On Darwin, allow XCTest to be missing if we're only building swift-testing tests.

This PR removes the constraint on Darwin that XCTest.framework must be present
in order to build tests using swift-testing. On Darwin, XCTest is included as a
framework inside Xcode, but if a developer installs the Xcode Command Line Tools
instead of the full IDE, XCTest is not included. They then get a diagnostic of
the form:

> error: XCTest not available: terminated(1): /usr/bin/xcrun --sdk macosx --show-sdk-platform-path output:
>    xcrun: error: unable to lookup item 'PlatformPath' from command line tools installation
>    xcrun: error: unable to lookup item 'PlatformPath' in SDK '/Library/Developer/CommandLineTools/SDKs/MacOSX15.0.sdk'

Which is a poor experience if they aren't even using XCTest.

This change, as a (positive) side effect, suppresses the same diagnostic when
running commands that are not usually dependent on the presence of XCTest such
as `swift build`.

Note that swift-corelibs-xctest is not supported on Darwin, so installing the
Xcode Command Line Tools and adding an explicit dependency on
swift-corelibs-xctest will not produce a functional test target bundle.
Supporting swift-corelibs-xctest on Darwin is a potential future direction.

Automated testing for this change is difficult because it relies on a build
environment that is not supported in CI (namely the presence of the CL tools but
not Xcode nor XCTest.framework.) I have manually tested the change against
swift-testing's own test target.

A separate PR will be necessary in swift-testing to remove some remaining XCTest
dependencies. Those changes are not covered by this PR.

Resolves rdar://125372431.

Author: grynspan

Sources/Commands/SwiftTestCommand.swift

@@ -40,7 +40,7 @@ private enum TestError: Swift.Error {
     case testProductNotFound(productName: String)
     case productIsNotTest(productName: String)
     case multipleTestProducts([String])
-    case xctestNotAvailable
+    case xctestNotAvailable(reason: String? = nil)
 }
 
 extension TestError: CustomStringConvertible {
@@ -57,8 +57,12 @@ extension TestError: CustomStringConvertible {
             return "invalid list test JSON structure, produced by \(context)\(underlying)"
         case .multipleTestProducts(let products):
             return "found multiple test products: \(products.joined(separator: ", ")); use --test-product to select one"
-        case .xctestNotAvailable:
-            return "XCTest not available"
+        case let .xctestNotAvailable(reason):
+            if let reason {
+                return "XCTest not available: \(reason)"
+            } else {
+                return "XCTest not available"
+            }
         }
     }
 }
@@ -203,9 +207,10 @@ package struct SwiftTestCommand: AsyncSwiftCommand {
     private func xctestRun(_ swiftCommandState: SwiftCommandState) async throws {
         // validate XCTest available on darwin based systems
         let toolchain = try swiftCommandState.getTargetToolchain()
-        let isHostTestingAvailable = try swiftCommandState.getHostToolchain().swiftSDK.supportsTesting
-        if (toolchain.targetTriple.isDarwin() && toolchain.xctestPath == nil) || !isHostTestingAvailable {
-            throw TestError.xctestNotAvailable
+        if case let .unsupported(reason) = try swiftCommandState.getHostToolchain().swiftSDK.xctestSupport {
+            throw TestError.xctestNotAvailable(reason: reason)
+        } else if toolchain.targetTriple.isDarwin() && toolchain.xctestPath == nil {
+            throw TestError.xctestNotAvailable()
         }
 
         let buildParameters = try swiftCommandState.buildParametersForTest(options: self.options, library: .xctest)
@@ -814,7 +819,7 @@ final class TestRunner {
         #if os(macOS)
         if library == .xctest {
             guard let xctestPath = self.toolchain.xctestPath else {
-                throw TestError.xctestNotAvailable
+                throw TestError.xctestNotAvailable()
             }
             args = [xctestPath.pathString]
             args += additionalArguments

Sources/Commands/Utilities/TestingSupport.swift

@@ -178,11 +178,12 @@ enum TestingSupport {
         #endif
         return env
         #else
-        // Add the sdk platform path if we have it. If this is not present, we might always end up failing.
-        let sdkPlatformFrameworksPath = try SwiftSDK.sdkPlatformFrameworkPaths()
-        // appending since we prefer the user setting (if set) to the one we inject
-        env.appendPath("DYLD_FRAMEWORK_PATH", value: sdkPlatformFrameworksPath.fwk.pathString)
-        env.appendPath("DYLD_LIBRARY_PATH", value: sdkPlatformFrameworksPath.lib.pathString)
+        // Add the sdk platform path if we have it.
+        if let sdkPlatformFrameworksPath = try? SwiftSDK.sdkPlatformFrameworkPaths() {
+            // appending since we prefer the user setting (if set) to the one we inject
+            env.appendPath("DYLD_FRAMEWORK_PATH", value: sdkPlatformFrameworksPath.fwk.pathString)
+            env.appendPath("DYLD_LIBRARY_PATH", value: sdkPlatformFrameworksPath.lib.pathString)
+        }
 
         // Fast path when no sanitizers are enabled.
         if sanitizers.isEmpty {

Sources/PackageModel/SwiftSDKs/SwiftSDK.swift

@@ -147,7 +147,29 @@ public struct SwiftSDK: Equatable {
     public var architectures: [String]? = nil
 
     /// Whether or not the receiver supports testing.
-    public let supportsTesting: Bool
+    @available(*, deprecated, message: "Use `xctestSupport` instead")
+    public var supportsTesting: Bool {
+        if case .supported = xctestSupport {
+            return true
+        }
+        return false
+    }
+
+    /// Whether or not the receiver supports testing using XCTest.
+    public enum XCTestSupport: Sendable, Equatable {
+        /// XCTest is supported.
+        case supported
+
+        /// XCTest is not supported.
+        ///
+        /// - Parameters:
+        ///     - reason: A string explaining why XCTest is not supported. If
+        ///         `nil`, no additional information is available.
+        case unsupported(reason: String?)
+    }
+
+    /// Whether or not the receiver supports using XCTest.
+    public let xctestSupport: XCTestSupport
 
     /// Root directory path of the SDK used to compile for the target triple.
     @available(*, deprecated, message: "use `pathsConfiguration.sdkRootPath` instead")
@@ -417,19 +439,44 @@ public struct SwiftSDK: Equatable {
         )
     }
 
+    /// Creates a Swift SDK with the specified properties.
+    @available(*, deprecated, message: "use `init(hostTriple:targetTriple:toolset:pathsConfiguration:xctestSupport:)` instead")
+    public init(
+        hostTriple: Triple? = nil,
+        targetTriple: Triple? = nil,
+        toolset: Toolset,
+        pathsConfiguration: PathsConfiguration,
+        supportsTesting: Bool
+    ) {
+        let xctestSupport: XCTestSupport
+        if supportsTesting {
+            xctestSupport = .supported
+        } else {
+            xctestSupport = .unsupported(reason: nil)
+        }
+
+        self.init(
+            hostTriple: hostTriple,
+            targetTriple: targetTriple,
+            toolset: toolset,
+            pathsConfiguration: pathsConfiguration,
+            xctestSupport: xctestSupport
+        )
+    }
+
     /// Creates a Swift SDK with the specified properties.
     public init(
         hostTriple: Triple? = nil,
         targetTriple: Triple? = nil,
         toolset: Toolset,
         pathsConfiguration: PathsConfiguration,
-        supportsTesting: Bool = true
+        xctestSupport: XCTestSupport = .supported
     ) {
         self.hostTriple = hostTriple
         self.targetTriple = targetTriple
         self.toolset = toolset
         self.pathsConfiguration = pathsConfiguration
-        self.supportsTesting = supportsTesting
+        self.xctestSupport = xctestSupport
     }
 
     /// Returns the bin directory for the host.
@@ -496,7 +543,7 @@ public struct SwiftSDK: Equatable {
         #endif
 
         // Compute common arguments for clang and swift.
-        let supportsTesting: Bool
+        let xctestSupport: XCTestSupport
         var extraCCFlags: [String] = []
         var extraSwiftCFlags: [String] = []
         #if os(macOS)
@@ -506,13 +553,12 @@ public struct SwiftSDK: Equatable {
             extraSwiftCFlags += ["-F", sdkPaths.fwk.pathString]
             extraSwiftCFlags += ["-I", sdkPaths.lib.pathString]
             extraSwiftCFlags += ["-L", sdkPaths.lib.pathString]
-            supportsTesting = true
+            xctestSupport = .supported
         } catch {
-            supportsTesting = false
-            observabilityScope?.emit(warning: "could not determine XCTest paths: \(error)")
+            xctestSupport = .unsupported(reason: String(describing: error))
         }
         #else
-        supportsTesting = true
+        xctestSupport = .supported
         #endif
 
         #if !os(Windows)
@@ -528,7 +574,7 @@ public struct SwiftSDK: Equatable {
                 rootPaths: [binDir]
             ),
             pathsConfiguration: .init(sdkRootPath: sdkPath),
-            supportsTesting: supportsTesting
+            xctestSupport: xctestSupport
         )
     }