[CompilationCaching] Write .cas-config file

Sources/SWBTaskConstruction/CMakeLists.txt

@@ -51,6 +51,7 @@ add_library(SWBTaskConstruction
   TaskProducers/StandardTaskProducer.swift
   TaskProducers/TaskProducer.swift
   TaskProducers/TaskProducerExtensionPoint.swift
+  TaskProducers/WorkspaceTaskProducers/CASConfigFileTaskProducer.swift
   TaskProducers/WorkspaceTaskProducers/BuildDependencyInfoTaskProducer.swift
   TaskProducers/WorkspaceTaskProducers/CreateBuildDirectoryTaskProducer.swift
   TaskProducers/WorkspaceTaskProducers/HeadermapVFSTaskProducer.swift

Sources/SWBTaskConstruction/ProductPlanning/ProductPlanner.swift

@@ -82,6 +82,7 @@ private struct WorkspaceProductPlanBuilder {
             HeadermapVFSTaskProducer(context: globalTaskProducerContext, targetContexts: targetContexts),
             PCHModuleMapTaskProducer(context: globalTaskProducerContext, targetContexts: targetContexts),
             BuildDependencyInfoTaskProducer(context: globalTaskProducerContext, targetContexts: targetContexts),
+            CASConfigFileTaskProducer(context: globalTaskProducerContext, targetContexts: targetContexts),
         ] + (globalProductPlan.planRequest.buildRequest.enableIndexBuildArena ? [IndexBuildVFSDirectoryRemapTaskProducer(context: globalTaskProducerContext)] : [])
 
         for taskProducerExtension in await taskProducerExtensions(globalTaskProducerContext.workspaceContext) {

Sources/SWBTaskConstruction/TaskProducers/WorkspaceTaskProducers/CASConfigFileTaskProducer.swift

@@ -0,0 +1,70 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 SWBCore
+import SWBUtil
+import SWBMacro
+import Foundation
+import SWBProtocol
+
+final class CASConfigFileTaskProducer: StandardTaskProducer, TaskProducer {
+    private let targetContexts: [TaskProducerContext]
+
+    init(context globalContext: TaskProducerContext, targetContexts: [TaskProducerContext]) {
+        self.targetContexts = targetContexts
+        super.init(globalContext)
+    }
+
+    func generateTasks() async -> [any SWBCore.PlannedTask] {
+        var tasks = [any PlannedTask]()
+        do {
+            let casConfigFiles = try Dictionary(try await targetContexts.concurrentMap(maximumParallelism: 100) { (targetContext: TaskProducerContext) async throws -> (Path, ByteString)? in
+                let scope = targetContext.settings.globalScope
+
+                // If compilation caching is not on, then there is no file to write.
+                // The condition here is more relax than the actual check in the compile task generation
+                // since it won't hurt if the file is not used.
+                guard scope.evaluate(BuiltinMacros.CLANG_ENABLE_COMPILE_CACHE) || scope.evaluate(BuiltinMacros.SWIFT_ENABLE_COMPILE_CACHE) else {
+                    return nil
+                }
+
+                // FIXME: we need consistent CAS configuration across all languages.
+                if !scope.evaluate(BuiltinMacros.COMPILATION_CACHE_REMOTE_SERVICE_PATH).isEmpty && !scope.evaluate(BuiltinMacros.COMPILATION_CACHE_REMOTE_SUPPORTED_LANGUAGES).isEmpty {
+                    return nil
+                }
+
+                let casOpts = try CASOptions.create(scope, .compiler(.other(dialectName: "swift")))
+                struct CASConfig: Encodable {
+                    let CASPath: String
+                    let PluginPath: String?
+                }
+                let content = try JSONEncoder().encode(CASConfig(CASPath: casOpts.casPath.str, PluginPath: casOpts.pluginPath?.str))
+                let path = scope.evaluate(BuiltinMacros.TARGET_TEMP_DIR).join(".cas-config")
+                return (path, ByteString(content))
+            }.compactMap { $0 }, uniquingKeysWith: { first, second in
+                guard first == second else {
+                    throw StubError.error("Unexpected difference in CAS config file.\nPath: \(first.asString)\nContent:\(second.asString)")
+                }
+                return first
+            })
+
+            for (configFilePath, configFileContent) in casConfigFiles {
+                await appendGeneratedTasks(&tasks) { delegate in
+                    context.writeFileSpec.constructFileTasks(CommandBuildContext(producer: context, scope: context.settings.globalScope, inputs: [], output: configFilePath), delegate, contents: configFileContent, permissions: nil, preparesForIndexing: true, additionalTaskOrderingOptions: [.immediate])
+                }
+            }
+        } catch {
+            self.context.error(error.localizedDescription)
+        }
+        return tasks
+    }
+}

Tests/SWBBuildSystemTests/ClangCompilationCachingTests.swift

@@ -246,6 +246,13 @@ fileprivate struct ClangCompilationCachingTests: CoreBasedTests {
             }
             #expect(try readMetrics("one") == #"{"global":{"clangCacheHits":0,"clangCacheMisses":1,"swiftCacheHits":0,"swiftCacheMisses":0},"tasks":{"CompileC":{"cacheMisses":1,"headerDependenciesNotValidatedTasks":1,"moduleDependenciesNotValidatedTasks":1}}}"#)
 
+            let CASConfigPath = tmpDirPath.join("Test/aProject/build/aProject.build/Debug\(runDestination == .macOS ? "": "-" + runDestination.platform)/Library.build/.cas-config")
+
+            #expect(try tester.fs.read(CASConfigPath).asString.contains("\"CASPath\":"))
+            if usePlugin {
+                #expect(try tester.fs.read(CASConfigPath).asString.contains("\"PluginPath\":"))
+            }
+
             // Touch the source file to trigger a new scan.
             try await tester.fs.updateTimestamp(testWorkspace.sourceRoot.join("aProject/file.c"))
 

Tests/SWBBuildSystemTests/SwiftCompilationCachingTests.swift

@@ -116,6 +116,8 @@ fileprivate struct SwiftCompilationCachingTests: CoreBasedTests {
             }
             #expect(try readMetrics("one").contains("\"swiftCacheHits\":0,\"swiftCacheMisses\":\(numCompile)"))
 
+            #expect(try tester.fs.read(tmpDirPath.join("Test/aProject/build/aProject.build/Debug-iphoneos/Application.build/.cas-config")).asString.contains("\"CASPath\":"))
+
             // touch a file, clean build folder, and rebuild.
             try await tester.fs.updateTimestamp(testWorkspace.sourceRoot.join("aProject/App.swift"))
             try await tester.checkBuild(runDestination: .anyiOSDevice, buildCommand: .cleanBuildFolder(style: .regular), body: { _ in })