Generated code for XCTest on non-Darwin needs to be actor-isolated. (#7566)

On Linux, Windows, etc. (anywhere that uses swift-corelibs-xctest
instead of XCTest.framework), SwiftPM is responsible for generating an
entry point function that passes in all tests discovered at compile
time. The compiler cannot tell whether the generated code is
concurrency-safe. This PR modifies the generated code and makes it
main-actor-isolated. Since it's only ever used in the program's main
function, this is safe.

Resolves #7556.

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
 }