[DNM] Enable exit test value capturing

This PR enables the value capture subfeature of exit tests and adds documentation for it.

> [!WARNING]
> Do not merge this PR unless/until [ST-NNNN](swiftlang/swift-evolution#2886) has been approved.

Author: grynspan

Package.swift

@@ -95,13 +95,6 @@ let package = Package(
     return result
   }(),
 
-  traits: [
-    .trait(
-      name: "ExperimentalExitTestValueCapture",
-      description: "Enable experimental support for capturing values in exit tests"
-    ),
-  ],
-
   dependencies: [
     .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "602.0.0-latest"),
   ],
@@ -340,14 +333,6 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .define("SWT_NO_LIBDISPATCH", .whenEmbedded()),
     ]
 
-    // Unconditionally enable 'ExperimentalExitTestValueCapture' when building
-    // for development.
-    if buildingForDevelopment {
-      result += [
-        .define("ExperimentalExitTestValueCapture")
-      ]
-    }
-
     return result
   }
 

Sources/Testing/Testing.docc/exit-testing.md

@@ -67,21 +67,7 @@ The parent process doesn't call the body of the exit test. Instead, the child
 process treats the body of the exit test as its `main()` function and calls it
 directly.
 
-- Note: Because the body acts as the `main()` function of a new process, it
-  can't capture any state originating in the parent process or from its lexical
-  context. For example, the following exit test will fail to compile because it
-  captures a variable declared outside the exit test itself:
-
-  ```swift
-  @Test func `Customer won't eat food unless it's nutritious`() async {
-    let isNutritious = false
-    await #expect(processExitsWith: .failure) {
-      var food = ...
-      food.isNutritious = isNutritious // ❌ ERROR: trying to capture state here
-      Customer.current.eat(food)
-    }
-  }
-  ```
+<!-- TODO: discuss @MainActor isolation or lack thereof -->
 
 If the body returns before the child process exits, the process exits as if
 `main()` returned normally. If the body throws an error, Swift handles it as if
@@ -106,6 +92,56 @@ status of the child process against the expected exit condition you passed. If
 they match, the exit test passes; otherwise, it fails and the testing library
 records an issue.
 
+### Capture state from the parent process
+
+To pass information from the parent process to the child process, you specify
+the Swift values you want to pass in a [capture list](https://docs.swift.org/swift-book/documentation/the-swift-programming-language/closures/#Capturing-Values)
+on the exit test's body:
+
+```swift
+@Test(arguments: Food.allJunkFood)
+func `Customer won't eat food unless it's nutritious`(_ food: Food) async {
+  await #expect(processExitsWith: .failure) { [food] in
+    Customer.current.eat(food)
+  }
+}
+```
+
+If a captured value is an argument to the current function or is `self`, its
+type is inferred at compile time. Otherwise, explicitly specify the type of the
+value using the `as` operator:
+
+```swift
+@Test func `Customer won't eat food unless it's nutritious`() async {
+  var food = ...
+  food.isNutritious = false
+  await #expect(processExitsWith: .failure) { [self, food = food as Food] in
+    self.prepare(food)
+    Customer.current.eat(food)
+  }
+}
+```
+
+Every value you capture in an exit test must conform to [`Sendable`](https://developer.apple.com/documentation/swift/sendable)
+and [`Codable`](https://developer.apple.com/documentation/swift/codable). Each
+value is encoded by the parent process using [`encode(to:)`](https://developer.apple.com/documentation/swift/encodable/encode(to:))
+and is decoded by the child process [`init(from:)`](https://developer.apple.com/documentation/swift/decodable/init(from:))
+before being passed to the exit test body.
+
+If a captured value's type does not conform to both `Sendable` and `Codable`, or
+if the value is not explicitly specified in the exit test body's capture list,
+the compiler emits an error:
+
+```swift
+@Test func `Customer won't eat food unless it's nutritious`() async {
+  var food = ...
+  food.isNutritious = false
+  await #expect(processExitsWith: .failure) {
+    Customer.current.eat(food) // ❌ ERROR: implicitly capturing 'food'
+  }
+}
+```
+
 ### Gather output from the child process
 
 The ``expect(processExitsWith:observing:_:sourceLocation:performing:)`` and

Sources/TestingMacros/ConditionMacro.swift

@@ -630,15 +630,6 @@ extension ExitTestConditionMacro {
   ) -> Bool {
     var diagnostics = [DiagnosticMessage]()
 
-    if let closureExpr = bodyArgumentExpr.as(ClosureExprSyntax.self),
-       let captureClause = closureExpr.signature?.capture,
-       !captureClause.items.isEmpty {
-      // Disallow capture lists if the experimental feature is not enabled.
-      if !ExitTestExpectMacro.isValueCapturingEnabled {
-        diagnostics.append(.captureClauseUnsupported(captureClause, in: closureExpr, inExitTest: macro))
-      }
-    }
-
     // Disallow exit tests in generic types and functions as they cannot be
     // correctly expanded due to the use of a nested type with static members.
     for lexicalContext in context.lexicalContext {
@@ -664,22 +655,6 @@ extension ExitTestConditionMacro {
   }
 }
 
-extension ExitTestExpectMacro {
-  /// Whether or not experimental value capturing via explicit capture lists is
-  /// enabled.
-  ///
-  /// This member is declared on ``ExitTestExpectMacro`` but also applies to
-  /// ``ExitTestRequireMacro``.
-  @TaskLocal
-  static var isValueCapturingEnabled: Bool = {
-#if ExperimentalExitTestValueCapture
-    return true
-#else
-    return false
-#endif
-  }()
-}
-
 /// A type describing the expansion of the `#expect(processExitsWith:)` macro.
 ///
 /// This type checks for nested invocations of `#expect()` and `#require()` and

Sources/TestingMacros/Support/DiagnosticMessage.swift

@@ -845,50 +845,6 @@ extension DiagnosticMessage {
     )
   }
 
-  /// Create a diagnostic message stating that a capture clause cannot be used
-  /// in an exit test.
-  ///
-  /// - Parameters:
-  ///   - captureClause: The invalid capture clause.
-  ///   - closure: The closure containing `captureClause`.
-  ///   - exitTestMacro: The containing exit test macro invocation.
-  ///
-  /// - Returns: A diagnostic message.
-  static func captureClauseUnsupported(_ captureClause: ClosureCaptureClauseSyntax, in closure: ClosureExprSyntax, inExitTest exitTestMacro: some FreestandingMacroExpansionSyntax) -> Self {
-    let changes: [FixIt.Change]
-    if let signature = closure.signature,
-       Array(signature.with(\.capture, nil).tokens(viewMode: .sourceAccurate)).count == 1 {
-      // The only remaining token in the signature is `in`, so remove the whole
-      // signature tree instead of just the capture clause.
-      changes = [
-        .replaceTrailingTrivia(token: closure.leftBrace, newTrivia: ""),
-        .replace(
-          oldNode: Syntax(signature),
-          newNode: Syntax("" as ExprSyntax)
-        )
-      ]
-    } else {
-      changes = [
-        .replace(
-          oldNode: Syntax(captureClause),
-          newNode: Syntax("" as ExprSyntax)
-        )
-      ]
-    }
-
-    return Self(
-      syntax: Syntax(captureClause),
-      message: "Cannot specify a capture clause in closure passed to \(_macroName(exitTestMacro))",
-      severity: .error,
-      fixIts: [
-        FixIt(
-          message: MacroExpansionFixItMessage("Remove '\(captureClause.trimmed)'"),
-          changes: changes
-        ),
-      ]
-    )
-  }
-
   /// Create a diagnostic message stating that an expression macro is not
   /// supported in a generic context.
   ///

Tests/TestingMacrosTests/ConditionMacroTests.swift

@@ -452,7 +452,6 @@ struct ConditionMacroTests {
     }
   }
 
-#if ExperimentalExitTestValueCapture
   @Test("#expect(processExitsWith:) produces a diagnostic for a bad capture",
         arguments: [
           "#expectExitTest(processExitsWith: x) { [weak a] in }":
@@ -470,34 +469,12 @@ struct ConditionMacroTests {
         ]
   )
   func exitTestCaptureDiagnostics(input: String, expectedMessage: String) throws {
-    try ExitTestExpectMacro.$isValueCapturingEnabled.withValue(true) {
-      let (_, diagnostics) = try parse(input)
-
-      #expect(diagnostics.count > 0)
-      for diagnostic in diagnostics {
-        #expect(diagnostic.diagMessage.severity == .error)
-        #expect(diagnostic.message == expectedMessage)
-      }
-    }
-  }
-#endif
+    let (_, diagnostics) = try parse(input)
 
-  @Test(
-    "Capture list on an exit test produces a diagnostic",
-    arguments: [
-      "#expectExitTest(processExitsWith: x) { [a] in }":
-        "Cannot specify a capture clause in closure passed to '#expectExitTest(processExitsWith:_:)'"
-    ]
-  )
-  func exitTestCaptureListProducesDiagnostic(input: String, expectedMessage: String) throws {
-    try ExitTestExpectMacro.$isValueCapturingEnabled.withValue(false) {
-      let (_, diagnostics) = try parse(input)
-
-      #expect(diagnostics.count > 0)
-      for diagnostic in diagnostics {
-        #expect(diagnostic.diagMessage.severity == .error)
-        #expect(diagnostic.message == expectedMessage)
-      }
+    #expect(diagnostics.count > 0)
+    for diagnostic in diagnostics {
+      #expect(diagnostic.diagMessage.severity == .error)
+      #expect(diagnostic.message == expectedMessage)
     }
   }
 

Tests/TestingTests/ExitTestTests.swift

@@ -381,7 +381,6 @@ private import _TestingInternals
     }
   }
 
-#if ExperimentalExitTestValueCapture
   @Test("Capture list")
   func captureList() async {
     let i = 123
@@ -559,7 +558,6 @@ private import _TestingInternals
     }
   }
 #endif
-#endif
 }
 
 // MARK: - Fixtures