Make Worker.Output Sendable

[g-mark]

Adds Sendable conformance as a constraint on Workflow.Output.

Checklist

• Unit Tests
• UI Tests
• Snapshot Tests (iOS only)
• I have made corresponding changes to the documentation

[CLAassistant]

All committers have signed the CLA.

[mjohnson12]

Will this break register? I would be surprised if all Workers Outputs were Sendable.

[g-mark]

Will this break register? I would be surprised if all Workers Outputs were Sendable.

I ran a local test a couple of weeks ago with no issues.

• I'll set up an integration branch.

[g-mark]

Integration passed 👍

[jamieQ]

before we proceed further with this effort, i think we should have a bit of discussion around the motivation and future direction of this type of change to better expose things to the concurrency system. two thoughts that come to mind:

1. imposing a Sendable conformance on a type is a fairly substantial ask in general since it's asserting that type must be able to be passed across arbitrary isolation domains. is this the right approach?
2. is this potentially a source-break for existing clients, so would necessitate a major semver bump? (granted register is probably the primary one)

[g-mark]

before we proceed further with this effort, i think we should have a bit of discussion around the motivation and future direction of this type of change to better expose things to the concurrency system.

That makes sense. For background, this came up because of a warning currently in WorkflowConcurrency while calling await send(output).

1. imposing a Sendable conformance on a type is a fairly substantial ask in general since it's asserting that type must be able to be passed across arbitrary isolation domains. is this the right approach?

This is probably the right approach, given what Sendable types are. Although I struggle to think of another approach, I would love to hear all other ideas.

2. is this potentially a source-break for existing clients, so would necessitate a major semver bump? (granted register is probably the primary one)

Yes it does have the potential to break clients if their Output types do not conform to Sendable, so I agree a major bump would be appropriate (side note: Alamofire changed a bunch of things to Sendable in a minor bump, and it broke some of our code).