Hi everyone! I have a pitch for a minor API addition to update AttributedString APIs for the new SendableMetatype protocol in Swift 6.2. Let me know if you have any comments/suggestions/concerns!
AttributedString & SendableMetatype
- Proposal: SF-NNNN
- Authors: Jeremy Schonfeld
- Review Manager: TBD
- Status: Pitch
Introduction/Motivation
SE-0470 Global-actor isolated conformances introduced the concept of a conformance isolated to a global actor. This in turn introduced the ability for metatypes to be non-Sendable as a generic metatype's conformance to a protocol may be isolated to a global actor. Since metatypes are not guaranteed to be Sendable, generic functions that pass metatypes across isolation boundaries now need to specify that they require Sendable metatypes (i.e. the conformance to the specified protocol must be nonisolated). One common API in Foundation that needs to pass metatypes across actor isolations is AttributedString via its AttributedStringKey and AttributeScope protocols. AttributedString contains a variety of APIs that accept generic metatypes constrained to these protocols. In many cases, these metatypes need to be passed across actor isolations (such as storing the contents of a scope in a cache, storing attribute keys in Sendable AttributedStrings and related types, etc.). We need to update Foundation's APIs to ensure that these assumptions around Sendable AttributedStringKey and AttributeScope metatypes still hold.
Proposed solution and example
Since Foundation needs to pass these metatypes across isolation boundaries and there is no benefit to providing an isolated conformance to AttributeScope/AttributedStringKey as the types themselves should never access global state and are never initialized, I propose annotating AttributedStringKey and AttributeScope as conforming to SendableMetatype to prevent isolated conformances to these protocols. Developers that do not bind their keys/scopes to global actors will see no impact, but developers that declare isolated conformances to either of these protocols will now see a compilation warning/error:
struct MyAttributeKey : @MainActor AttributedStringKey {
typealias Value = Int
static let name = "MyFramework.MyAttributeKey"
}
var myString = AttributedString()
myString[MyAttributeKey.self] = 2 // Main actor-isolated conformance of 'MyAttributeKey' to 'AttributedStringKey' cannot satisfy conformance requirement for a 'Sendable' type parameter
Detailed design
The SendableMetatype conformance will be added to the pre-existing AttributedStringKey and AttributeScope protocols:
@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
public protocol AttributedStringKey : SendableMetatype {
// ...
}
@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
public protocol AttributeScope : DecodingConfigurationProviding, EncodingConfigurationProviding, SendableMetatype {
// ...
}
Source compatibility
Isolated conformances are new to Swift 6.2 and therefore all existing code (which must use nonisolated conformances) will not be impacted. Any code that has already adopted an isolated conformance here in Swift 6.2 will now see a warning (or error in Swift 6 mode) when providing these key/scope types to AttributedString APIs.
Since SendableMetatype is a marker protocol, this change does not impact ABI and does not require availability adjustments.
Implications on adoption
Modules that define conformances to AttributedStringKey or AttributeScope will not be able to define them as isolated conformances. The conformances must be nonisolated.
Future directions
None.
Alternatives considered
Adding a SendableMetatype annotation to all applicable AttributedString APIs
Instead, we could add the conformance to every AttributedString API that accepts a key or scope in case these keys/scope types are used in other APIs that do not require a nonisolated conformance. I chose to not pursue this route as it required more complex changes, it has mostly the same effect, and I don't expect any client to realistically require an isolated conformance to these protocols.
Check out the proposal document on this swift-foundation repo PR.