[Concurrency] Global actors #34201
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[Concurrency] Global actors #34201
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The globalActor attribute indicates that a particular type describes a global actor. Global actors allow the notion of actor state isolation to be spread across various declarations throughout a program, rather than being centered around a single actor class. There are useful primarily for existing notions such as "main thread" or subsystems accessed through global/singleton state.
Global actor types can be used as attributes on various kinds of declarations to indicate that those declarations are part of the isolated state of that global actor. Allow such annotation and perform basic correctness checks.
Extend the actor isolation checking rules to account for global actors. For example, a function annotated with a given global actor can invoke synchronous methods from the same global actor, but not from a different global actor or a particular actor instance. Similarly, a method of an (instance) actor that is annotated with a global actor attribute is not part of the (instance) actor and, therefore, cannot operate on its actor-isolated state.
The "privileged" cases for actor instance and global actor isolation covered the case where the entity itself is within the particular actor but can be freely used from outside the actor, e.g., because it is asynchronous or an asynchronous handler. This is the wrong modeling for the problem, because it's only the first part of that---what the isolation of the particular entity is---that is needed for most clients. There will be a different abstraction for that.
DougGregor
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Rename IsolationRestriction to ActorIsolationRestriction. This type governs the *use* of a potentially actor-isolated entity, which includes the notion that (e.g.) an asychronous function can be used from any context even when it is actor-isolated. Use this for @objc checking and protocol conformance checking, in addition to its original use for enforcing actor isolation in statements and expressions.
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@swift-ci please smoke test |
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@swift-ci please test source compatibility |
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This was getting big enough, so I went ahead and merged it. The global actor propagation rules, as well as checking for consistency of witnesses and requirements, is still to come. |
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Global actor types can be used as attributes on various kinds of declarations to indicate that those declarations are part of the isolated state of that global actor. This pull request introduces support for global actors into the concurrency model:
@globalActorattribute indicates a type that can act as a global actor. Such types must have astaticproperty namedsharedthat provides a globally shared actor instance identified by the type. Add the attribute and check for this condition.Implements most of rdar://70101562.