Swift/Ninja: Serialize compile edge after emit-module within target

Since commit b6367f723b (Swift/Ninja: Emit modules separately from
compilation, 2026-04-10) the compile and emit-module edges share the
same -output-file-map, which references a single module-level
.swiftdeps file.  When ninja schedules both edges concurrently, both
swift invocations write that file and corrupt it.

Add the .swiftmodule output as an order-only dependency of the object
compile edge so the two edges within a target run sequentially.
Downstream targets are unaffected: they still unblock as soon as the
emit-module edge finishes, preserving cross-target parallelism.

Move the object-build WriteBuild call to after the emit-module block
so the order-only dep is added after modBuild is copied from objBuild,
avoiding a self-reference on the emit-module edge's own output.

Issue: #27748

Source/cmNinjaTargetGenerator.cxx

@@ -2276,11 +2276,6 @@ void cmNinjaTargetGenerator::WriteSwiftObjectBuildStatement(
 
   objBuild.OrderOnlyDeps.push_back(this->OrderDependsTargetForTarget(config));
 
-  // Write object build
-  this->GetGlobalGenerator()->WriteBuild(this->GetImplFileStream(fileConfig),
-                                         objBuild,
-                                         this->ForceResponseFile() ? -1 : 0);
-
   // Write a separate emit-module build edge that produces .swiftmodule
   // without compile outputs. This allows downstream Swift targets to start
   // compiling as soon as the module interface is ready, overlapping with
@@ -2311,7 +2306,16 @@ void cmNinjaTargetGenerator::WriteSwiftObjectBuildStatement(
     this->GetGlobalGenerator()->WriteBuild(this->GetImplFileStream(fileConfig),
                                            modBuild,
                                            this->ForceResponseFile() ? -1 : 0);
+
+    // Both edges share the same -output-file-map; serialize the compile
+    // edge after emit-module so they do not race on the module .swiftdeps.
+    objBuild.OrderOnlyDeps.push_back(moduleFilepath);
   }
+
+  // Write object build
+  this->GetGlobalGenerator()->WriteBuild(this->GetImplFileStream(fileConfig),
+                                         objBuild,
+                                         this->ForceResponseFile() ? -1 : 0);
 }
 
 void cmNinjaTargetGenerator::WriteTargetDependInfo(std::string const& lang,

Tests/RunCMake/Swift/EmitModuleSeparatelyOrdering-check.cmake

@@ -0,0 +1,28 @@
+# The Swift object compile edge must order-only depend on the .swiftmodule
+# produced by the separate emit-module edge.  Both edges share the same
+# -output-file-map; running them concurrently corrupts the module-level
+# swift-dependencies file referenced by that map.
+if(RunCMake_GENERATOR_IS_MULTI_CONFIG)
+  set(path "${RunCMake_TEST_BINARY_DIR}/CMakeFiles/impl-Debug.ninja")
+else()
+  set(path "${RunCMake_TEST_BINARY_DIR}/build.ninja")
+endif()
+file(READ "${path}" build_ninja)
+
+# Find the object compile edge for L and confirm L.swiftmodule appears
+# after the order-only `||` separator.  The edge ends at the next `build `
+# at the start of a line (or end of file).
+string(REGEX MATCH
+  "build [^\n]*L\\.swift\\.o[^\n]*:[^\n]*(\n [^\n]+)*"
+  obj_edge "${build_ninja}")
+
+if(NOT obj_edge)
+  string(APPEND RunCMake_TEST_FAILED
+    "Could not find object compile edge for L.swift.\n")
+  return()
+endif()
+
+if(NOT obj_edge MATCHES "\\|\\|[^\n]*L\\.swiftmodule")
+  string(APPEND RunCMake_TEST_FAILED
+    "Object compile edge for L.swift is missing an order-only dependency on L.swiftmodule.\nEdge:\n${obj_edge}\n")
+endif()

Tests/RunCMake/Swift/EmitModuleSeparatelyOrdering.cmake

@@ -0,0 +1,14 @@
+cmake_policy(SET CMP0157 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)
+
+# The compile and emit-module edges share the same -output-file-map.
+# Within the target, the object compile edge must order-only depend on the
+# .swiftmodule so the two edges do not run concurrently and corrupt the
+# shared module-level swift-dependencies entry.
+add_library(L STATIC L.swift)

Tests/RunCMake/Swift/RunCMakeTest.cmake

@@ -178,6 +178,10 @@ if(RunCMake_GENERATOR MATCHES "Ninja")
     run_cmake_command(EmitModuleSeparatelyExistingModulePath-build ${CMAKE_COMMAND} --build . -- -vn)
   endblock()
 
+  block()
+    run_cmake(EmitModuleSeparatelyOrdering)
+  endblock()
+
   block()
     if(CMAKE_SYSTEM_NAME MATCHES Windows)
       set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/ImportLibraryFlags-build)