Make the ExitTest type move-only. (#712)

This PR adds `~Copyable` to `ExitTest`, which is a tools-only bit of SPI
representing a (gasp) exit test. This change has the effect of making it
impossible to accidentally invoke an exit test body in the parent
process, and also impossible to accidentally invoke twice in the child
process. Phew!

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ExitTests/ExitTest.swift

@@ -20,12 +20,12 @@ private import _TestingInternals
 /// Instances of this type describe an exit test defined by the test author and
 /// discovered or called at runtime.
 @_spi(Experimental) @_spi(ForToolsIntegrationOnly)
-public struct ExitTest: Sendable {
+public struct ExitTest: Sendable, ~Copyable {
   /// The expected exit condition of the exit test.
   public var expectedExitCondition: ExitCondition
 
   /// The body closure of the exit test.
-  fileprivate var body: @Sendable () async throws -> Void
+  fileprivate var body: @Sendable () async throws -> Void = {}
 
   /// The source location of the exit test.
   ///
@@ -73,7 +73,7 @@ public struct ExitTest: Sendable {
   /// to terminate the process; if it does not, the testing library will
   /// terminate the process in a way that causes the corresponding expectation
   /// to fail.
-  public func callAsFunction() async -> Never {
+  public consuming func callAsFunction() async -> Never {
     Self._disableCrashReporting()
 
     do {
@@ -146,7 +146,6 @@ extension ExitTest {
 ///
 /// - Parameters:
 ///   - expectedExitCondition: The expected exit condition.
-///   - body: The exit test body.
 ///   - expression: The expression, corresponding to `condition`, that is being
 ///     evaluated (if available at compile time.)
 ///   - comments: An array of comments describing the expectation. This array
@@ -161,7 +160,7 @@ extension ExitTest {
 /// convention.
 func callExitTest(
   exitsWith expectedExitCondition: ExitCondition,
-  performing body: @escaping @Sendable () async throws -> Void,
+  performing _: @escaping @Sendable () async throws -> Void,
   expression: __Expression,
   comments: @autoclosure () -> [Comment],
   isRequired: Bool,
@@ -173,7 +172,7 @@ func callExitTest(
 
   let actualExitCondition: ExitCondition
   do {
-    let exitTest = ExitTest(expectedExitCondition: expectedExitCondition, body: body, sourceLocation: sourceLocation)
+    let exitTest = ExitTest(expectedExitCondition: expectedExitCondition, sourceLocation: sourceLocation)
     actualExitCondition = try await configuration.exitTestHandler(exitTest)
   } catch {
     // An error here would indicate a problem in the exit test handler such as a
@@ -242,9 +241,11 @@ extension ExitTest {
   /// configurations is undefined.
   static func findInEnvironmentForEntryPoint() -> Self? {
     if var sourceLocationString = Environment.variable(named: "SWT_EXPERIMENTAL_EXIT_TEST_SOURCE_LOCATION") {
-      return try? sourceLocationString.withUTF8 { sourceLocationBuffer in
+      let sourceLocation = try? sourceLocationString.withUTF8 { sourceLocationBuffer in
         let sourceLocationBuffer = UnsafeRawBufferPointer(sourceLocationBuffer)
-        let sourceLocation = try JSON.decode(SourceLocation.self, from: sourceLocationBuffer)
+        return try JSON.decode(SourceLocation.self, from: sourceLocationBuffer)
+      }
+      if let sourceLocation {
         return find(at: sourceLocation)
       }
     }