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Add SPI-based withMainSerialExecutor (#81)
* Add SPI-based `_withMainSerialExecutor` This helper makes it easier to [reliably test Swift concurrency](https://forums.swift.org/t/reliably-testing-code-that-adopts-swift-concurrency/57304/81). * wip
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‎Package.swift‎

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@@ -23,9 +23,11 @@ let package = Package(
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.package(url: "https://github.com/pointfreeco/xctest-dynamic-overlay", from: "0.8.0"),
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],
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targets: [
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.systemLibrary(name: "_CAsyncSupport"),
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.target(
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name: "Dependencies",
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dependencies: [
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"_CAsyncSupport",
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.product(name: "CombineSchedulers", package: "combine-schedulers"),
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.product(name: "Clocks", package: "swift-clocks"),
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.product(name: "XCTestDynamicOverlay", package: "xctest-dynamic-overlay"),

‎Sources/Dependencies/ConcurrencySupport/MainSerialExecutor.swift‎

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import _CAsyncSupport
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@_spi(Concurrency) public func withMainSerialExecutor<T>(
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@_implicitSelfCapture operation: () async throws -> T
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) async rethrows -> T {
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let hook = swift_task_enqueueGlobal_hook
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defer { swift_task_enqueueGlobal_hook = hook }
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swift_task_enqueueGlobal_hook = { job, original in
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MainActor.shared.enqueue(unsafeBitCast(job, to: UnownedJob.self))
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}
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return try await operation()
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}

‎Sources/_CAsyncSupport/_CAsyncSupport.h‎

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//===----------------------------------------------------------------------===//
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//
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// This source file is part of the Swift Async Algorithms open source project
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//
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// Copyright (c) 2022 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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//
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//===----------------------------------------------------------------------===//
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#include <stdint.h>
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#if !defined(__has_feature)
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#define __has_feature(x) 0
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#endif
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#if !defined(__has_attribute)
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#define __has_attribute(x) 0
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#endif
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#if !defined(__has_builtin)
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#define __has_builtin(builtin) 0
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#endif
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#if !defined(__has_cpp_attribute)
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#define __has_cpp_attribute(attribute) 0
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#endif
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// TODO: These macro definitions are duplicated in BridgedSwiftObject.h. Move
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// them to a single file if we find a location that both Visibility.h and
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// BridgedSwiftObject.h can import.
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#if __has_feature(nullability)
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// Provide macros to temporarily suppress warning about the use of
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// _Nullable and _Nonnull.
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# define SWIFT_BEGIN_NULLABILITY_ANNOTATIONS \
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_Pragma("clang diagnostic push") \
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_Pragma("clang diagnostic ignored \"-Wnullability-extension\"")
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# define SWIFT_END_NULLABILITY_ANNOTATIONS \
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_Pragma("clang diagnostic pop")
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#else
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// #define _Nullable and _Nonnull to nothing if we're not being built
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// with a compiler that supports them.
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# define _Nullable
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# define _Nonnull
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# define SWIFT_BEGIN_NULLABILITY_ANNOTATIONS
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# define SWIFT_END_NULLABILITY_ANNOTATIONS
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#endif
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#define SWIFT_MACRO_CONCAT(A, B) A ## B
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#define SWIFT_MACRO_IF_0(IF_TRUE, IF_FALSE) IF_FALSE
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#define SWIFT_MACRO_IF_1(IF_TRUE, IF_FALSE) IF_TRUE
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#define SWIFT_MACRO_IF(COND, IF_TRUE, IF_FALSE) \
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SWIFT_MACRO_CONCAT(SWIFT_MACRO_IF_, COND)(IF_TRUE, IF_FALSE)
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#if __has_attribute(pure)
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#define SWIFT_READONLY __attribute__((__pure__))
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#else
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#define SWIFT_READONLY
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#endif
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#if __has_attribute(const)
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#define SWIFT_READNONE __attribute__((__const__))
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#else
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#define SWIFT_READNONE
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#endif
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#if __has_attribute(always_inline)
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#define SWIFT_ALWAYS_INLINE __attribute__((always_inline))
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#else
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#define SWIFT_ALWAYS_INLINE
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#endif
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#if __has_attribute(noinline)
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#define SWIFT_NOINLINE __attribute__((__noinline__))
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#else
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#define SWIFT_NOINLINE
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#endif
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#if __has_attribute(noreturn)
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#define SWIFT_NORETURN __attribute__((__noreturn__))
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#else
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#define SWIFT_NORETURN
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#endif
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#if __has_attribute(used)
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#define SWIFT_USED __attribute__((__used__))
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#else
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#define SWIFT_USED
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#endif
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#if __has_attribute(unavailable)
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#define SWIFT_ATTRIBUTE_UNAVAILABLE __attribute__((__unavailable__))
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#else
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#define SWIFT_ATTRIBUTE_UNAVAILABLE
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#endif
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#if (__has_attribute(weak_import))
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#define SWIFT_WEAK_IMPORT __attribute__((weak_import))
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#else
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#define SWIFT_WEAK_IMPORT
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#endif
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// Define the appropriate attributes for sharing symbols across
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// image (executable / shared-library) boundaries.
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//
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// SWIFT_ATTRIBUTE_FOR_EXPORTS will be placed on declarations that
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// are known to be exported from the current image. Typically, they
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// are placed on header declarations and then inherited by the actual
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// definitions.
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//
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// SWIFT_ATTRIBUTE_FOR_IMPORTS will be placed on declarations that
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// are known to be exported from a different image. This never
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// includes a definition.
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//
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// Getting the right attribute on a declaratioon can be pretty awkward,
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// but it's necessary under the C translation model. All of this
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// ceremony is familiar to Windows programmers; C/C++ programmers
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// everywhere else usually don't bother, but since we have to get it
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// right for Windows, we have everything set up to get it right on
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// other targets as well, and doing so lets the compiler use more
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// efficient symbol access patterns.
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#if defined(__MACH__) || defined(__wasi__)
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// On Mach-O and WebAssembly, we use non-hidden visibility. We just use
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// default visibility on both imports and exports, both because these
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// targets don't support protected visibility but because they don't
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// need it: symbols are not interposable outside the current image
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// by default.
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# define SWIFT_ATTRIBUTE_FOR_EXPORTS __attribute__((__visibility__("default")))
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# define SWIFT_ATTRIBUTE_FOR_IMPORTS __attribute__((__visibility__("default")))
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#elif defined(__ELF__)
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// On ELF, we use non-hidden visibility. For exports, we must use
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// protected visibility to tell the compiler and linker that the symbols
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// can't be interposed outside the current image. For imports, we must
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// use default visibility because protected visibility guarantees that
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// the symbol is defined in the current library, which isn't true for
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// an import.
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//
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// The compiler does assume that the runtime and standard library can
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// refer to each other's symbols as DSO-local, so it's important that
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// we get this right or we can get linker errors.
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# define SWIFT_ATTRIBUTE_FOR_EXPORTS __attribute__((__visibility__("protected")))
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# define SWIFT_ATTRIBUTE_FOR_IMPORTS __attribute__((__visibility__("default")))
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#elif defined(__CYGWIN__)
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// For now, we ignore all this on Cygwin.
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# define SWIFT_ATTRIBUTE_FOR_EXPORTS
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# define SWIFT_ATTRIBUTE_FOR_IMPORTS
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// FIXME: this #else should be some sort of #elif Windows
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#else // !__MACH__ && !__ELF__
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// On PE/COFF, we use dllimport and dllexport.
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# define SWIFT_ATTRIBUTE_FOR_EXPORTS __declspec(dllexport)
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# define SWIFT_ATTRIBUTE_FOR_IMPORTS __declspec(dllimport)
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#endif
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// CMake conventionally passes -DlibraryName_EXPORTS when building
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// code that goes into libraryName. This isn't the best macro name,
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// but it's conventional. We do have to pass it explicitly in a few
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// places in the build system for a variety of reasons.
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//
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// Unfortunately, defined(D) is a special function you can use in
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// preprocessor conditions, not a macro you can use anywhere, so we
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// need to manually check for all the libraries we know about so that
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// we can use them in our condition below.s
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#if defined(swiftCore_EXPORTS)
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#define SWIFT_IMAGE_EXPORTS_swiftCore 1
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#else
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#define SWIFT_IMAGE_EXPORTS_swiftCore 0
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#endif
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#if defined(swift_Concurrency_EXPORTS)
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#define SWIFT_IMAGE_EXPORTS_swift_Concurrency 1
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#else
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#define SWIFT_IMAGE_EXPORTS_swift_Concurrency 0
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#endif
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#if defined(swift_Distributed_EXPORTS)
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#define SWIFT_IMAGE_EXPORTS_swift_Distributed 1
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#else
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#define SWIFT_IMAGE_EXPORTS_swift_Distributed 0
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#endif
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#if defined(swift_Differentiation_EXPORTS)
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#define SWIFT_IMAGE_EXPORTS_swift_Differentiation 1
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#else
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#define SWIFT_IMAGE_EXPORTS_swift_Differentiation 0
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#endif
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#define SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY) \
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SWIFT_MACRO_IF(SWIFT_IMAGE_EXPORTS_##LIBRARY, \
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SWIFT_ATTRIBUTE_FOR_EXPORTS, \
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SWIFT_ATTRIBUTE_FOR_IMPORTS)
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// SWIFT_EXPORT_FROM(LIBRARY) declares something to be a C-linkage
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// entity exported by the given library.
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//
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// SWIFT_RUNTIME_EXPORT is just SWIFT_EXPORT_FROM(swiftCore).
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//
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// TODO: use this in shims headers in overlays.
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#if defined(__cplusplus)
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#define SWIFT_EXPORT_FROM(LIBRARY) extern "C" SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY)
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#define SWIFT_EXPORT extern "C"
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#else
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#define SWIFT_EXPORT extern
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#define SWIFT_EXPORT_FROM(LIBRARY) SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY)
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#endif
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#define SWIFT_RUNTIME_EXPORT SWIFT_EXPORT_FROM(swiftCore)
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// Define mappings for calling conventions.
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// Annotation for specifying a calling convention of
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// a runtime function. It should be used with declarations
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// of runtime functions like this:
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// void runtime_function_name() SWIFT_CC(swift)
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#define SWIFT_CC(CC) SWIFT_CC_##CC
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// SWIFT_CC(c) is the C calling convention.
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#define SWIFT_CC_c
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// SWIFT_CC(swift) is the Swift calling convention.
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// FIXME: the next comment is false.
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// Functions outside the stdlib or runtime that include this file may be built
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// with a compiler that doesn't support swiftcall; don't define these macros
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// in that case so any incorrect usage is caught.
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#if __has_attribute(swiftcall)
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#define SWIFT_CC_swift __attribute__((swiftcall))
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#define SWIFT_CONTEXT __attribute__((swift_context))
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#define SWIFT_ERROR_RESULT __attribute__((swift_error_result))
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#define SWIFT_INDIRECT_RESULT __attribute__((swift_indirect_result))
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#else
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#define SWIFT_CC_swift
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#define SWIFT_CONTEXT
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#define SWIFT_ERROR_RESULT
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#define SWIFT_INDIRECT_RESULT
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#endif
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typedef struct _Job* JobRef;
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typedef SWIFT_CC(swift) void (*swift_task_enqueueGlobal_original)(JobRef _Nonnull job);
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SWIFT_EXPORT_FROM(swift_Concurrency)
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SWIFT_CC(swift) void (* _Nullable swift_task_enqueueGlobal_hook)(
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JobRef _Nonnull job, swift_task_enqueueGlobal_original _Nonnull original);
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‎Sources/_CAsyncSupport/module.modulemap‎

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module _CAsyncSupport [system] {
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header "_CAsyncSupport.h"
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export *
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}

‎Tests/DependenciesTests/MainSerialExecutorTests.swift‎

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@_spi(Concurrency) import Dependencies
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import XCTest
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final class MainSerialExecutorTests: XCTestCase {
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func testSerializedExecution() async {
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let xs = LockIsolated<[Int]>([])
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await withMainSerialExecutor {
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await withTaskGroup(of: Void.self) { group in
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for x in 1...1000 {
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group.addTask {
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xs.withValue { $0.append(x) }
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}
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}
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}
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}
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xs.withValue { XCTAssertEqual(Array(1...1000), $0) }
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}
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}

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