Statically linking Swift Stdlib in SwiftBuild if set

Sources/SwiftBuildSupport/Diagnostics+Extensions.swift

@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+
+extension Basics.Diagnostic {
+    package static var swiftBackDeployWarning: Self {
+        .warning(
+            """
+            Swift compiler no longer supports statically linking the Swift libraries. They're included in the OS by \
+            default starting with macOS Mojave 10.14.4 beta 3. For macOS Mojave 10.14.3 and earlier, there's an \
+            optional "Swift 5 Runtime Support for Command Line Tools" package that can be downloaded from \"More Downloads\" \
+            for Apple Developers at https://developer.apple.com/download/more/
+            """
+        )
+    }
+}

Sources/SwiftBuildSupport/SwiftBuildSystem.swift

@@ -1055,7 +1055,7 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
         }
         try settings.merge(Self.constructDebuggingSettingsOverrides(from: buildParameters.debuggingParameters), uniquingKeysWith: reportConflict)
         try settings.merge(Self.constructDriverSettingsOverrides(from: buildParameters.driverParameters), uniquingKeysWith: reportConflict)
-        try settings.merge(Self.constructLinkerSettingsOverrides(from: buildParameters.linkingParameters), uniquingKeysWith: reportConflict)
+        try settings.merge(self.constructLinkerSettingsOverrides(from: buildParameters.linkingParameters, triple: buildParameters.triple), uniquingKeysWith: reportConflict)
         try settings.merge(Self.constructTestingSettingsOverrides(from: buildParameters.testingParameters), uniquingKeysWith: reportConflict)
         try settings.merge(Self.constructAPIDigesterSettingsOverrides(from: buildParameters.apiDigesterMode), uniquingKeysWith: reportConflict)
 
@@ -1142,7 +1142,10 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
         return settings
     }
 
-    private static func constructLinkerSettingsOverrides(from parameters: BuildParameters.Linking) -> [String: String] {
+    private func constructLinkerSettingsOverrides(
+        from parameters: BuildParameters.Linking,
+        triple: Triple,
+    ) -> [String: String] {
         var settings: [String: String] = [:]
 
         if parameters.linkerDeadStrip {
@@ -1160,7 +1163,19 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
             break
         }
 
-        // TODO: shouldLinkStaticSwiftStdlib
+        if triple.isDarwin() && parameters.shouldLinkStaticSwiftStdlib {
+            self.observabilityScope.emit(.swiftBackDeployWarning)
+        } else {
+            if parameters.shouldLinkStaticSwiftStdlib {
+                settings["SWIFT_FORCE_STATIC_LINK_STDLIB"] = "YES"
+            } else {
+                settings["SWIFT_FORCE_STATIC_LINK_STDLIB"] = "NO"
+            }
+        }
+
+        if let resourcesPath = self.buildParameters.toolchain.swiftResourcesPath(isStatic: parameters.shouldLinkStaticSwiftStdlib) {
+            settings["SWIFT_RESOURCE_DIR"] = resourcesPath.pathString
+        }
 
         return settings
     }

Sources/_InternalTestSupport/SwiftTesting+Tags.swift

@@ -34,6 +34,7 @@ extension Tag.FunctionalArea {
     @Tag public static var PIF: Tag
     @Tag public static var IndexMode: Tag
     @Tag public static var Sanitizer: Tag
+    @Tag public static var LinkSwiftStaticStdlib: Tag
 }
 
 extension Tag.Feature {

Tests/SwiftBuildSupportTests/SwiftBuildSystemTests.swift

@@ -197,6 +197,87 @@ struct SwiftBuildSystemTests {
         }
     }
 
+    @Suite(
+        .tags(
+            .FunctionalArea.LinkSwiftStaticStdlib,
+        ),
+    )
+    struct SwiftStaticStdlibSettingTests {
+        @Test
+        func makingBuildParametersRaisesAWarningWhenRunOnDarwin() async throws {
+            // GIVEN we have a Darwin triple
+            let triple = try Triple("x86_64-apple-macosx")
+            // AND we want to statically link Swift sdtlib
+            let shouldLinkStaticSwiftStdlib = true
+            try await withInstantiatedSwiftBuildSystem(
+                fromFixture: "PIFBuilder/Simple",
+                buildParameters: mockBuildParameters(
+                    destination: .host,
+                    shouldLinkStaticSwiftStdlib: shouldLinkStaticSwiftStdlib,
+                    triple: triple,
+                ),
+            ) { swiftBuild, session, observabilityScope, buildParameters in
+                // WHEN we make the build parameter
+                let _: SWBBuildParameters = try await swiftBuild.makeBuildParameters(
+                    session: session,
+                    symbolGraphOptions: nil,
+                    setToolchainSetting: false, // Set this to false as SwiftBuild checks the toolchain path
+                )
+
+                // THEN we expect a warning to be emitted
+                let warnings = observabilityScope.diagnostics.filter {
+                    $0.severity == .warning
+                }
+                #expect(warnings.count == 1)
+
+                let diagnostic = try #require(warnings.first)
+                // AND we expect the diagnostic message, severity and description to be as expected
+                #expect(diagnostic.message == Basics.Diagnostic.swiftBackDeployWarning.message)
+                #expect(diagnostic.severity == Basics.Diagnostic.swiftBackDeployWarning.severity)
+                #expect(diagnostic.description == Basics.Diagnostic.swiftBackDeployWarning.description)
+            }
+        }
+
+        @Test(
+            arguments: [
+                (shouldLinkStaticSwiftStdlib: true, expectedValue: "YES"),
+                (shouldLinkStaticSwiftStdlib: false, expectedValue: "NO"),
+            ]
+        )
+        func swiftStaticStdLibSettingIsSetCorrectly(
+            shouldLinkStaticSwiftStdlib: Bool,
+            expectedValue: String
+        ) async throws {
+            // GIVEN we have a non-darwin triple AND we want statically link Swift sdtlib or not
+            let nonDarwinTriple = try Triple("i686-pc-windows-cygnus")
+            try await withInstantiatedSwiftBuildSystem(
+                fromFixture: "PIFBuilder/Simple",
+                buildParameters: mockBuildParameters(
+                    destination: .host,
+                    shouldLinkStaticSwiftStdlib: shouldLinkStaticSwiftStdlib,
+                    triple: nonDarwinTriple,
+                ),
+            ) { swiftBuild, session, observabilityScope, buildParameters in
+                // WHEN we make the build parameter
+                let buildSettings = try await swiftBuild.makeBuildParameters(
+                    session: session,
+                    symbolGraphOptions: nil,
+                    setToolchainSetting: false, // Set this to false as SwiftBuild checks the toolchain path
+                )
+
+                // THEN we don't expect any warnings to be emitted
+                let warnings = observabilityScope.diagnostics.filter {
+                    $0.severity == .warning
+                }
+                #expect(warnings.isEmpty)
+
+                // AND we expect the build setting to be set correctly
+                let synthesizedArgs = try #require(buildSettings.overrides.synthesized)
+                #expect(synthesizedArgs.table["SWIFT_FORCE_STATIC_LINK_STDLIB"] == expectedValue)
+            }
+        }
+    }
+
     @Test(
         arguments: BuildParameters.IndexStoreMode.allCases,
         // arguments: [BuildParameters.IndexStoreMode.on],