Finding a way to allow this in-reality-safe isolation violation

[vanvoorden (Rick van Voorden)]

@Nickolas_Pohilets

But under the hood, this requires compiler to distinguish between isolation of the variable (nonisolated) and isolation of the type (isolated to self).

Something along these lines I've been looking for is some kind of strategy to enforce "dimensions" of sendability… where I can specify that a variable is "unsafe" WRT the access of that variable but also "safe" WRT the value itself.

How to make a copy-on-write struct Sendable and thread-safe?

This actually got me thinking… I have a similar pattern for my own CoW types… but I'm seeing a (potential) missing piece here WRT Sendable. Suppose you start with your example and then try to make the CoW Sendable. Here is the error: public struct MyCopyOnWriteStruct : Sendable { public init() {} public var value: Int { get { storage.value } set { copyStorageIfNecessary() storage.value = newValue } } private var storage = Storage() // `- …