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
When overriding a method or witnessing a requirement that comes from
Objective-C and has no actor annotation, allow the overriding method
or witness to specify a global actor (any global actor, but probably
almost always the main actor) without triggering the actor-isolation
errors one would normally get if both entities were written in Swift.
This opens up a hole in actor-isolation checking, because nothing
guarantees that code won't call these methods through the superclass
or protocol from the wrong actor. On the other hand, it's a very
convenient hole, because it allows us to state when we "know" that an
Objective-C framework will only call a method on (e.g.) the main
actor, and make that known to Swift to be enforced everywhere else in
Swift. If this is a good idea, it's plausible to introduce runtime
assertions of some form to tell the user when such an annotation is
wrong.
Review over a large number of APIs has found that most of the time, the
result type of an Objective-C completion handler method that becomes
"async throws" should be optional. Change the default behavior to
match this, and replace _Nullable_on_error with _Nullable_result to
capture the case where the result should be optional.
When concurrency is enabled, the same Objective-C method on a protocol can
be imported as two different protocol requirements, both of which have the
same selector. When performing conformance checking, only treat a
given @objc protocol requirement as "unsatisfied" by a conformance if
none of the requirements with the same Objective-C selector (+
instance/class designation) are satisfied.
