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🍒[6.0][Concurrency] Reimplement @TaskLocal as a macro (#73100)
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‎CHANGELOG.md

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## Swift 6.0
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* Since its introduction in Swift 5.1 the @TaskLocal property wrapper was used to
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create and access task-local value bindings. Property wrappers introduce mutable storage,
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which was now properly flagged as potential source of concurrency unsafety.
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In order for Swift 6 language mode to not flag task-locals as potentially thread-unsafe,
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task locals are now implemented using a macro. The macro has the same general semantics
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and usage patterns, however there are two source-break situations which the Swift 6
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task locals cannot handle:
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Using an implicit default `nil` value for task local initialization, when combined with a type alias:
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```swift
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// allowed in Swift 5.x, not allowed in Swift 6.x
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typealias MyValue = Optional<Int>
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@TaskLocal
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static var number: MyValue // Swift 6: error, please specify default value explicitly
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// Solution 1: Specify the default value
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@TaskLocal
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static var number: MyValue = nil
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// Solution 2: Avoid the type-alias
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@TaskLocal
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static var number: Optional<Int>
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```
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At the same time, task locals can now be declared as global properties, which wasn't possible before.
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* Swift 5.10 missed a semantic check from [SE-0309][]. In type context, a reference to a
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protocol `P` that has associated types or `Self` requirements should use
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the `any` keyword, but this was not enforced in nested generic argument positions.

‎lib/Macros/Sources/SwiftMacros/CMakeLists.txt

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OptionSetMacro.swift
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DebugDescriptionMacro.swift
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DistributedResolvableMacro.swift
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SWIFT_DEPENDENCIES
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TaskLocalMacro.swift
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SWIFT_DEPENDENCIES
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SwiftDiagnostics
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SwiftSyntax
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SwiftSyntaxBuilder

‎lib/Macros/Sources/SwiftMacros/TaskLocalMacro.swift

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//===----------------------------------------------------------------------===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2022-2023 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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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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import SwiftSyntax
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import SwiftSyntaxMacros
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import SwiftDiagnostics
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/// Macro implementing the TaskLocal functionality.
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///
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/// It introduces a peer `static let $name: TaskLocal<Type>` as well as a getter
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/// that accesses the task local storage.
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public enum TaskLocalMacro {}
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extension TaskLocalMacro: PeerMacro {
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public static func expansion(
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of node: AttributeSyntax,
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providingPeersOf declaration: some DeclSyntaxProtocol,
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in context: some MacroExpansionContext
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) throws -> [DeclSyntax] {
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guard let varDecl = try requireVar(declaration, diagnose: false) else {
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return []
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}
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guard try requireStaticContext(varDecl, in: context, diagnose: false) else {
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return []
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}
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guard varDecl.bindings.count == 1 else {
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throw DiagnosticsError(
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syntax: declaration,
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message: "'@TaskLocal' property must have exactly one binding", id: .incompatibleDecl)
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}
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guard let firstBinding = varDecl.bindings.first else {
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throw DiagnosticsError(
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syntax: declaration,
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message: "'@TaskLocal' property must have declared binding", id: .incompatibleDecl)
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}
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guard let name = firstBinding.pattern.as(IdentifierPatternSyntax.self)?.identifier else {
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throw DiagnosticsError(
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syntax: declaration,
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message: "'@TaskLocal' property must have name", id: .incompatibleDecl)
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}
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let type = firstBinding.typeAnnotation?.type
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let explicitTypeAnnotation: TypeAnnotationSyntax?
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if let type {
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explicitTypeAnnotation = TypeAnnotationSyntax(type: TypeSyntax("TaskLocal<\(type.trimmed)>"))
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} else {
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explicitTypeAnnotation = nil
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}
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let initialValue: ExprSyntax
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if let initializerValue = firstBinding.initializer?.value {
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initialValue = ExprSyntax(initializerValue)
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} else if let type, type.isOptional {
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initialValue = ExprSyntax(NilLiteralExprSyntax())
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} else {
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throw DiagnosticsError(
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syntax: declaration,
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message: "'@TaskLocal' property must have default value, or be optional", id: .mustBeVar)
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}
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// If the property is global, do not prefix the synthesised decl with 'static'
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let isGlobal = context.lexicalContext.isEmpty
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let staticKeyword: TokenSyntax?
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if isGlobal {
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staticKeyword = nil
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} else {
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staticKeyword = TokenSyntax.keyword(.static, trailingTrivia: .space)
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}
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return [
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"""
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\(staticKeyword)let $\(name)\(explicitTypeAnnotation) = TaskLocal(wrappedValue: \(initialValue))
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"""
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]
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}
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}
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extension TaskLocalMacro: AccessorMacro {
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public static func expansion(
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of node: AttributeSyntax,
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providingAccessorsOf declaration: some DeclSyntaxProtocol,
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in context: some MacroExpansionContext
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) throws -> [AccessorDeclSyntax] {
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// We very specifically have to fail and diagnose in the accessor macro,
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// rather than in the peer macro, since returning [] from the accessor
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// macro adds another series of errors about it missing to emit a decl.
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guard let varDecl = try requireVar(declaration) else {
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return []
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}
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try requireStaticContext(varDecl, in: context)
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guard let firstBinding = varDecl.bindings.first else {
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return []
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}
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guard let name = firstBinding.pattern.as(IdentifierPatternSyntax.self)?.identifier else {
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return []
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}
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return ["get { $\(name).get() }"]
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}
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}
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@discardableResult
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private func requireVar(_ decl: some DeclSyntaxProtocol,
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diagnose: Bool = true) throws -> VariableDeclSyntax? {
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if let varDecl = decl.as(VariableDeclSyntax.self) {
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return varDecl
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}
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if diagnose {
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throw DiagnosticsError(
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syntax: decl,
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message: "'@TaskLocal' can only be applied to properties", id: .mustBeVar)
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}
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return nil
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}
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@discardableResult
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private func requireStaticContext(_ decl: VariableDeclSyntax,
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in context: some MacroExpansionContext,
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diagnose: Bool = true) throws -> Bool {
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let isStatic = decl.modifiers.contains { modifier in
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modifier.name.text == "\(Keyword.static)"
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}
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if isStatic {
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return true
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}
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let isGlobal = context.lexicalContext.isEmpty
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if isGlobal {
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return true
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}
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if diagnose {
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throw DiagnosticsError(
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syntax: decl,
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message: "'@TaskLocal' can only be applied to 'static' property, or global variables", id: .mustBeStatic)
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}
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return false
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}
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extension TypeSyntax {
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// This isn't great since we can't handle type aliases since the macro
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// has no type information, but at least for the common case for Optional<T>
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// and T? we can detect the optional.
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fileprivate var isOptional: Bool {
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switch self.as(TypeSyntaxEnum.self) {
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case .optionalType:
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return true
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case .identifierType(let identifierType):
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return identifierType.name.text == "Optional"
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case .memberType(let memberType):
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guard let baseIdentifier = memberType.baseType.as(IdentifierTypeSyntax.self),
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baseIdentifier.name.text == "Swift" else {
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return false
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}
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return memberType.name.text == "Optional"
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default: return false
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}
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}
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}
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struct TaskLocalMacroDiagnostic: DiagnosticMessage {
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enum ID: String {
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case mustBeVar = "must be var"
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case mustBeStatic = "must be static"
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case incompatibleDecl = "incompatible declaration"
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}
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var message: String
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var diagnosticID: MessageID
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var severity: DiagnosticSeverity
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init(message: String, diagnosticID: SwiftDiagnostics.MessageID, severity: SwiftDiagnostics.DiagnosticSeverity = .error) {
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self.message = message
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self.diagnosticID = diagnosticID
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self.severity = severity
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}
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init(message: String, domain: String, id: ID, severity: SwiftDiagnostics.DiagnosticSeverity = .error) {
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self.message = message
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self.diagnosticID = MessageID(domain: domain, id: id.rawValue)
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self.severity = severity
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}
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}
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extension DiagnosticsError {
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init(
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syntax: some SyntaxProtocol,
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message: String,
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domain: String = "Swift",
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id: TaskLocalMacroDiagnostic.ID,
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severity: SwiftDiagnostics.DiagnosticSeverity = .error) {
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self.init(diagnostics: [
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Diagnostic(
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node: Syntax(syntax),
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message: TaskLocalMacroDiagnostic(
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message: message,
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domain: domain,
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id: id,
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severity: severity))
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])
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}
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}

‎lib/Sema/TypeCheckConcurrency.cpp

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if (var->isGlobalStorage() && !isActorType) {
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auto *diagVar = var;
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if (auto *originalVar = var->getOriginalWrappedProperty()) {
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// temporary 5.10 checking bypass for @TaskLocal <rdar://120907014>
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// TODO: @TaskLocal should be a macro <rdar://120914014>
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if (auto *classDecl =
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var->getInterfaceType()->getClassOrBoundGenericClass()) {
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auto &ctx = var->getASTContext();
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if (classDecl == ctx.getTaskLocalDecl()) {
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return isolation;
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}
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}
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diagVar = originalVar;
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}
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if (var->isLet()) {

‎stdlib/public/Concurrency/TaskLocal.swift

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import Swift
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@_implementationOnly import _SwiftConcurrencyShims
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/// Property wrapper that defines a task-local value key.
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// Macros are disabled when Swift is built without swift-syntax.
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#if $Macros && hasAttribute(attached)
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/// Macro that introduces a ``TaskLocal-class`` binding.
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///
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/// For information about task-local bindings, see ``TaskLocal-class``.
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///
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/// - SeeAlso: ``TaskLocal-class``
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@available(SwiftStdlib 5.1, *)
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@attached(accessor)
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@attached(peer, names: prefixed(`$`))
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public macro TaskLocal() =
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#externalMacro(module: "SwiftMacros", type: "TaskLocalMacro")
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#endif
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/// Wrapper type that defines a task-local value key.
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///
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/// A task-local value is a value that can be bound and read in the context of a
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/// `Task`. It is implicitly carried with the task, and is accessible by any
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/// child tasks the task creates (such as TaskGroup or `async let` created tasks).
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/// ``Task``. It is implicitly carried with the task, and is accessible by any
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/// child tasks it creates (such as TaskGroup or `async let` created tasks).
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///
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/// ### Task-local declarations
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///
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/// Task locals must be declared as static properties (or global properties,
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/// once property wrappers support these), like this:
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/// Task locals must be declared as static properties or global properties, like this:
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///
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/// enum Example {
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/// @TaskLocal
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/// static let traceID: TraceID?
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/// }
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///
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/// // Global task local properties are supported since Swift 6.0:
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/// @TaskLocal
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/// var contextualNumber: Int = 12
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///
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/// ### Default values
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/// Reading a task local value when no value was bound to it results in returning
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/// its default value. For a task local declared as optional (such as e.g. `TraceID?`),
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/// read() // traceID: nil
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/// }
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/// }
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@propertyWrapper
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///
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/// - SeeAlso: ``TaskLocal-macro``
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@available(SwiftStdlib 5.1, *)
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public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible {
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let defaultValue: Value

‎test/Concurrency/Runtime/async_task_locals_spawn_let.swift renamed to ‎test/Concurrency/Runtime/async_task_locals_async_let.swift

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// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library) | %FileCheck %s
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// RUN: %target-run-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library) | %FileCheck %s
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// REQUIRES: executable_test
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// REQUIRES: concurrency

‎test/Concurrency/Runtime/async_task_locals_basic.swift

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// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// RUN: %target-run-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// REQUIRES: executable_test
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// REQUIRES: concurrency

‎test/Concurrency/Runtime/async_task_locals_copy_to_async.swift

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// REQUIRES: rdar80824152
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// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// RUN: %target-run-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// REQUIRES: executable_test
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// REQUIRES: concurrency

‎test/Concurrency/Runtime/async_task_locals_copy_to_sync.swift

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// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// RUN: %target-run-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// REQUIRES: executable_test
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// REQUIRES: concurrency

‎test/Concurrency/Runtime/async_task_locals_groups.swift

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// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// RUN: %target-run-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) | %FileCheck %s
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// REQUIRES: executable_test
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// REQUIRES: concurrency

‎test/Concurrency/Runtime/async_task_locals_prevent_illegal_use_discarding_taskgroup.swift

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// RUN: %target-fail-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) 2>&1 | %FileCheck %s
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// RUN: %target-fail-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) 2>&1 | %FileCheck %s
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//
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// // TODO: could not figure out how to use 'not --crash' it never is used with target-run-simple-swift
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// This test is intended to *crash*, so we're using target-fail-simple-swift

‎test/Concurrency/Runtime/async_task_locals_prevent_illegal_use_taskgroup.swift

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// RUN: %target-fail-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) 2>&1 | %FileCheck %s
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// RUN: %target-fail-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library %import-libdispatch) 2>&1 | %FileCheck %s
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//
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// // TODO: could not figure out how to use 'not --crash' it never is used with target-run-simple-swift
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// This test is intended to *crash*, so we're using target-fail-simple-swift

‎test/Concurrency/Runtime/async_task_locals_synchronous_bind.swift

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// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library) | %FileCheck %s
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// RUN: %target-run-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library) | %FileCheck %s
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// REQUIRES: executable_test
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// REQUIRES: concurrency

‎test/Concurrency/Runtime/async_task_locals_wrapper.swift

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// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library) | %FileCheck %s
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// RUN: %target-run-simple-swift( -plugin-path %swift-plugin-dir -Xfrontend -disable-availability-checking -parse-as-library) | %FileCheck %s
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// REQUIRES: executable_test
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// REQUIRES: concurrency

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