Why is this variable first set to null then the new value

[taylorswift]

@Luminaron

My best guess is that this avoids Copy-On-Write by keeping the number of references to the underlying collection type to only one. However, I couldn’t find any material that explicitly confirms my guess.

i agree with your guess, that pattern was a ritual from the Swift 5.4 days and you still see it in a lot of code bases today. in theory the consume/consuming keywords are supposed to replace this, but there are some bugs to be ironed out with those, so the ritual lives on.

these threads also discusses a similar topic: Appending to an Array stored in an enum case payload: O(1) or O(n)? - #3 by lukasa

Expanding the capabilities of enum cases

One of the biggest issues I have had with enum cases is ensuring in-place mutation. You have to spell things in a very particular way to avoid making extra copies: struct TreeNode { enum Storage { case internal(children: [TreeNode]) case leaf(values: [Int]) } var _storage: Storage mutating func insert(value: Int) { switch _storage { // The optimizer recognizes this magic incantation and eliminates redundant copies. case .leaf(var values) { // 1: capture payload …