How can I use String substring in Swift 4? ‘substring(to:)’ is deprecated: Please use String slicing subscript with a ‘partial range from’ operator

I have the following simple code written in Swift 3:

let str = "Hello, playground"
let index = str.index(of: ",")!
let newStr = str.substring(to: index)

From Xcode 9 beta 5, I get the following warning:

‘substring(to:)‘ is deprecated: Please use String slicing subscript with a ‘partial range from’ operator.

How can this slicing subscript with partial range from be used in Swift 4?