willSet causing copy-on-write types to copy on mutations?

[xwu (Xiaodi Wu)]

SE-0268 (Review #2) — Review didSet Semantics

There are two ways to give willSet semantics for a non-instantaneous modification (e.g. calling a mutating method on the property): You can do what Swift does today, which is to force the modification to happen to a temporary copy and then call willSet immediately before writing back. You can call willSet before the modification begins. The second makes more sense to me abstractly, but it's totally incompatible with willSet getting passed the new value, and it's a radically different point in…