Implement -debug-module-path for implicit modules in swift-driver

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -109,12 +109,15 @@ extension Driver {
       jobNeedPathRemap = false
     }
 
-    if isPlanJobForExplicitModule && forObject && isFrontendArgSupported(.debugModulePath),
-       let explicitModulePlanner {
-      let mainModule = explicitModulePlanner.dependencyGraph.mainModule
-      let pathHandle = moduleOutputInfo.output?.outputPath ?? mainModule.modulePath.path
-      let path = VirtualPath.lookup(pathHandle)
-      try addPathOption(option: .debugModulePath, path: path, to: &commandLine, remap: jobNeedPathRemap)
+    // Add the -debug-module-path option to the compile job.
+    // If modulePathHandle is nil, this build doesn't produce a Swift module.
+    if forObject && isFrontendArgSupported(.debugModulePath),
+       var modulePathHandle = moduleOutputInfo.output?.outputPath {
+      // Recompute the module path based on the module name, because this is effectively passing the output
+      // of the module merge action which depends on the compile action.
+      let moduleBase : VirtualPath = VirtualPath.lookup(modulePathHandle).parentDirectory.appending(component: moduleOutputInfo.name)
+      modulePathHandle = try moduleBase.replacingExtension(with: .swiftModule).intern()
+      try addPathOption(option: .debugModulePath, path: VirtualPath.lookup(modulePathHandle), to: &commandLine, remap: jobNeedPathRemap)
     }
 
     // Check if dependency scanner has put the job into direct clang cc1 mode.

Tests/SwiftDriverTests/ExplicitModuleBuildTests.swift

@@ -716,10 +716,6 @@ final class ExplicitModuleBuildTests: XCTestCase {
       for job in jobs {
         XCTAssertEqual(job.outputs.count, 1)
         let outputFilePath = job.outputs[0].file
-        if job.kind == .compile && driver.isFrontendArgSupported(.debugModulePath) {
-          XCTAssertTrue(job.commandLine.contains(subsequence: ["-debug-module-path", try toPathOption("testExplicitModuleBuildJobs.swiftmodule")]))
-
-        }
 
         // Swift dependencies
         if let outputFileExtension = outputFilePath.extension,
@@ -782,6 +778,38 @@ final class ExplicitModuleBuildTests: XCTestCase {
     }
   }
 
+  /// Test the -debug-module-path option in expilicit builds.
+  func testExplicitModuleBuildDebugModulePath() throws {
+    let (stdlibPath, shimsPath, _, _) = try getDriverArtifactsForScanning()
+    try withTemporaryDirectory { path in
+      let main = path.appending(component: "testExplicitModuleBuildJobs.swift")
+      try localFileSystem.writeFileContents(main, bytes:
+        """
+        import C;
+        """
+      )
+
+      let swiftModuleInterfacesPath: AbsolutePath =
+          try testInputsPath.appending(component: "ExplicitModuleBuilds")
+                            .appending(component: "Swift")
+      let sdkArgumentsForTesting = (try? Driver.sdkArgumentsForTesting()) ?? []
+      var driver = try Driver(args: ["swiftc",
+                                     "-g",
+                                     "-I", swiftModuleInterfacesPath.nativePathString(escaped: false),
+                                     "-I", stdlibPath.nativePathString(escaped: false),
+                                     "-I", shimsPath.nativePathString(escaped: false),
+                                     "-explicit-module-build",
+                                     "-disable-implicit-concurrency-module-import",
+                                     "-disable-implicit-string-processing-module-import",
+                                     main.nativePathString(escaped: false)] + sdkArgumentsForTesting)
+      guard driver.isFrontendArgSupported(.debugModulePath) else { return }
+      let jobs = try driver.planBuild()
+      try jobs.filter { $0.kind == .compile }.forEach { job in
+          XCTAssertTrue(job.commandLine.contains(subsequence: ["-debug-module-path", try toPathOption("testExplicitModuleBuildJobs.swiftmodule")]))
+      }
+    }
+  }
+
   func testRegisterModuleDependencyFlag() throws {
     let (stdlibPath, shimsPath, _, _) = try getDriverArtifactsForScanning()
     try withTemporaryDirectory { path in
@@ -1100,24 +1128,6 @@ final class ExplicitModuleBuildTests: XCTestCase {
               let baseName = "testExplicitModuleVerifyInterfaceJobs"
               XCTAssertTrue(matchTemporary(outputFilePath, basename: baseName, fileExtension: "o") ||
                             matchTemporary(outputFilePath, basename: baseName, fileExtension: "autolink"))
-            if outputFilePath.extension == FileType.object.rawValue && driver.isFrontendArgSupported(.debugModulePath) {
-              // Check that this is an absolute path pointing to the temporary directory.
-              var found : Bool = false
-              for arg in job.commandLine {
-                if !found && arg == "-debug-module-path" {
-                  found = true
-                } else if found {
-                  if case let .path(vpath) = arg {
-                    XCTAssertTrue(vpath.isTemporary)
-                    XCTAssertTrue(vpath.extension == FileType.swiftModule.rawValue)
-                  } else {
-                    XCTFail("argument is not a path")
-                  }
-                    break
-                }
-              }
-              XCTAssertTrue(found)
-            }
             default:
               XCTFail("Unexpected module dependency build job output: \(outputFilePath)")
           }

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -4129,6 +4129,26 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+
+  func testDebugModulePath() throws {
+    do {
+      var driver = try Driver(args: ["swiftc", "-target", "arm64-unknown-macos26", "-g", "foo.swift"])
+      let plannedJobs = try driver.planBuild()
+      let compileJob = try plannedJobs.findJob(.compile)
+      // This needs to be "foo.swiftmodule", not the unmerged "foo-1.swiftmodule".
+      XCTAssertJobInvocationMatches(compileJob, .flag("-debug-module-path"), .path(try .temporary(RelativePath(validating: "foo")).replacingExtension(with: .swiftModule)))
+    }
+    // Unfortunately this depends on dsymutil.
+    #if os(macOS)
+    do {
+      var driver = try Driver(args: ["swiftc", "-target", "arm64-unknown-macos26", "-g", "foo.swift"])
+      let plannedJobs = try driver.planBuild()
+      let compileJob = try plannedJobs.findJob(.compile)
+      XCTAssertJobInvocationMatches(compileJob, .flag("-debug-module-path"), .path(try .temporary(RelativePath(validating: "foo")).replacingExtension(with: .swiftModule)))
+    }
+  #endif
+  }
+
   func testModuleWrapJob() throws {
     // FIXME: These tests will fail when run on macOS, because
     // swift-autolink-extract is not present