Should there be BidirectionalCollection.dropLast(while:)?

Using Swift
2

Avoid calling count on a non-RandomAccess collection, by leveraging the fact that a slice shares indices with its base collection:

return self[..<suffix.startIndex]

Or, using only standard library methods:

let i = try lastIndex{ try !predicate($0) }

guard let j = i else {
  return self[..<startIndex]
}

return self[...j]