- Introduce an UnownedSerialExecutor type into the concurrency library.
- Create a SerialExecutor protocol which allows an executor type to
change how it executes jobs.
- Add an unownedExecutor requirement to the Actor protocol.
- Change the ABI for ExecutorRef so that it stores a SerialExecutor
witness table pointer in the implementation field. This effectively
makes ExecutorRef an `unowned(unsafe) SerialExecutor`, except that
default actors are represented without a witness table pointer (just
a bit-pattern).
- Synthesize the unownedExecutor method for default actors (i.e. actors
that don't provide an unownedExecutor property).
- Make synthesized unownedExecutor properties `final`, and give them
a semantics attribute specifying that they're for default actors.
- Split `Builtin.buildSerialExecutorRef` into a few more precise
builtins. We're not using the main-actor one yet, though.
Pitch thread:
https://forums.swift.org/t/support-custom-executors-in-swift-concurrency/44425
This patch updates the `actor class` spelling to `actor` in almost all
of the tests. There are places where I verify that we sanely handle
`actor` as an attribute though. These include:
- test/decl/class/actor/basic.swift
- test/decl/protocol/special/Actor.swift
- test/SourceKit/CursorInfo/cursor_info_concurrency.swift
- test/attr/attr_objc_async.swift
- test/ModuleInterface/actor_protocol.swift
