[CompilationCaching] Write .cas-config file

Write a .cas-config file when compilation caching is enabled. This
allows tools that might not be orchestrated by the build system directly
but might need to read CAS to find the CAS by searching for the
configuration file.

rdar://166398186

Author: cachemeifyoucan

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 +1,77 @@
+//
+//  File.swift
+//  SwiftBuild
+//
+//  Created by Steven Wu on 12/13/25.
+//
+
+//===----------------------------------------------------------------------===//
+//
+// 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 })