[WIP][DNM] add is identical methods to concrete types

[vanvoorden]

Many types in Standard Library are “copy-on-write” data structures. These types present as value types, but can leverage a reference to some shared state to optimize for performance. When we copy this value we copy a reference to shared storage. If we perform a mutation on a copy we can preserve value semantics by copying the storage reference to a unique value before we write our mutation: we “copy” on “write”.

This means that many types in Standard Library already have some private reference that can be checked in constant-time to determine if two values are identical. Because these types copy before writing, two values that are identical by their shared storage must be equal by value.

[vanvoorden]

@lorentey Here is my new attempt at a pitch only for identical functions on concrete types. My plan is to propose the update on Equatable in a separate pitch. Want to take a look and let me know what you think? How do you feel about being named as a coauthor? I expect to have enough time to complete all the implementations and unit tests myself and I do not need you to commit to allocating any engineering time.

I was planning to keep working on this over the next week and have it live on the forums 2025-06-16 after WWDC.

[lorentey]

This looks good! Thank you for pushing these. 👍🏼

We may need to subset out the Foundation APIs and run them through the evolution process specific to that project. Cc @parkera

[vanvoorden]

We may need to subset out the Foundation APIs and run them through the evolution process specific to that project.

Ahh… that makes sense. Keeping them together in one place would be nice… but I'm not opposed to breaking them apart.

[vanvoorden]

Does anyone have a very strong opinion on adding availability to a symbol on a type and the extension that declares that symbol?

[1] Like this:

extension String {
  @available(SwiftStdlib 6.3, *)
  public func isIdentical(to other: Self) -> Bool {
    self._guts.rawBits == other._guts.rawBits
  }
}

[2] Or this:

@available(SwiftStdlib 6.3, *)
extension String {
  @available(SwiftStdlib 6.3, *)
  public func isIdentical(to other: Self) -> Bool {
    self._guts.rawBits == other._guts.rawBits
  }
}

[3] Or this:

@available(SwiftStdlib 6.3, *)
extension String {
  public func isIdentical(to other: Self) -> Bool {
    self._guts.rawBits == other._guts.rawBits
  }
}

I'm generally seeing number [2] in std lib… but it also looks like I found [1] and [3] here and there… any strong opinions or consensus about that?