Swift/Ninja: Split compilation model

Splitting the Swift build into an object build and a separate link step,
instead of building and linking in one step. The immediate benefit is
LSP support because we are able to emit compile-commands for Swift files
now. Additionally, it is possible to specify flags to the compile step,
enabling folks to emit C and C++ headers from their Swift builds for
C/C++ interop, without needing custom commands. Eventually, this gives
us a path toward working object libraries.

Object Libraries:
 - Object libraries don't work today because CMake doesn't emit targets
   for object libraries into the Ninja build file.
 - tl;dr: Object libraries work if they aren't WMO. Still need work to
   make WMO'd object libraries work.

   Object libraries still don't completely work with this patch because,
   while we emit the targets, the `TARGET_OBJECTS` generator expression
   expansion has a separate mechanism for determining what the names of
   the objects are based on the input source files, so targets that
   depend on an object library built with a whole-module optimization
   will depend on objects based on the name of the source file instead
   of the actual emitted object file.

These features require being able to accurately model wholemodule builds
though, because we actually need to track object files and WMO affects
what objects are emitted. For that, we require CMP0157 use the NEW
policy. When it's OLD, we have to fall back on the old behavior and
cannot provide object libraries or the compile-commands for LSP.

Issue: #25308

Modules/CMakeSwiftInformation.cmake

@@ -116,36 +116,66 @@ endif()
 
 cmake_initialize_per_config_variable(CMAKE_Swift_FLAGS "Swift Compiler Flags")
 
-# NOTE(compnerd) we do not have an object compile rule since we build the objects as part of the link step
-if(NOT CMAKE_Swift_COMPILE_OBJECT)
-  set(CMAKE_Swift_COMPILE_OBJECT ":")
-endif()
-
 if(NOT CMAKE_Swift_NUM_THREADS MATCHES "^[0-9]+$")
   cmake_host_system_information(RESULT CMAKE_Swift_NUM_THREADS QUERY NUMBER_OF_LOGICAL_CORES)
 endif()
 
-if(NOT CMAKE_Swift_CREATE_SHARED_LIBRARY)
-  set(CMAKE_Swift_CREATE_SHARED_LIBRARY "<CMAKE_Swift_COMPILER> -j ${CMAKE_Swift_NUM_THREADS} -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <SONAME_FLAG> <TARGET_INSTALLNAME_DIR><TARGET_SONAME> ${CMAKE_Swift_IMPLIB_LINKER_FLAGS} <LINK_LIBRARIES>")
-endif()
+# Swift split-compilation requires CMP0157 NEW policy
+if(CMAKE_Swift_COMPILATION_MODE_DEFAULT)
+  set(CMAKE_Swift_PARALLEL_FLAGS "-j ${CMAKE_Swift_NUM_THREADS} -num-threads ${CMAKE_Swift_NUM_THREADS}")
+  if(NOT CMAKE_Swift_COMPILE_OBJECT)
+    # Omit the object output. The output is controlled by the output-file-map
+    # for normal builds. For wholemodule builds, CMake appends the appropriate
+    # flags.
+    set(CMAKE_Swift_COMPILE_OBJECT "<CMAKE_Swift_COMPILER> ${CMAKE_Swift_PARALLEL_FLAGS} -c <DEFINES> <FLAGS> <INCLUDES> <SOURCE>")
+  endif()
 
-if(NOT CMAKE_Swift_CREATE_SHARED_MODULE)
-  set(CMAKE_Swift_CREATE_SHARED_MODULE ${CMAKE_Swift_CREATE_SHARED_LIBRARY})
-endif()
+  if(NOT CMAKE_Swift_CREATE_SHARED_LIBRARY)
+    set(CMAKE_Swift_CREATE_SHARED_LIBRARY "<CMAKE_Swift_COMPILER> ${CMAKE_Swift_PARALLEL_FLAGS} -emit-library <CMAKE_SHARED_LIBRARY_Swift_FLAGS> <LANGUAGE_COMPILE_FLAGS> <LINK_FLAGS> <SONAME_FLAG> <TARGET_SONAME> -o <TARGET> <OBJECTS> <LINK_LIBRARIES>")
+  endif()
 
-if(NOT CMAKE_Swift_LINK_EXECUTABLE)
-  set(CMAKE_Swift_LINK_EXECUTABLE "<CMAKE_Swift_COMPILER> -j ${CMAKE_Swift_NUM_THREADS} -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-executable -o <TARGET> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
-endif()
+  if(NOT CMAKE_Swift_CREATE_SHARED_MODULE)
+    set(CMAKE_Swift_CREATE_SHARED_MODULE ${CMAKE_Swift_CREATE_SHARED_LIBRARY})
+  endif()
 
-if(NOT CMAKE_Swift_LINK_EXECUTABLE_WITH_EXPORTS)
-  set(CMAKE_Swift_LINK_EXECUTABLE_WITH_EXPORTS "${CMAKE_Swift_LINK_EXECUTABLE} -emit-module -emit-module-path <SWIFT_MODULE> ${CMAKE_Swift_IMPLIB_LINKER_FLAGS}")
-endif()
+  if(NOT CMAKE_Swift_LINK_EXECUTABLE)
+    set(CMAKE_Swift_LINK_EXECUTABLE "<CMAKE_Swift_COMPILER> ${CMAKE_Swift_PARALLEL_FLAGS} -emit-executable -o <TARGET> <FLAGS> <OBJECTS> <LINK_FLAGS> <LINK_LIBRARIES>")
+  endif()
 
-if(NOT CMAKE_Swift_CREATE_STATIC_LIBRARY)
-  set(CMAKE_Swift_CREATE_STATIC_LIBRARY "<CMAKE_Swift_COMPILER> -j ${CMAKE_Swift_NUM_THREADS} -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -static -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+  if(NOT CMAKE_Swift_CREATE_STATIC_LIBRARY)
+    set(CMAKE_Swift_CREATE_STATIC_LIBRARY "<CMAKE_Swift_COMPILER> -emit-library -static -o <TARGET> <OBJECTS> <LINK_FLAGS>")
+    set(CMAKE_Swift_ARCHIVE_CREATE "<CMAKE_AR> crs <TARGET> <OBJECTS>")
+    set(CMAKE_Swift_ARCHIVE_FINISH "")
+  endif()
+  unset(CMAKE_Swift_PARALLEL_FLAGS)
+else()
+  # NOTE(compnerd) we do not have an object compile rule since we build the objects as part of the link step
+  if(NOT CMAKE_Swift_COMPILE_OBJECT)
+    set(CMAKE_Swift_COMPILE_OBJECT ":")
+  endif()
+
+  if(NOT CMAKE_Swift_CREATE_SHARED_LIBRARY)
+    set(CMAKE_Swift_CREATE_SHARED_LIBRARY "<CMAKE_Swift_COMPILER> -j ${CMAKE_Swift_NUM_THREADS} -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <SONAME_FLAG> <TARGET_INSTALLNAME_DIR><TARGET_SONAME> ${CMAKE_Swift_IMPLIB_LINKER_FLAGS} <LINK_LIBRARIES>")
+  endif()
 
-  set(CMAKE_Swift_ARCHIVE_CREATE "<CMAKE_AR> crs <TARGET> <OBJECTS>")
-  set(CMAKE_Swift_ARCHIVE_FINISH "")
+  if(NOT CMAKE_Swift_CREATE_SHARED_MODULE)
+    set(CMAKE_Swift_CREATE_SHARED_MODULE ${CMAKE_Swift_CREATE_SHARED_LIBRARY})
+  endif()
+
+  if(NOT CMAKE_Swift_LINK_EXECUTABLE)
+    set(CMAKE_Swift_LINK_EXECUTABLE "<CMAKE_Swift_COMPILER> -j ${CMAKE_Swift_NUM_THREADS} -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-executable -o <TARGET> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+  endif()
+
+  if(NOT CMAKE_Swift_LINK_EXECUTABLE_WITH_EXPORTS)
+    set(CMAKE_Swift_LINK_EXECUTABLE_WITH_EXPORTS "${CMAKE_Swift_LINK_EXECUTABLE} -emit-module -emit-module-path <SWIFT_MODULE> ${CMAKE_Swift_IMPLIB_LINKER_FLAGS}")
+  endif()
+
+  if(NOT CMAKE_Swift_CREATE_STATIC_LIBRARY)
+    set(CMAKE_Swift_CREATE_STATIC_LIBRARY "<CMAKE_Swift_COMPILER> -j ${CMAKE_Swift_NUM_THREADS} -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -static -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+
+    set(CMAKE_Swift_ARCHIVE_CREATE "<CMAKE_AR> crs <TARGET> <OBJECTS>")
+    set(CMAKE_Swift_ARCHIVE_FINISH "")
+  endif()
 endif()
 
 set(CMAKE_Swift_INFORMATION_LOADED 1)

Source/cmLocalGenerator.cxx

@@ -1366,7 +1366,7 @@ std::vector<BT<std::string>> cmLocalGenerator::GetStaticLibraryFlags(
 {
   const std::string configUpper = cmSystemTools::UpperCase(config);
   std::vector<BT<std::string>> flags;
-  if (linkLanguage != "Swift") {
+  if (linkLanguage != "Swift" && !this->IsSplitSwiftBuild()) {
     std::string staticLibFlags;
     this->AppendFlags(
       staticLibFlags,
@@ -3016,6 +3016,12 @@ cm::optional<cmSwiftCompileMode> cmLocalGenerator::GetSwiftCompileMode(
   return cmSwiftCompileMode::Unknown;
 }
 
+bool cmLocalGenerator::IsSplitSwiftBuild() const
+{
+  return cmNonempty(this->GetMakefile()->GetDefinition(
+    "CMAKE_Swift_COMPILATION_MODE_DEFAULT"));
+}
+
 namespace {
 
 inline void RegisterUnitySources(cmGeneratorTarget* target, cmSourceFile* sf,

Source/cmLocalGenerator.h

@@ -562,6 +562,10 @@ public:
   cm::optional<cmSwiftCompileMode> GetSwiftCompileMode(
     cmGeneratorTarget const* target, std::string const& config);
 
+  // Can we build Swift with a separate object build and link step
+  // (If CMP0157 is NEW, we can do a split build)
+  bool IsSplitSwiftBuild() const;
+
 protected:
   // The default implementation converts to a Windows shortpath to
   // help older toolchains handle spaces and such.  A generator may

Source/cmNinjaNormalTargetGenerator.cxx

@@ -1209,7 +1209,10 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement(
     globalGen->GetByproductsForCleanTarget(config).push_back(targetOutputReal);
   }
 
-  if (this->TargetLinkLanguage(config) == "Swift") {
+  // If we can't split the Swift build model (CMP0157 is OLD or unset), fall
+  // back on the old one-step "build/link" logic.
+  if (!this->GetLocalGenerator()->IsSplitSwiftBuild() &&
+      this->TargetLinkLanguage(config) == "Swift") {
     vars["SWIFT_LIBRARY_NAME"] = [this, config]() -> std::string {
       cmGeneratorTarget::Names targetNames =
         this->GetGeneratorTarget()->GetLibraryNames(config);
@@ -1222,12 +1225,12 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement(
       cmOutputConverter::SHELL);
 
     vars["SWIFT_SOURCES"] = [this, config]() -> std::string {
-      std::vector<cmSourceFile const*> sources;
+      std::vector<cmSourceFile const*> sourceFiles;
       std::stringstream oss;
 
-      this->GetGeneratorTarget()->GetObjectSources(sources, config);
+      this->GetGeneratorTarget()->GetObjectSources(sourceFiles, config);
       cmLocalGenerator const* LocalGen = this->GetLocalGenerator();
-      for (const auto& source : sources) {
+      for (const auto& source : sourceFiles) {
         const std::string sourcePath = source->GetLanguage() == "Swift"
           ? this->GetCompiledSourceNinjaPath(source)
           : this->GetObjectFilePath(source, config);
@@ -1245,10 +1248,8 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement(
     vars["FLAGS"] = this->GetFlags("Swift", config);
     vars["INCLUDES"] = this->GetIncludes("Swift", config);
     this->GenerateSwiftOutputFileMap(config, vars["FLAGS"]);
-  }
 
-  // Compute specific libraries to link with.
-  if (this->TargetLinkLanguage(config) == "Swift") {
+    // Compute specific libraries to link with.
     std::vector<cmSourceFile const*> sources;
     gt->GetObjectSources(sources, config);
     for (const auto& source : sources) {

Source/cmNinjaTargetGenerator.cxx

@@ -14,6 +14,7 @@
 #include <utility>
 
 #include <cm/memory>
+#include <cm/optional>
 #include <cm/string_view>
 #include <cmext/algorithm>
 #include <cmext/string_view>
@@ -31,6 +32,7 @@
 #include "cmGlobalCommonGenerator.h"
 #include "cmGlobalNinjaGenerator.h"
 #include "cmList.h"
+#include "cmListFileCache.h"
 #include "cmLocalGenerator.h"
 #include "cmLocalNinjaGenerator.h"
 #include "cmMakefile.h"
@@ -47,6 +49,7 @@
 #include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 #include "cmTarget.h"
+#include "cmTargetDepend.h"
 #include "cmValue.h"
 #include "cmake.h"
 
@@ -145,7 +148,7 @@ std::string cmNinjaTargetGenerator::LanguageScanRule(
 bool cmNinjaTargetGenerator::NeedExplicitPreprocessing(
   std::string const& lang) const
 {
-  return lang == "Fortran";
+  return lang == "Fortran" || lang == "Swift";
 }
 
 bool cmNinjaTargetGenerator::CompileWithDefines(std::string const& lang) const
@@ -1136,9 +1139,18 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements(
     std::vector<cmSourceFile const*> objectSources;
     this->GeneratorTarget->GetObjectSources(objectSources, config);
 
+    std::vector<cmSourceFile const*> swiftSources;
+
     for (cmSourceFile const* sf : objectSources) {
-      this->WriteObjectBuildStatement(sf, config, fileConfig, firstForConfig);
+      if (sf->GetLanguage() == "Swift") {
+        swiftSources.push_back(sf);
+      } else {
+        this->WriteObjectBuildStatement(sf, config, fileConfig,
+                                        firstForConfig);
+      }
     }
+    WriteSwiftObjectBuildStatement(swiftSources, config, fileConfig,
+                                   firstForConfig);
   }
 
   {
@@ -1234,9 +1246,11 @@ void cmNinjaTargetGenerator::GenerateSwiftOutputFileMap(
     if (cmValue name = target->GetProperty("Swift_DEPENDENCIES_FILE")) {
       return *name;
     }
-    return this->ConvertToNinjaPath(cmStrCat(target->GetSupportDirectory(),
-                                             '/', config, '/',
-                                             target->GetName(), ".swiftdeps"));
+    return this->GetLocalGenerator()->ConvertToOutputFormat(
+      this->ConvertToNinjaPath(cmStrCat(target->GetSupportDirectory(), '/',
+                                        config, '/', target->GetName(),
+                                        ".swiftdeps")),
+      cmOutputConverter::SHELL);
   }();
 
   std::string mapFilePath =
@@ -1254,8 +1268,10 @@ void cmNinjaTargetGenerator::GenerateSwiftOutputFileMap(
 
   // Add flag
   this->LocalGenerator->AppendFlags(flags, "-output-file-map");
-  this->LocalGenerator->AppendFlagEscape(flags,
-                                         ConvertToNinjaPath(mapFilePath));
+  this->LocalGenerator->AppendFlagEscape(
+    flags,
+    this->GetLocalGenerator()->ConvertToOutputFormat(
+      ConvertToNinjaPath(mapFilePath), cmOutputConverter::SHELL));
 }
 
 namespace {
@@ -1862,6 +1878,214 @@ void cmNinjaTargetGenerator::WriteCxxModuleBmiBuildStatement(
                                          bmiBuild, commandLineLengthLimit);
 }
 
+void cmNinjaTargetGenerator::WriteSwiftObjectBuildStatement(
+  std::vector<cmSourceFile const*> const& sources, std::string const& config,
+  std::string const& fileConfig, bool firstForConfig)
+{
+  // Swift sources are compiled as a module, not individually like with C/C++.
+  // Flags, header search paths, and definitions are passed to the entire
+  // module build, but we still need to emit compile-commands for each source
+  // file in order to support CMAKE_EXPORT_COMPILE_COMMANDS.
+  // In whole-module mode, with a single thread, the Swift compiler will
+  // only emit a single object file, but if more than one thread is specified,
+  // or building in other modes, the compiler will emit multiple object files.
+  // When building a single-output, we do not provide an output-file-map (OFM),
+  // and instead pass `-o` to tell the compiler where to write the object.
+  // When building multiple outputs, we provide an OFM to tell the compiler
+  // where to put each object.
+  //
+  //
+  // Per-Target (module):
+  //  - Flags
+  //  - Definitions
+  //  - Include paths
+  //  - (single-output) output object filename
+  //  - Swiftmodule
+  //
+  //  Per-File:
+  //  - compile-command
+  //  - (multi-output) OFM data
+  //  - (multi-output) output object filename
+  //
+  //  Note: Due to the differences in the build models, we are only able to
+  //  build the object build-graph if we know what mode the target is built in.
+  //  For that, we need the "NEW" behavior for CMP0157. Otherwise, we have to
+  //  fall back on the old "linker" build. Otherwise, this should be
+  //  indistinguishable from the old behavior.
+  //
+  //  FIXME(#25490): Add response file support to Swift object build step
+  //  FIXME(#25491): Include all files in module in compile_commands.json
+
+  if (sources.empty()) {
+    return;
+  }
+
+  cmSwiftCompileMode compileMode;
+  if (cm::optional<cmSwiftCompileMode> optionalCompileMode =
+        this->LocalGenerator->GetSwiftCompileMode(this->GeneratorTarget,
+                                                  config)) {
+    compileMode = *optionalCompileMode;
+  } else {
+    // CMP0157 is not NEW, bailing early!
+    return;
+  }
+
+  auto getTargetPropertyOrDefault =
+    [](cmGeneratorTarget const& target, std::string const& property,
+       std::string defaultValue) -> std::string {
+    if (cmValue value = target.GetProperty(property)) {
+      return *value;
+    }
+    return defaultValue;
+  };
+
+  std::string const language = "Swift";
+  std::string const objectDir = this->ConvertToNinjaPath(
+    cmStrCat(this->GeneratorTarget->GetSupportDirectory(),
+             this->GetGlobalGenerator()->ConfigDirectory(config)));
+
+  cmGeneratorTarget const& target = *this->GeneratorTarget;
+  cmNinjaBuild objBuild(
+    this->LanguageCompilerRule(language, config, WithScanning::No));
+  cmNinjaVars& vars = objBuild.Variables;
+
+  std::string const moduleName =
+    getTargetPropertyOrDefault(target, "Swift_MODULE_NAME", target.GetName());
+  std::string const moduleDirectory = getTargetPropertyOrDefault(
+    target, "Swift_MODULE_DIRECTORY",
+    target.LocalGenerator->GetCurrentBinaryDirectory());
+  std::string const moduleFilename = getTargetPropertyOrDefault(
+    target, "Swift_MODULE", cmStrCat(moduleName, ".swiftmodule"));
+  std::string const moduleFilepath =
+    this->ConvertToNinjaPath(cmStrCat(moduleDirectory, '/', moduleFilename));
+
+  bool const isSingleOutput = [this, compileMode]() -> bool {
+    bool isMultiThread = false;
+    if (cmValue numThreadStr =
+          this->GetMakefile()->GetDefinition("CMAKE_Swift_NUM_THREADS")) {
+      unsigned long numThreads;
+      cmStrToULong(*numThreadStr, &numThreads);
+      // numThreads == 1 is multi-threaded according to swiftc
+      isMultiThread = numThreads > 0;
+    }
+    return !isMultiThread && compileMode == cmSwiftCompileMode::Wholemodule;
+  }();
+
+  // Swift modules only make sense to emit from things that can be imported.
+  // Executables that don't export symbols can't be imported, so don't try to
+  // emit a swiftmodule for them. It will break.
+  if (target.GetType() != cmStateEnums::EXECUTABLE ||
+      target.IsExecutableWithExports()) {
+    std::string const emitModuleFlag = "-emit-module";
+    std::string const modulePathFlag = "-emit-module-path";
+    this->LocalGenerator->AppendFlags(
+      vars["FLAGS"], { emitModuleFlag, modulePathFlag, moduleFilepath });
+    objBuild.Outputs.push_back(moduleFilepath);
+
+    std::string const moduleNameFlag = "-module-name";
+    this->LocalGenerator->AppendFlags(
+      vars["FLAGS"], cmStrCat(moduleNameFlag, ' ', moduleName));
+  }
+
+  if (target.GetType() != cmStateEnums::EXECUTABLE) {
+    std::string const libraryLinkNameFlag = "-module-link-name";
+    std::string const libraryLinkName =
+      this->GetGeneratorTarget()->GetLibraryNames(config).Base;
+    this->LocalGenerator->AppendFlags(
+      vars["FLAGS"], cmStrCat(libraryLinkNameFlag, ' ', libraryLinkName));
+  }
+
+  // Without `-emit-library` or `-emit-executable`, targets with a single
+  // source file parse as a Swift script instead of like normal source. For
+  // non-executable targets, append this to ensure that they are parsed like a
+  // normal source.
+  if (target.GetType() != cmStateEnums::EXECUTABLE) {
+    this->LocalGenerator->AppendFlags(vars["FLAGS"], "-parse-as-library");
+  }
+
+  this->LocalGenerator->AppendFlags(vars["FLAGS"],
+                                    this->GetFlags(language, config));
+  vars["DEFINES"] = this->GetDefines(language, config);
+  vars["INCLUDES"] = this->GetIncludes(language, config);
+
+  // target-level object filename
+  std::string const targetObjectFilename = this->ConvertToNinjaPath(cmStrCat(
+    objectDir, '/', moduleName,
+    this->GetGlobalGenerator()->GetLanguageOutputExtension(language)));
+
+  if (isSingleOutput) {
+    this->LocalGenerator->AppendFlags(vars["FLAGS"],
+                                      cmStrCat("-o ", targetObjectFilename));
+    objBuild.Outputs.push_back(targetObjectFilename);
+    this->Configs[config].Objects.push_back(targetObjectFilename);
+  }
+
+  for (cmSourceFile const* sf : sources) {
+    // Add dependency to object build on each source file
+    std::string const sourceFilePath = this->GetCompiledSourceNinjaPath(sf);
+    objBuild.ExplicitDeps.push_back(sourceFilePath);
+
+    if (isSingleOutput) {
+      if (firstForConfig) {
+        this->ExportObjectCompileCommand(
+          language, sourceFilePath, objectDir, targetObjectFilename,
+          cmSystemTools::GetFilenamePath(targetObjectFilename), vars["FLAGS"],
+          vars["DEFINES"], vars["INCLUDES"],
+          /*compile pdb*/ "", /*target pdb*/ "", config);
+      }
+    } else {
+      // Object outputs
+      std::string const objectFilepath =
+        this->ConvertToNinjaPath(this->GetObjectFilePath(sf, config));
+      this->EnsureParentDirectoryExists(objectFilepath);
+      objBuild.Outputs.push_back(objectFilepath);
+      this->Configs[config].Objects.push_back(objectFilepath);
+
+      // Add OFM data
+      this->EmitSwiftDependencyInfo(sf, config);
+
+      // Emit compile commands
+      if (firstForConfig) {
+        this->ExportObjectCompileCommand(
+          language, sourceFilePath, objectDir, objectFilepath,
+          cmSystemTools::GetFilenamePath(objectFilepath), vars["FLAGS"],
+          vars["DEFINES"], vars["INCLUDES"],
+          /*compile pdb*/ "",
+          /*target pdb*/ "", config);
+      }
+    }
+  }
+
+  if (!isSingleOutput) {
+    this->GenerateSwiftOutputFileMap(config, vars["FLAGS"]);
+  }
+
+  for (cmTargetDepend const& dep :
+       this->GetGlobalGenerator()->GetTargetDirectDepends(&target)) {
+    if (!dep->IsLanguageUsed("Swift", config)) {
+      continue;
+    }
+    // Add dependencies on the emitted swiftmodule file from swift targets we
+    // depend on
+    std::string const depModuleName =
+      getTargetPropertyOrDefault(*dep, "Swift_MODULE_NAME", dep->GetName());
+    std::string const depModuleDir = getTargetPropertyOrDefault(
+      *dep, "Swift_MODULE_DIRECTORY",
+      dep->LocalGenerator->GetCurrentBinaryDirectory());
+    std::string const depModuleFilename = getTargetPropertyOrDefault(
+      *dep, "Swift_MODULE", cmStrCat(depModuleName, ".swiftmodule"));
+    std::string const depModuleFilepath =
+      this->ConvertToNinjaPath(cmStrCat(depModuleDir, '/', depModuleFilename));
+    objBuild.ImplicitDeps.push_back(depModuleFilepath);
+  }
+
+  objBuild.OrderOnlyDeps.push_back(this->OrderDependsTargetForTarget(config));
+
+  // Write object build
+  this->GetGlobalGenerator()->WriteBuild(this->GetImplFileStream(fileConfig),
+                                         objBuild);
+}
+
 void cmNinjaTargetGenerator::WriteTargetDependInfo(std::string const& lang,
                                                    const std::string& config)
 {

Source/cmNinjaTargetGenerator.h

@@ -171,6 +171,9 @@ protected:
                                        const std::string& config,
                                        const std::string& fileConfig,
                                        bool firstForConfig);
+  void WriteSwiftObjectBuildStatement(
+    std::vector<cmSourceFile const*> const& sources, const std::string& config,
+    const std::string& fileConfig, bool firstForConfig);
   void WriteObjectBuildStatement(cmSourceFile const* source,
                                  const std::string& config,
                                  const std::string& fileConfig,

Tests/RunCMake/Swift/IncrementalSwift-second-stdout.txt

@@ -1,3 +1,3 @@
-.*Linking Swift static library libA.a
-.*Linking Swift static library libB.a
-FAILED: libB.a CMakeFiles/B.dir/b.swift.o B.swiftmodule
+.*Building Swift object A.swiftmodule CMakeFiles/A.dir/a.swift.o
+.*Building Swift object B.swiftmodule CMakeFiles/B.dir/b.swift.o
+FAILED: B.swiftmodule CMakeFiles/B.dir/b.swift.o

Tests/SwiftOnly/CMakeLists.txt

@@ -43,6 +43,13 @@ add_library(N N.swift)
 target_link_libraries(N PUBLIC
   M)
 
+if(NOT XCODE_VERSION OR XCODE_VERSION VERSION_GREATER_EQUAL 9.0)
+  # TODO: Add a wholemodule object-library test once that is working
+  add_library(O OBJECT O.swift L.swift)
+  target_link_libraries(N PUBLIC O)
+  set_target_properties(O PROPERTIES Swift_COMPILATION_MODE "incremental")
+endif()
+
 # Dummy to make sure generation works with such targets.
 add_library(SwiftIface INTERFACE)
 target_link_libraries(SwiftOnly PRIVATE SwiftIface)
@@ -59,14 +66,15 @@ if(CMAKE_Swift_COMPILER_VERSION VERSION_GREATER_EQUAL 5.2)
 endif()
 
 function(test_cmp0157_default mode)
-
-  cmake_policy(GET CMP0157 cmp0157_wmo)
-  if(cmp0157_wmo STREQUAL "NEW")
-    set(CMAKE_Swift_COMPILATION_MODE "${mode}")
-    add_executable(hi_${mode} main.swift)
-    get_target_property(${mode}_swift_comp_mode hi_${mode} "Swift_COMPILATION_MODE")
-    if(NOT ${mode}_swift_comp_mode STREQUAL ${mode})
-      message(SEND_ERROR "expected ${mode} -- found ${${mode}_swift_comp_mode}")
+  if(POLICY CMP0157)
+    cmake_policy(GET CMP0157 cmp0157_wmo)
+    if(cmp0157_wmo STREQUAL "NEW")
+      set(CMAKE_Swift_COMPILATION_MODE "${mode}")
+      add_executable(hi_${mode} main.swift)
+      get_target_property(${mode}_swift_comp_mode hi_${mode} "Swift_COMPILATION_MODE")
+      if(NOT ${mode}_swift_comp_mode STREQUAL ${mode})
+        message(SEND_ERROR "expected ${mode} -- found ${${mode}_swift_comp_mode}")
+      endif()
     endif()
   endif()
 endfunction()