Don't link libc++ or libstd++ for Embedded Swift (#7357)

These libraries are not usually available for embedded platforms. A
workaround of passing this flag via `unsafeFlags` in `Package.swift`
still remains after this change.

Author: MaxDesiatov

Sources/Build/BuildPlan/BuildPlan+Product.swift

@@ -17,7 +17,10 @@ import struct PackageGraph.ResolvedProduct
 import struct PackageGraph.ResolvedTarget
 import class PackageModel.BinaryTarget
 import class PackageModel.ClangTarget
+
+@_spi(SwiftPMInternal)
 import class PackageModel.Target
+
 import class PackageModel.SwiftTarget
 import class PackageModel.SystemLibraryTarget
 import struct SPMBuildCore.BuildParameters
@@ -28,7 +31,10 @@ extension BuildPlan {
     /// Plan a product.
     func plan(buildProduct: ProductBuildDescription) throws {
         // Compute the product's dependency.
-        let dependencies = try computeDependencies(of: buildProduct.product, buildParameters: buildProduct.buildParameters)
+        let dependencies = try computeDependencies(
+            of: buildProduct.product,
+            buildParameters: buildProduct.buildParameters
+        )
 
         // Add flags for system targets.
         for systemModule in dependencies.systemModules {
@@ -50,19 +56,24 @@ extension BuildPlan {
             }
         }
 
-        // Link C++ if needed.
-        // Note: This will come from build settings in future.
-        for target in dependencies.staticTargets {
-            if case let target as ClangTarget = target.underlying, target.isCXX {
-                let triple = buildProduct.buildParameters.triple
-                if triple.isDarwin() {
-                    buildProduct.additionalFlags += ["-lc++"]
-                } else if triple.isWindows() {
-                    // Don't link any C++ library.
-                } else {
-                    buildProduct.additionalFlags += ["-lstdc++"]
+        // Don't link libc++ or libstd++ when building for Embedded Swift.
+        // Users can still link it manually for embedded platforms when needed,
+        // by providing `-Xlinker -lc++` options via CLI or `Package.swift`.
+        if !buildProduct.product.targets.contains(where: \.underlying.isEmbeddedSwiftTarget) {
+            // Link C++ if needed.
+            // Note: This will come from build settings in future.
+            for target in dependencies.staticTargets {
+                if case let target as ClangTarget = target.underlying, target.isCXX {
+                    let triple = buildProduct.buildParameters.triple
+                    if triple.isDarwin() {
+                        buildProduct.additionalFlags += ["-lc++"]
+                    } else if triple.isWindows() {
+                        // Don't link any C++ library.
+                    } else {
+                        buildProduct.additionalFlags += ["-lstdc++"]
+                    }
+                    break
                 }
-                break
             }
         }
 

Sources/PackageModel/Target/Target.swift

@@ -339,6 +339,15 @@ public class Target: PolymorphicCodableProtocol {
         self.pluginUsages = []
         self.usesUnsafeFlags = try container.decode(Bool.self, forKey: .usesUnsafeFlags)
     }
+
+    @_spi(SwiftPMInternal)
+    public var isEmbeddedSwiftTarget: Bool {
+        for case .enableExperimentalFeature("Embedded") in self.buildSettingsDescription.swiftSettings.map(\.kind) {
+            return true
+        }
+
+        return false
+    }
 }
 
 extension Target: Hashable {
@@ -371,3 +380,10 @@ public extension Sequence where Iterator.Element == Target {
         }
     }
 }
+
+extension [TargetBuildSettingDescription.Setting] {
+    @_spi(SwiftPMInternal)
+    public var swiftSettings: Self {
+        self.filter { $0.tool == .swift }
+    }
+}

Sources/SPMTestSupport/MockPackageGraphs.swift

@@ -156,3 +156,46 @@ public func trivialPackageGraph(pkgRootPath: AbsolutePath) throws -> MockPackage
 
     return (graph, fs, observability.topScope)
 }
+
+@_spi(SwiftPMInternal)
+public func embeddedCxxInteropPackageGraph(pkgRootPath: AbsolutePath) throws -> MockPackageGraph {
+    let fs = InMemoryFileSystem(
+        emptyFiles:
+        "/Pkg/Sources/app/main.swift",
+        "/Pkg/Sources/lib/lib.cpp",
+        "/Pkg/Sources/lib/include/lib.h",
+        "/Pkg/Tests/test/TestCase.swift"
+    )
+
+    let observability = ObservabilitySystem.makeForTesting()
+    let graph = try loadPackageGraph(
+        fileSystem: fs,
+        manifests: [
+            Manifest.createRootManifest(
+                displayName: "Pkg",
+                path: "/Pkg",
+                targets: [
+                    TargetDescription(
+                        name: "app",
+                        dependencies: ["lib"],
+                        settings: [.init(tool: .swift, kind: .enableExperimentalFeature("Embedded"))]
+                    ),
+                    TargetDescription(
+                        name: "lib",
+                        dependencies: [],
+                        settings: [.init(tool: .swift, kind: .interoperabilityMode(.Cxx))]
+                    ),
+                    TargetDescription(
+                        name: "test",
+                        dependencies: ["lib"],
+                        type: .test
+                    ),
+                ]
+            ),
+        ],
+        observabilityScope: observability.topScope
+    )
+    XCTAssertNoDiagnostics(observability.diagnostics)
+
+    return (graph, fs, observability.topScope)
+}

Tests/BuildTests/CrossCompilationBuildPlanTests.swift

@@ -20,6 +20,9 @@ import class PackageModel.Manifest
 import struct PackageModel.TargetDescription
 import func SPMTestSupport.loadPackageGraph
 
+@_spi(SwiftPMInternal)
+import func SPMTestSupport.embeddedCxxInteropPackageGraph
+
 @_spi(SwiftPMInternal)
 import func SPMTestSupport.macrosPackageGraph
 
@@ -37,13 +40,12 @@ import XCTest
 
 final class CrossCompilationBuildPlanTests: XCTestCase {
     func testEmbeddedWasmTarget() throws {
-        let pkgPath = AbsolutePath("/Pkg")
-        let (graph, fs, observabilityScope) = try trivialPackageGraph(pkgRootPath: pkgPath)
+        var (graph, fs, observabilityScope) = try trivialPackageGraph(pkgRootPath: "/Pkg")
 
         let triple = try Triple("wasm32-unknown-none-wasm")
         var parameters = mockBuildParameters(targetTriple: triple)
         parameters.linkingParameters.shouldLinkStaticSwiftStdlib = true
-        let result = try BuildPlanResult(plan: BuildPlan(
+        var result = try BuildPlanResult(plan: BuildPlan(
             buildParameters: parameters,
             graph: graph,
             fileSystem: fs,
@@ -55,7 +57,34 @@ final class CrossCompilationBuildPlanTests: XCTestCase {
         result.checkTargetsCount(5)
 
         let buildPath = result.plan.productsBuildPath
-        let appBuildDescription = try result.buildProduct(for: "app")
+        var appBuildDescription = try result.buildProduct(for: "app")
+        XCTAssertEqual(
+            try appBuildDescription.linkArguments(),
+            [
+                result.plan.destinationBuildParameters.toolchain.swiftCompilerPath.pathString,
+                "-L", buildPath.pathString,
+                "-o", buildPath.appending(components: "app.wasm").pathString,
+                "-module-name", "app", "-static-stdlib", "-emit-executable",
+                "@\(buildPath.appending(components: "app.product", "Objects.LinkFileList"))",
+                "-target", triple.tripleString,
+                "-g",
+            ]
+        )
+
+        (graph, fs, observabilityScope) = try embeddedCxxInteropPackageGraph(pkgRootPath: "/Pkg")
+
+        result = try BuildPlanResult(plan: BuildPlan(
+            buildParameters: parameters,
+            graph: graph,
+            fileSystem: fs,
+            observabilityScope: observabilityScope
+        ))
+        result.checkProductsCount(2)
+        // There are two additional targets on non-Apple platforms, for test discovery and
+        // test entry point
+        result.checkTargetsCount(5)
+
+        appBuildDescription = try result.buildProduct(for: "app")
         XCTAssertEqual(
             try appBuildDescription.linkArguments(),
             [
@@ -64,6 +93,7 @@ final class CrossCompilationBuildPlanTests: XCTestCase {
                 "-o", buildPath.appending(components: "app.wasm").pathString,
                 "-module-name", "app", "-static-stdlib", "-emit-executable",
                 "@\(buildPath.appending(components: "app.product", "Objects.LinkFileList"))",
+                "-enable-experimental-feature", "Embedded",
                 "-target", triple.tripleString,
                 "-g",
             ]