[6.0] Generated code for XCTest on non-Darwin needs to be actor-isolated. (#7569)

Explanation: Ensure that the synthesized entry point for
swift-corelibs-xctest is concurrency-safe by marking it `@MainActor`.
Scope: All XCTest-based tests built/run on non-Darwin platforms.
Original PR: #7566
Risk: Low. The change is necessary to suppress a concurrency diagnostic.
The synthesized `main()` function is implicitly main-actor-isolated.
Testing: Added unit test to check for the diagnostic, verified it did
not occur anymore with the change in place.
Reviewer: @bnbarham, @MaxDesiatov, @stmontgomery, @briancroom

Author: grynspan

Sources/Build/BuildOperationBuildSystemDelegateHandler.swift

@@ -87,6 +87,7 @@ final class TestDiscoveryCommand: CustomLLBuildCommand, TestBuildCommand {
 
                 fileprivate extension \#(className) {
                     @available(*, deprecated, message: "Not actually deprecated. Marked as deprecated to allow inclusion of deprecated tests (which test deprecated functionality) without warnings")
+                    @MainActor
                     static let __allTests__\#(className) = [
                         \#(testMethods.map { $0.allTestsEntry }.joined(separator: ",\n        "))
                     ]
@@ -98,6 +99,7 @@ final class TestDiscoveryCommand: CustomLLBuildCommand, TestBuildCommand {
         content +=
         #"""
         @available(*, deprecated, message: "Not actually deprecated. Marked as deprecated to allow inclusion of deprecated tests (which test deprecated functionality) without warnings")
+        @MainActor
         func __\#(module)__allTests() -> [XCTestCaseEntry] {
             return [
                 \#(testsByClassNames.map { "testCase(\($0.key).__allTests__\($0.key))" }
@@ -166,6 +168,7 @@ final class TestDiscoveryCommand: CustomLLBuildCommand, TestBuildCommand {
                 import XCTest
 
                 @available(*, deprecated, message: "Not actually deprecated. Marked as deprecated to allow inclusion of deprecated tests (which test deprecated functionality) without warnings")
+                @MainActor
                 public func __allDiscoveredTests() -> [XCTestCaseEntry] {
                     \#(testsKeyword) tests = [XCTestCaseEntry]()
 
@@ -264,18 +267,15 @@ final class TestEntryPointCommand: CustomLLBuildCommand, TestBuildCommand {
                 @main
                 @available(*, deprecated, message: "Not actually deprecated. Marked as deprecated to allow inclusion of deprecated tests (which test deprecated functionality) without warnings")
                 struct Runner {
-                    #if os(WASI)
-                    /// On WASI, we can't block the main thread, so XCTestMain is defined as async.
                     static func main() async {
                         \#(testObservabilitySetup)
+                #if os(WASI)
+                        /// On WASI, we can't block the main thread, so XCTestMain is defined as async.
                         await XCTMain(__allDiscoveredTests()) as Never
-                    }
-                    #else
-                    static func main() {
-                        \#(testObservabilitySetup)
+                #else
                         XCTMain(__allDiscoveredTests()) as Never
+                #endif
                     }
-                    #endif
                 }
                 """#
             )

Tests/CommandsTests/TestCommandTests.swift

@@ -293,4 +293,14 @@ final class TestCommandTests: CommandsTestCase {
             }
         }
     }
+
+#if !canImport(Darwin)
+    func testGeneratedMainIsConcurrencySafe_XCTest() throws {
+        let strictConcurrencyFlags = ["-Xswiftc", "-strict-concurrency=complete"]
+        try fixture(name: "Miscellaneous/TestDiscovery/Simple") { fixturePath in
+            let (_, stderr) = try SwiftPM.Test.execute(strictConcurrencyFlags, packagePath: fixturePath)
+            XCTAssertNoMatch(stderr, .contains("is not concurrency-safe"))
+        }
+    }
+#endif
 }