[swift-evolution] [Concurrency] async/await + actors

[Brent Royal-Gordon]

> On Aug 19, 2017, at 7:41 AM, Matthew Johnson <matthew at anandabits.com> wrote:
> 
> Regardless of which approach we take, it feels like something that needs to be implicit for structs and enums where value semantics is trivially provable by way of transitivity. When that is not the case we could require an explicit `value` or `nonvalue` annotation (specific keywords subject to bikeshedding of course).

There is no such thing as "trivially provable by way of transitivity". This type is comprised of only value types, and yet it has reference semantics:

	struct EntryRef {
		private var index: Int
		
		var entry: Entry {
			get { return entries[index] }
			set { entries[index] = newValue }
		}
	}

This type is comprised of only reference types, and yet it has value semantics:

	struct OpaqueToken: Equatable {
		class Token {}
		private let token: Token
		
		static func == (lhs: OpaqueToken, rhs: OpaqueToken) -> Bool {
			return lhs.token === rhs.token
		}
	}

I think it's better to have types explicitly declare that they have value semantics if they want to make that promise, and otherwise not have the compiler make any assumptions either way. Safety features should not be *guessing* that your code is safe. If you can somehow *prove* it safe, go ahead—but I don't see how that can work without a lot of manual annotations on bridged code.

-- 
Brent Royal-Gordon
Architechies

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