Create an alternate SwiftPM build system called Swift Build

Add basic test coverage for the new build system for the supported platforms:
Linux, macOS, and Windows. Provide basic progress reporting, and an option
for "very verbose" mode so that that individual build tasks and the arguments
can be inspected for correctness.

Build a package with the new build system like this:

    swift build --build-system=swiftbuild

Enable very verbose task outputs with their flags, options, and arguments:

    swift build --build-system=swiftbuild --vv

IntegrationTests/Tests/IntegrationTests/SwiftPMTests.swift

@@ -49,6 +49,22 @@ final class SwiftPMTests: XCTestCase {
         }
     }
 
+    func testSwiftBuild() throws {
+        #if os(Linux)
+        if FileManager.default.contents(atPath: "/etc/system-release").map { String(decoding: $0, as: UTF8.self) == "Amazon Linux release 2 (Karoo)\n" } ?? false {
+            throw XCTSkip("Skipping SwiftBuild testing on Amazon Linux because of platform issues.")
+        }
+        #endif
+
+        // Test SwiftBuildSystem
+        try withTemporaryDirectory { tmpDir in
+            let packagePath = tmpDir.appending(component: "foo")
+            try localFileSystem.createDirectory(packagePath)
+            try sh(swiftPackage, "--package-path", packagePath, "init", "--type", "executable")
+            try sh(swiftBuild, "--package-path", packagePath, "--build-system", "swiftbuild")
+        }
+    }
+
     func testArchCustomization() throws {
         #if !os(macOS)
         try XCTSkip("Test requires macOS")

Package.swift

@@ -452,6 +452,13 @@ let package = Package(
                 .unsafeFlags(["-static"]),
             ]
         ),
+        .target(
+            name: "SwiftBuildSupport",
+            dependencies: [
+                "SPMBuildCore",
+                "PackageGraph",
+            ]
+        ),
         .target(
             /** High level functionality */
             name: "Workspace",
@@ -500,6 +507,7 @@ let package = Package(
                 "PackageGraph",
                 "Workspace",
                 "XCBuildSupport",
+                "SwiftBuildSupport",
             ],
             exclude: ["CMakeLists.txt"],
             swiftSettings: [
@@ -521,6 +529,7 @@ let package = Package(
                 "SourceControl",
                 "Workspace",
                 "XCBuildSupport",
+                "SwiftBuildSupport",
             ] + swiftSyntaxDependencies(["SwiftIDEUtils"]),
             exclude: ["CMakeLists.txt", "README.md"],
             swiftSettings: [
@@ -619,6 +628,7 @@ let package = Package(
                 "PackageLoading",
                 "PackageModel",
                 "XCBuildSupport",
+                "SwiftBuildSupport",
             ],
             exclude: ["CMakeLists.txt"]
         ),
@@ -698,6 +708,7 @@ let package = Package(
                 dependencies: [
                     "Build",
                     "XCBuildSupport",
+                    "SwiftBuildSupport",
                     "_InternalTestSupport"
                 ],
                 swiftSettings: [
@@ -1010,3 +1021,27 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
         .package(path: "../swift-toolchain-sqlite"),
     ]
 }
+
+if ProcessInfo.processInfo.environment["SWIFTPM_SWBUILD_FRAMEWORK"] == nil &&
+    ProcessInfo.processInfo.environment["SWIFTPM_NO_SWBUILD_DEPENDENCY"] == nil {
+
+    let swiftbuildsupport: Target = package.targets.first(where: { $0.name == "SwiftBuildSupport" } )!
+    swiftbuildsupport.dependencies += [
+        .product(name: "SwiftBuild", package: "swift-build"),
+    ]
+
+    swiftbuildsupport.dependencies += [
+        // This is here to statically link the build service in the same executable as SwiftPM
+        .product(name: "SWBBuildService", package: "swift-build"),
+    ]
+
+    if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
+        package.dependencies += [
+            .package(url: "https://github.com/swiftlang/swift-build.git", branch: relatedDependenciesBranch),
+        ]
+    } else {
+        package.dependencies += [
+            .package(path: "../swift-build"),
+        ]
+    }
+}

Sources/CMakeLists.txt

@@ -40,3 +40,4 @@ add_subdirectory(swift-test)
 add_subdirectory(SwiftSDKCommand)
 add_subdirectory(Workspace)
 add_subdirectory(XCBuildSupport)
+add_subdirectory(SwiftBuildSupport)

Sources/CoreCommands/BuildSystemSupport.swift

@@ -14,6 +14,7 @@ import Basics
 import Build
 import SPMBuildCore
 import XCBuildSupport
+import SwiftBuildSupport
 import PackageGraph
 
 import class Basics.ObservabilityScope
@@ -93,10 +94,40 @@ private struct XcodeBuildSystemFactory: BuildSystemFactory {
     }
 }
 
+private struct SwiftBuildSystemFactory: BuildSystemFactory {
+    let swiftCommandState: SwiftCommandState
+
+    func makeBuildSystem(
+        explicitProduct: String?,
+        traitConfiguration: TraitConfiguration,
+        cacheBuildManifest: Bool,
+        productsBuildParameters: BuildParameters?,
+        toolsBuildParameters: BuildParameters?,
+        packageGraphLoader: (() async throws -> ModulesGraph)?,
+        outputStream: OutputByteStream?,
+        logLevel: Diagnostic.Severity?,
+        observabilityScope: ObservabilityScope?
+    ) throws -> any BuildSystem {
+        return try SwiftBuildSystem(
+            buildParameters: productsBuildParameters ?? self.swiftCommandState.productsBuildParameters,
+            packageGraphLoader: packageGraphLoader ?? {
+                try await self.swiftCommandState.loadPackageGraph(
+                    explicitProduct: explicitProduct
+                )
+            },
+            outputStream: outputStream ?? self.swiftCommandState.outputStream,
+            logLevel: logLevel ?? self.swiftCommandState.logLevel,
+            fileSystem: self.swiftCommandState.fileSystem,
+            observabilityScope: observabilityScope ?? self.swiftCommandState.observabilityScope
+        )
+    }
+}
+
 extension SwiftCommandState {
     public var defaultBuildSystemProvider: BuildSystemProvider {
         .init(providers: [
             .native: NativeBuildSystemFactory(swiftCommandState: self),
+            .swiftbuild: SwiftBuildSystemFactory(swiftCommandState: self),
             .xcode: XcodeBuildSystemFactory(swiftCommandState: self)
         ])
     }

Sources/CoreCommands/CMakeLists.txt

@@ -19,7 +19,8 @@ target_link_libraries(CoreCommands PUBLIC
   TSCBasic
   TSCUtility
   Workspace
-  XCBuildSupport)
+  XCBuildSupport
+  SwiftBuildSupport)
 target_link_libraries(CoreCommands PRIVATE
   DriverSupport
   $<$<NOT:$<PLATFORM_ID:Darwin>>:FoundationXML>)

Sources/CoreCommands/SwiftCommandState.swift

@@ -728,7 +728,7 @@ public final class SwiftCommandState {
         toolsBuildParameters: BuildParameters? = .none,
         packageGraphLoader: (() async throws -> ModulesGraph)? = .none,
         outputStream: OutputByteStream? = .none,
-        logLevel: Basics.Diagnostic.Severity? = .none,
+        logLevel: Basics.Diagnostic.Severity? = nil,
         observabilityScope: ObservabilityScope? = .none
     ) async throws -> BuildSystem {
         guard let buildSystemProvider else {
@@ -747,7 +747,7 @@ public final class SwiftCommandState {
             toolsBuildParameters: toolsBuildParameters,
             packageGraphLoader: packageGraphLoader,
             outputStream: outputStream,
-            logLevel: logLevel,
+            logLevel: logLevel ?? self.logLevel,
             observabilityScope: observabilityScope
         )
 

Sources/PackageGraph/ResolvedPackagesStore.swift

@@ -560,7 +560,12 @@ extension ResolvedPackagesStore.ResolvedPackage {
         let location = mirrors.effective(for: pin.location)
         switch pin.kind {
         case .localSourceControl:
-            packageRef = try .localSourceControl(identity: identity, path: AbsolutePath(validating: location))
+            // FIXME investigate why local source control packages resolve to a non-local URL location
+            if !location.hasPrefix("https://") {
+                packageRef = try .localSourceControl(identity: identity, path: AbsolutePath(validating: location))
+            } else {
+                packageRef = .remoteSourceControl(identity: identity, url: SourceControlURL(location))
+            }
         case .remoteSourceControl:
             packageRef = .remoteSourceControl(identity: identity, url: SourceControlURL(location))
         case .registry:

Sources/PackageModel/SwiftLanguageVersion.swift

@@ -39,6 +39,11 @@ public struct SwiftLanguageVersion: Hashable, Sendable {
         v3, v4, v4_2, v5, v6
     ]
 
+    /// The list of supported Swift language versions for this toolchain.
+    public static let supportedSwiftLanguageVersions = [
+        v4, v4_2, v5, v6
+    ]
+
     /// The raw value of the language version.
     //
     // This should be passed as a value to Swift compiler's -swift-version flag.

Sources/SPMBuildCore/BuildSystem/BuildSystem.swift

@@ -33,7 +33,7 @@ public enum BuildSubset {
 }
 
 /// A protocol that represents a build system used by SwiftPM for all build operations. This allows factoring out the
-/// implementation details between SwiftPM's `BuildOperation` and the XCBuild backed `XCBuildSystem`.
+/// implementation details between SwiftPM's `BuildOperation` and the Swift Build backed `SwiftBuildSystem`.
 public protocol BuildSystem: Cancellable {
 
     /// The delegate used by the build system.
@@ -130,6 +130,7 @@ public struct BuildSystemProvider {
     // TODO: In the future, we may want this to be about specific capabilities of a build system rather than choosing a concrete one.
     public enum Kind: String, CaseIterable {
         case native
+        case swiftbuild
         case xcode
     }
 
@@ -172,6 +173,7 @@ extension BuildSystemProvider.Kind {
     public var usesXcodeBuildEngine: Bool {
         switch self {
             case .native: return false
+            case .swiftbuild: return false
             case .xcode: return true
         }
     }

Sources/SwiftBuildSupport/BuildSystem.swift

@@ -0,0 +1,26 @@
+import SPMBuildCore
+import PackageModel
+
+extension BuildConfiguration {
+    public var swiftbuildName: String {
+        switch self {
+        case .debug: "Debug"
+        case .release: "Release"
+        }
+    }
+}
+
+extension BuildSubset {
+    var pifTargetName: String {
+        switch self {
+        case .product(let name, _):
+            PackagePIFProjectBuilder.targetName(for: name)
+        case .target(let name, _):
+            name
+        case .allExcludingTests:
+            PIFBuilder.allExcludingTestsTargetName
+        case .allIncludingTests:
+            PIFBuilder.allIncludingTestsTargetName
+        }
+    }
+}

Sources/SwiftBuildSupport/CMakeLists.txt

@@ -0,0 +1,23 @@
+# 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 Swift project authors
+
+add_library(SwiftBuildSupport STATIC
+  PIF.swift
+  PIFBuilder.swift
+  BuildSystem.swift
+  SwiftBuildSystem.swift)
+target_link_libraries(SwiftBuildSupport PUBLIC
+  Build
+  DriverSupport
+  TSCBasic
+  TSCUtility
+  PackageGraph
+)
+
+set_target_properties(SwiftBuildSupport PROPERTIES
+  INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})