Add a draft proposal for specifying default isolation via typealias.

Author: hborla

proposals/NNNN-default-isolation-typealias.md

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+# Default actor isolation typealias
+
+* Proposal: [SE-NNNN](NNNN-default-isolation-typealias.md)
+* Authors: [Holly Borla](https://github.com/hborla)
+* Review Manager: TBD
+* Status: **Awaiting review**
+* Vision: [Improving the approachability of data-race safety](/visions/approachable-concurrency.md)
+* Implementation: [swiftlang/swift#80572](https://github.com/swiftlang/swift/pull/80572)
+* Experimental Feature Flag: `DefaultIsolationTypealias`
+* Previous Proposal: [SE-0466: Control default actor isolation inference][SE-0466]
+* Review: ([pitch](https://forums.swift.org/t/pitch-a-typealias-for-per-file-default-actor-isolation/79150))
+
+## Introduction
+
+[SE-0466: Control default actor isolation inference][SE-0466] introduced the ability to specify default actor isolation on a per-module basis. This proposal introduces a new typealias for specifying default actor isolation in individual source files within a module. This allows specific files to opt out of main actor isolation within a main-actor-by-default module, and opt into main actor isolation within a nonisolated-by-default module.
+
+## Motivation
+
+SE-0466 allows code to opt in to being “single-threaded” by default by isolating everything in the module to the main actor. When the programmer really wants concurrency, they can request it explicitly by marking a function or type as `nonisolated`, or they can define it in a module that does not default to main-actor isolation. However, it's very common to group multiple declarations used in concurrent code into one source file or a small set of source files. Instead of choosing between writing `nonisolated` on each individual declaration or splitting those files into a separate module, it's desirable to state that all declarations in those files default to `nonisolated`.
+
+## Proposed solution
+
+This proposal allows writing a private typealias named `DefaultIsolation` to specify the default actor isolation for a file.
+
+An underlying type of `MainActor` specifies that all declarations in the file default to main actor isolated:
+
+```swift
+// main.swift
+
+private typealias DefaultIsolation = MainActor
+
+// Implicitly '@MainActor'
+var global = 0
+
+// Implicitly '@MainActor'
+func main() { ... }
+
+main()
+```
+
+An underlying type of `nonisolated` specifies that all declarations in the file default to `nonisolated`:
+
+```swift
+// Point.swift
+
+private typealias DefaultIsolation = nonisolated
+
+// Implicitly 'nonisolated'
+struct Point {
+  var x: Int
+  var y: Int
+}
+```
+
+## Detailed design
+
+ A typealias named `DefaultIsolation` can specify the actor isolation to use for the source file it's written in under the following conditions:
+
+* The typealias is written at the top-level.
+* The typealias is `private` or `fileprivate`; the `DefaultIsolation` typealias cannot be used to set the default isolation for the entire module, so its access level cannot be `internal` or above.
+* The underlying type is either `MainActor` or `nonisolated`.
+
+ It is not invalid to write a typealias called `DefaultIsolation` that does not meet the above conditions. Any typealias named `DefaultIsolation` that does not meet the above conditions will be skipped when looking up the default isolation for the source file. The compiler will emit a warning for any `DefaultIsolation` typealias that is not considered for default actor isolation along with the reason why:
+
+```swift
+@globalActor
+actor CustomGlobalActor {
+  static let shared = CustomGlobalActor()
+}
+
+private typealias DefaultIsolation = CustomGlobalActor // warning: not used for default actor isolation
+```
+
+To allow writing `nonisolated` as the underlying type of a typealias, this proposal adds a typealias named `nonisolated` to the Concurrency library:
+
+```swift
+public typealias nonisolated = Never
+```
+
+This typealias serves no purpose beyond specifying default actor isolation.
+
+## Source compatibility
+
+Technically source breaking if someone happens to have written a private `DefaultIsolation` typealias with an underlying type of `MainActor`, which will start to infer every declaration in that file as `@MainActor`-isolated after this change. This seems extremely unlikely.
+
+## ABI compatibility
+
+This proposal has no ABI impact on existing code.
+
+## Implications on adoption
+
+This proposal does not change the adoption implications of adding `@MainActor` to a declaration that was previously nonisolated and vice versa. The source and ABI compatibility implications of changing actor isolation are documented in the Swift migration guide's [Library Evolution](https://github.com/apple/swift-migration-guide/blob/29d6e889e3bd43c42fe38a5c3f612141c7cefdf7/Guide.docc/LibraryEvolution.md#main-actor-annotations) article.
+
+## Alternatives considered
+
+Adding a typealias named `nonisolated` to `Never` to the Concurrency library to enable writing it as the underlying type of a typealias is pretty strange; this approach leveraging the fact that `nonisolated` is a contextual keyword, so it's valid to use `nonisolated` as an identifier. This proposal uses a typealias instead of an empty struct or enum type to avoid the complications of having a new type be only available with the Swift 6.2 standard library.
+
+It's extremely valuable to have a consistent way to spell `nonisolated`. Introducing a type that follows standard naming conventions, such as `Nonisolated`, or using an existing type like `Never` is more consistent with recommended style, but overall complicates the concurrency model because it means you need to spell `nonisolated` differently when specifying it per file versus writing it on a declaration. And because the underlying type of this typealias is used to infer actor isolation, it's not used as a type in the same way that other typealiases are.
+
+Another alternative is to introduce a bespoke syntax such as `using MainActor` or `using nonisolated`. This approach preserves a consistent spelling for `nonisolated`, but at the cost of adding new language syntax that deviates from other defaulting rules such as the default literal types and the default actor system types.
+
+Having a `nonisolated` typealias may also allow us to improve the package manifest APIs for specifying default isolation, allowing us to move away from using `nil` to specify `nonisolated`:
+
+```swift
+SwiftSetting.defaultIsolation(nonisolated.self)
+```
+
+We can also pursue allowing bare metatypes without `.self` to allow:
+
+```swift
+SwiftSetting.defaultIsolation(nonisolated)
+SwiftSetting.defaultIsolation(MainActor)
+```
+
+[SE-0466]: /proposals/0466-control-default-actor-isolation.md