a VarDecl or Expr.
This generalization exposed a bug where distributed actor isolation checking
was skipped in some cases, including for the isolated call in `whenLocal`.
The `whenLocal` implementation violated distributed actor isolation because
despite the `__isLocal` dynamic check, the `self` value passed to the `body`
function argument is still not statically local. To workaround this, I
applied the `_local` modifier explicitly to `self` before the call, which
also necessitated allowing `_local` to be written explicitly in the Distributed
library.
When an actual instance of a distributed actor is on the local node, it is
has the capabilities of `Actor`. This isn't expressible directly in the type
system, because not all `DistributedActor`s are `Actor`s, nor is the
opposite true.
Instead, provide an API `DistributedActor.asLocalActor` that can only
be executed when the distributed actor is known to be local (because
this API is not itself `distributed`), and produces an existential
`any Actor` referencing that actor. The resulting existential value
carries with it a special witness table that adapts any type
conforming to the DistributedActor protocol into a type that conforms
to the Actor protocol. It is "as if" one had written something like this:
extension DistributedActor: Actor { }
which, of course, is not permitted in the language. Nonetheless, we
lovingly craft such a witness table:
* The "type" being extended is represented as an extension context,
rather than as a type context. This hasn't been done before, all Swift
runtimes support it uniformly.
* A special witness is provided in the Distributed library to implement
the `Actor.unownedExecutor` operation. This witness back-deploys to the
Swift version were distributed actors were introduced (5.7). On Swift
5.9 runtimes (and newer), it will use
`DistributedActor.unownedExecutor` to support custom executors.
* The conformance of `Self: DistributedActor` is represented as a
conditional requirement, which gets satisfied by the witness table
that makes the type a `DistributedActor`. This makes the special
witness work.
* The witness table is *not* visible via any of the normal runtime
lookup tables, because doing so would allow any
`DistributedActor`-conforming type to conform to `Actor`, which would
break the safety model.
* The witness table is emitted on demand in any client that needs it.
In back-deployment configurations, there may be several witness tables
for the same concrete distributed actor conforming to `Actor`.
However, this duplication can only be observed under fairly extreme
circumstances (where one is opening the returned existential and
instantiating generic types with the distributed actor type as an
`Actor`, then performing dynamic type equivalence checks), and will
not be present with a new Swift runtime.
All of these tricks together mean that we need no runtime changes, and
`asLocalActor` back-deploys as far as distributed actors, allowing it's
use in `#isolation` and the async for...in loop.
These metatypes are a gateway to more incorrect
uses of these noncopyable values because we don't
yet have the corresponding runtime support yet.
The other use cases of using metatypes of
noncopyable types in generics is not high enough to
warrant people using them yet.
resolves rdar://106452518
* [Executors][Distributed] custom executors for distributed actor
* harden ordering guarantees of synthesised fields
* the issue was that a non-default actor must implement the is remote check differently
* NonDefaultDistributedActor to complete support and remote flag handling
* invoke nonDefaultDistributedActorInitialize when necessary in SILGen
* refactor inline assertion into method
* cleanup
* [Executors][Distributed] Update module version for NonDefaultDistributedActor
* Minor docs cleanup
* we solved those fixme's
* add mangling test for non-def-dist-actor
