Adds a folder to the Finder sidebar.

addToSidebar.swift

import Foundation

func addToSidebar(path: String) {
    guard
        let listType = kLSSharedFileListFavoriteItems?.takeUnretainedValue(),
        let itemList = LSSharedFileListCreate(nil, listType, nil)?.takeUnretainedValue(),
        let items = LSSharedFileListCopySnapshot(itemList, nil)?.takeUnretainedValue()
    else {
        return
    }

    let lastIndex = CFArrayGetCount(items) - 1
    let lastItemPointer = CFArrayGetValueAtIndex(items, lastIndex)
    let lastItem = unsafeBitCast(lastItemPointer, to: LSSharedFileListItem.self)
    let url = NSURL(fileURLWithPath: path, isDirectory: true)
    LSSharedFileListInsertItemURL(itemList, lastItem, nil, nil, url, nil, nil)
}

Most of these functions are deprecated and display this warning:

'LSSharedFileListCopySnapshot' was deprecated in OS X 10.11: This functionality is no longer supported on OS X.

So this is no longer a good idea.