Doug Gregor b68f8555b3 [Concurrency] Introduce "unsafe" @Sendable and @MainActor parameters. ...

Introduce the notion of "unsafe" @Sendable parameters, indicated by the
hidden @_unsafeSendable parameter attribute. Closure arguments to such
parameters are treated as @Sendable within code that has already
adopted concurrency, but are otherwise enert, allowing them to be
applied to existing concurrency-related APIs to smooth the transition
path to concurrency.

Additionally, introduce the notion of an "unsafe" @MainActor closure,
for cases where we have determined that the closure will execute on
the main actor but it (also) isn't part of the type system.

Pattern-match uses of the Dispatch library's DispatchQueue to infer
both kinds of "unsafe" as appropriate, especially (e.g.) matching the pattern

  DispatchQueue.main.async { ... }

to treat the closure as unsafe @Sendable and @MainActor, allowing such
existing code to better integrate with concurrency.

Implements rdar://75988966.

2021-03-31 09:40:03 -07:00

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