SwiftBuild: add scratch path symbolic links

The native build system would create the `debug` and `release` symbolic
link in the scratch path as they were the original output location before
triple support was added.

Although this is not an officially support feature, add the `debug` and
`release` symbolic links when using Swift Build to ease the transition
until a better solution is avaiable.

Relates to: #9963
Issue: rdar://175144467

Author: bkhouri

Sources/CoreCommands/SwiftCommandState.swift

@@ -1001,6 +1001,7 @@ public final class SwiftCommandState {
             }
 
         return try BuildParameters(
+            scratchPath: self.scratchDirectory,
             destination: destination,
             dataPath: dataPath,
             configuration: self.options.build.configuration ?? self.preferredBuildConfiguration,

Sources/SPMBuildCore/BuildParameters/BuildParameters.swift

@@ -47,6 +47,9 @@ public struct BuildParameters: Encodable {
         case target
     }
 
+    // The scrath path
+    package let scratchPath: Basics.AbsolutePath
+
     /// The destination these parameters are going to be used for.
     public var destination: Destination
 
@@ -161,6 +164,7 @@ public struct BuildParameters: Encodable {
     public var apiDigesterMode: APIDigesterMode?
 
     public init(
+        scratchPath: Basics.AbsolutePath,
         destination: Destination,
         dataPath: Basics.AbsolutePath,
         configuration: BuildConfiguration,
@@ -185,6 +189,8 @@ public struct BuildParameters: Encodable {
         testingParameters: Testing = .init(),
         apiDigesterMode: APIDigesterMode? = nil
     ) throws {
+        self.scratchPath = scratchPath
+
         // Default to the unversioned triple if none is provided so that we defer to the package's requested deployment target, for Darwin platforms. For other platforms, continue to include the version since those don't have the concept of a package-specified version, and the version is meaningful for some platforms including Android and FreeBSD.
         let triple = try triple ?? {
             let hostTriple = try Triple.getHostTriple(

Sources/SwiftBuildSupport/SwiftBuildSystem.swift

@@ -406,6 +406,12 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
             replArguments: nil,
         )
 
+        defer {
+            createBuildSymbolicLinks(
+                scratchPath: self.buildParameters.scratchPath,
+                buildConfig: self.buildParameters.configuration,
+            )
+        }
         guard !buildParameters.shouldSkipBuilding else {
             result.serializedDiagnosticPathsByTargetName = .failure(StringError("Building was skipped"))
             return result
@@ -424,6 +430,38 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
         )
     }
 
+    func createBuildSymbolicLinks(
+        scratchPath: Basics.AbsolutePath,
+        buildConfig: BuildConfiguration,
+    ) {
+        let oldBuildPath = scratchPath.appending(component: buildConfig.dirname)
+        if self.fileSystem.exists(oldBuildPath) {
+            do {
+                // This does not delete the directory pointed to by the symbolic link
+                try self.fileSystem.removeFileTree(oldBuildPath)
+            }
+            catch {
+                self.observabilityScope.emit(
+                    warning: "unable to delete \(oldBuildPath), skip creating symbolic link",
+                    underlyingError: error
+                )
+            }
+        }
+
+        do {
+            try self.fileSystem.createSymbolicLink(
+                oldBuildPath,
+                pointingAt: self.buildParameters.buildPath,
+                relative: true
+            )
+        } catch {
+            self.observabilityScope.emit(
+                warning: "unable to create symbolic link at \(oldBuildPath)",
+                underlyingError: error
+            )
+        }
+    }
+
     /// Compiles any plugins specified or implied by the build subset, returning
     /// true if the build should proceed. Throws an error in case of failure. A
     /// reason why the build might not proceed even on success is if only plugins

Sources/_InternalTestSupport/MockBuildTestHelper.swift

@@ -88,6 +88,7 @@ public let defaultTargetTriple: String = hostTriple.tripleString
 #endif
 
 public func mockBuildParameters(
+    scratchDirectory: AbsolutePath? = nil,
     destination: BuildParameters.Destination,
     buildPath: AbsolutePath? = nil,
     config: BuildConfiguration = .debug,
@@ -110,6 +111,7 @@ public func mockBuildParameters(
     numberOfWorkers: UInt32 = 3,
 ) -> BuildParameters {
     try! BuildParameters(
+        scratchPath: scratchDirectory ?? AbsolutePath("/path/to/scratch"),
         destination: destination,
         dataPath: buildPath ?? AbsolutePath("/path/to/build").appending(triple.tripleString),
         configuration: config,

Sources/swift-bootstrap/main.swift

@@ -311,6 +311,7 @@ struct SwiftBootstrapBuildTool: AsyncParsableCommand {
             )
 
             return try BuildParameters(
+                scratchPath: scratchDirectory,
                 destination: .target,
                 dataPath: dataPath,
                 configuration: configuration,