Ownership pattern matching questions

I'm exploring ownership accessors by re-implementing Rust types in Swift. public enum Cow<B : ToOwned & ~Copyable> : Borrow, Clone, ~Escapable { /// Borrowed data. case borrowed(Ref<B>) /// Owned data. case owned(B.Owned) public var value: B { borrow { switch self { case let .borrowed(b): b.value case let .owned(b): b.value } } mutate { switch self { case let .borrowed(b): self = .owned(b.value.toOwned()) switch self { case .owned(var o): return &o.valu...