[swift-evolution] RFC: Preventing Retain Cycles (Memory Ownership Model)

[Haravikk]

This is very interesting, but it'll probably be a little while before I can fully get my head around it.

One query though; an example mentioned is adding @owns(Element) to the Generator protocol, but associated types feel a little different to me; you mention using the strong keyword as an alternative, but I wonder if that might actually be a better fit for the associated type case, while @owns works elsewhere, or perhaps strong could even be made a shorthand for simpler cases? Just thinking out loud I guess, it would be nice to hear the thought process around associated types; most (possibly all I don't remember) cases in which I've used associated types it has been for strong references, so it seems like that should be as easy as possible to do under the new opt-in approach.

But yeah, I generally agree that this ought to be opt-in, as it's an area I do stumble upon quite a bit myself (in many languages).

> On 30 Jul 2016, at 02:42, Andrew Bennett via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I'd like an opt-in way to verify and prevent unintentional strong references in Swift.
> 
> This can be used to verify ownership structures, and ultimately avoid retain cycles.
> 
> Read a draft proposal here:
> https://github.com/therealbnut/swift-evolution/blob/therealbnut-explicit-ownership/proposals/NNNN-explicit-ownership-type-attribute.md <https://github.com/therealbnut/swift-evolution/blob/therealbnut-explicit-ownership/proposals/NNNN-explicit-ownership-type-attribute.md>
> 
> TL;DR:
> 
> If you have any questions please read the proposal before asking here.
> 
> It's an opt-in attribute that defines a whitelist of types something can own. For example:
> 
> @owns(TypeA, TypeB) struct TypeC { ... }
> 
> I wrote this a few months ago, but we weren't accepting additive proposals. Now we're explicitly looking for something like this:
> 
> Memory ownership model: Adding an (opt-in) Cyclone/Rust inspired memory ownership model to Swift is highly desired by systems programmers and folks who want predictable and deterministic performance (for example, in real time audio processing code).  More pertinent to the goals of Swift 4, this feature is important because it fundamentally shapes the ABI.  It informs code generation for “inout", how low-level “addressors” work in the ABI, impacts the Swift runtime, and will have a significant impact on the type system and name mangling. 
> 
>  - Chris <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025676.html>
> 
> ----
> 
> Here's a link to the version of the proposal <https://github.com/therealbnut/swift-evolution/commit/6ab167825d802c7826804e1957eb515d3009743a> when I sent this email.
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