs().forEach { i in
    print(i)
}

I gotta ask: Snake case in Swift? Surely you need more boundaries in your life.