Swift/Ninja: Fix emit-module-path clobbering on separate emit-module edge

Since commit b6367f723b (Swift/Ninja: Emit modules separately from
compilation, 2026-04-10) the compile edge still appends
-emit-module -emit-module-path when emitModuleSeparately is active,
duplicating the emit-module edge.  For targets with directory-style
module paths (CMP0195=NEW), this causes a file/directory conflict
that fails the build.

Guard -emit-module -emit-module-path on both the compile and
emit-module edges so they are not appended when the flags already
contain -emit-module-path (e.g. from the target's compile options).

Discovered building swift-syntax on Windows where the directory-style
path was overwritten by the flat path from the compile edge.

Issue: #27748

Source/cmNinjaTargetGenerator.cxx

@@ -2200,8 +2200,8 @@ void cmNinjaTargetGenerator::WriteSwiftObjectBuildStatement(
   objBuild.RspFile = cmStrCat(targetObjectFilename, ".swift.rsp");
 
   // Importable targets keep -emit-module on compile so swiftc still emits
-  // .swiftdoc.  When splitting module emission, only the .swiftmodule output
-  // moves to the separate emit-module edge.
+  // .swiftdoc.  When splitting module emission, both the .swiftmodule output
+  // and -emit-module flags move entirely to the separate emit-module edge.
   if (targetIsImportable && !emitModuleSeparately) {
     objBuild.Outputs.push_back(moduleFilepath);
   }
@@ -2240,7 +2240,8 @@ void cmNinjaTargetGenerator::WriteSwiftObjectBuildStatement(
   std::string const moduleOutputPath =
     this->LocalGenerator->ConvertToOutputFormat(moduleFilepath,
                                                 cmOutputConverter::SHELL);
-  if (targetIsImportable) {
+  if (targetIsImportable && !emitModuleSeparately &&
+      commonFlags.find("-emit-module-path") == std::string::npos) {
     std::string const emitModuleFlag = "-emit-module";
     std::string const modulePathFlag = "-emit-module-path";
     this->LocalGenerator->AppendFlags(
@@ -2291,10 +2292,14 @@ void cmNinjaTargetGenerator::WriteSwiftObjectBuildStatement(
     // Start from common flags (shared with compile edge) and add
     // emit-module-specific flags.  The emit-module rule template already
     // contains -emit-module, so we only need -emit-module-path here.
+    // Skip if the flags already contain one (e.g. a directory-style path
+    // set by the target's compile options).
     modBuild.Variables["FLAGS"] = commonFlags;
-    this->LocalGenerator->AppendFlags(
-      modBuild.Variables["FLAGS"],
-      cmStrCat("-emit-module-path ", moduleOutputPath));
+    if (commonFlags.find("-emit-module-path") == std::string::npos) {
+      this->LocalGenerator->AppendFlags(
+        modBuild.Variables["FLAGS"],
+        cmStrCat("-emit-module-path ", moduleOutputPath));
+    }
 
     modBuild.RspFile = cmStrCat(moduleFilepath, ".rsp");
 

Tests/RunCMake/Swift/EmitModuleSeparately-build-check.cmake

@@ -2,8 +2,17 @@ string(
   REGEX MATCHALL
   "swiftc(\\.exe)?\"? [^\n]* -emit-module-path [^\n]*L\\.swiftmodule"
   swift_module_commands "${actual_stdout}")
-list(LENGTH swift_module_commands swift_module_command_count)
-if(swift_module_command_count LESS 2)
+
+set(emit_module_commands "${swift_module_commands}")
+list(FILTER emit_module_commands EXCLUDE REGEX " -c ")
+if(NOT emit_module_commands)
   string(APPEND RunCMake_TEST_FAILED
-    "Expected separate compile and emit-module commands for L, found ${swift_module_command_count} command(s) with '-emit-module-path ... L.swiftmodule'.\n")
+    "Expected an emit-module command with '-emit-module-path ... L.swiftmodule'.\n")
+endif()
+
+set(compile_commands "${swift_module_commands}")
+list(FILTER compile_commands INCLUDE REGEX " -c ")
+if(compile_commands)
+  string(APPEND RunCMake_TEST_FAILED
+    "Compile command (-c) should not contain '-emit-module-path' when emitting module separately.\n")
 endif()

Tests/RunCMake/Swift/EmitModuleSeparatelyDirectoryStyle-build-check.cmake

@@ -0,0 +1,26 @@
+string(
+  REGEX MATCHALL
+  "swiftc(\\.exe)?\"? [^\n]* -emit-module-path [^\n]*L\\.swiftmodule[^ \n]*"
+  swift_module_commands "${actual_stdout}")
+
+set(emit_module_commands "${swift_module_commands}")
+list(FILTER emit_module_commands EXCLUDE REGEX " -c ")
+if(NOT emit_module_commands)
+  string(APPEND RunCMake_TEST_FAILED
+    "Expected an emit-module command with '-emit-module-path ... L.swiftmodule'.\n")
+endif()
+
+set(compile_commands "${swift_module_commands}")
+list(FILTER compile_commands INCLUDE REGEX " -c ")
+if(compile_commands)
+  string(APPEND RunCMake_TEST_FAILED
+    "Compile command (-c) should not contain '-emit-module-path' when emitting module separately.\n")
+endif()
+
+# CMP0195=NEW: the emit-module path should be directory-style
+# (L.swiftmodule/<triple>.swiftmodule), not flat.
+list(FILTER emit_module_commands INCLUDE REGEX "L\\.swiftmodule(/|\\\\)[-_a-zA-Z0-9]+\\.swiftmodule")
+if(NOT emit_module_commands)
+  string(APPEND RunCMake_TEST_FAILED
+    "Expected directory-style -emit-module-path (L.swiftmodule/<triple>.swiftmodule) from CMP0195=NEW.\n")
+endif()

Tests/RunCMake/Swift/EmitModuleSeparatelyDirectoryStyle.cmake

@@ -0,0 +1,15 @@
+cmake_minimum_required(VERSION 4.0)
+
+cmake_policy(SET CMP0157 NEW)
+cmake_policy(SET CMP0195 NEW)
+cmake_policy(SET CMP0215 NEW)
+
+if(NOT CMAKE_GENERATOR MATCHES "Ninja")
+  message(SEND_ERROR "this test must use a Ninja generator, found ${CMAKE_GENERATOR}")
+endif()
+
+enable_language(Swift)
+
+add_library(L STATIC L.swift)
+add_library(LClient STATIC LClient.swift)
+target_link_libraries(LClient PRIVATE L)

Tests/RunCMake/Swift/EmitModuleSeparatelyExistingModulePath-build-check.cmake

@@ -0,0 +1,11 @@
+# When the target's flags already contain -emit-module-path, the emit-module
+# edge should not append a second one that clobbers it.
+# Match command lines that have two -emit-module-path flags on the same line.
+string(
+  REGEX MATCHALL
+  "-emit-module-path [^\n]*-emit-module-path"
+  duplicate_flags "${actual_stdout}")
+if(duplicate_flags)
+  string(APPEND RunCMake_TEST_FAILED
+    "Found command with duplicate -emit-module-path flags.\n")
+endif()

Tests/RunCMake/Swift/EmitModuleSeparatelyExistingModulePath.cmake

@@ -0,0 +1,22 @@
+cmake_minimum_required(VERSION 4.0)
+
+cmake_policy(SET CMP0157 NEW)
+cmake_policy(SET CMP0195 NEW)
+cmake_policy(SET CMP0215 NEW)
+
+if(NOT CMAKE_GENERATOR MATCHES "Ninja")
+  message(SEND_ERROR "this test must use a Ninja generator, found ${CMAKE_GENERATOR}")
+endif()
+
+enable_language(Swift)
+
+add_library(L STATIC L.swift)
+add_library(LClient STATIC LClient.swift)
+target_link_libraries(LClient PRIVATE L)
+
+# Simulate a target whose flags already contain -emit-module-path with a
+# directory-style path (as seen with swift-syntax's CMake config).  Use the
+# real module triple so the path matches GetSwiftModulePath() and the build
+# would succeed outside of dry-run.
+target_compile_options(L PRIVATE
+  -emit-module-path ${CMAKE_CURRENT_BINARY_DIR}/L.swiftmodule/${CMAKE_Swift_MODULE_TRIPLE}.swiftmodule)

Tests/RunCMake/Swift/RunCMakeTest.cmake

@@ -162,6 +162,22 @@ if(RunCMake_GENERATOR MATCHES "Ninja")
     run_cmake_command(EmitModuleSeparatelyResponseFile-build ${CMAKE_COMMAND} --build . -- -vn)
   endblock()
 
+  block()
+    set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/EmitModuleSeparatelyDirectoryStyle-build)
+    run_cmake(EmitModuleSeparatelyDirectoryStyle)
+    set(RunCMake_TEST_NO_CLEAN 1)
+    # -v: verbose to capture executed commands -n: dry-run to avoid actually compiling
+    run_cmake_command(EmitModuleSeparatelyDirectoryStyle-build ${CMAKE_COMMAND} --build . -- -vn)
+  endblock()
+
+  block()
+    set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/EmitModuleSeparatelyExistingModulePath-build)
+    run_cmake(EmitModuleSeparatelyExistingModulePath)
+    set(RunCMake_TEST_NO_CLEAN 1)
+    # -v: verbose to capture executed commands -n: dry-run to avoid actually compiling
+    run_cmake_command(EmitModuleSeparatelyExistingModulePath-build ${CMAKE_COMMAND} --build . -- -vn)
+  endblock()
+
   block()
     if(CMAKE_SYSTEM_NAME MATCHES Windows)
       set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/ImportLibraryFlags-build)