Swift/Ninja: Add link edge dependency on emit-module output

Since commit b6367f723b (Swift/Ninja: Emit modules separately from
compilation, 2026-04-10) the .swiftmodule output moved from the
compile edge to a separate emit-module edge.  The link edge does not
depend on the emit-module output, so ninja never runs the emit-module
edge when no other target in the build imports the module (e.g.
install-only targets).

Store the declared .swiftmodule output path during Swift object
statement generation and add it as an implicit dependency of the link
edge.

Issue: #27748

Source/cmNinjaNormalTargetGenerator.cxx

@@ -1652,6 +1652,14 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement(
     }
   }
 
+  // For split Swift builds, ensure the link edge depends on the target's own
+  // .swiftmodule so the emit-module edge runs even when no other target in
+  // the build depends on it (e.g. install-only targets).
+  std::string swiftModuleOutput = this->GetSwiftModuleOutput(config);
+  if (!swiftModuleOutput.empty()) {
+    linkBuild.ImplicitDeps.emplace_back(std::move(swiftModuleOutput));
+  }
+
   // Ninja should restat after linking if and only if there are byproducts.
   vars["RESTAT"] = byproducts.ExplicitOuts.empty() ? "" : "1";
 

Source/cmNinjaTargetGenerator.cxx

@@ -2202,6 +2202,9 @@ void cmNinjaTargetGenerator::WriteSwiftObjectBuildStatement(
   // Importable targets keep -emit-module on compile so swiftc still emits
   // .swiftdoc.  When splitting module emission, both the .swiftmodule output
   // and -emit-module flags move entirely to the separate emit-module edge.
+  if (targetIsImportable) {
+    this->Configs[config].SwiftModuleOutput = moduleFilepath;
+  }
   if (targetIsImportable && !emitModuleSeparately) {
     objBuild.Outputs.push_back(moduleFilepath);
   }
@@ -2673,6 +2676,16 @@ cmNinjaDeps cmNinjaTargetGenerator::GetObjects(std::string const& config) const
   return {};
 }
 
+std::string cmNinjaTargetGenerator::GetSwiftModuleOutput(
+  std::string const& config) const
+{
+  auto const it = this->Configs.find(config);
+  if (it != this->Configs.end()) {
+    return it->second.SwiftModuleOutput;
+  }
+  return {};
+}
+
 void cmNinjaTargetGenerator::EnsureDirectoryExists(
   std::string const& path) const
 {

Source/cmNinjaTargetGenerator.h

@@ -216,6 +216,7 @@ protected:
   void AdditionalCleanFiles(std::string const& config);
 
   cmNinjaDeps GetObjects(std::string const& config) const;
+  std::string GetSwiftModuleOutput(std::string const& config) const;
 
   void EnsureDirectoryExists(std::string const& dir) const;
   void EnsureParentDirectoryExists(std::string const& path) const;
@@ -278,6 +279,8 @@ private:
     Json::Value SwiftOutputMap;
     cmNinjaDeps ExtraFiles;
     std::unique_ptr<MacOSXContentGeneratorType> MacOSXContentGenerator;
+    // Path declared as the .swiftmodule output (compile or emit-module edge).
+    std::string SwiftModuleOutput;
   };
 
   std::map<std::string, ByConfig> Configs;

Tests/RunCMake/Swift/EmitModuleSeparatelyLinkDep-check.cmake

@@ -0,0 +1,14 @@
+# The link edge for a Swift library must implicitly depend on the
+# .swiftmodule so the emit-module edge runs even when no other target
+# in the build depends on it.
+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)
+
+if(NOT build_ninja MATCHES "build [^\n]*(libL\\.a|L\\.lib)[^\n]*:.*\\|[^\n]*L\\.swiftmodule")
+  string(APPEND RunCMake_TEST_FAILED
+    "Link edge for L does not depend on L.swiftmodule.\n")
+endif()

Tests/RunCMake/Swift/EmitModuleSeparatelyLinkDep.cmake

@@ -0,0 +1,12 @@
+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)
+
+# A standalone library with no dependent Swift targets.  The link edge
+# must still depend on the .swiftmodule so the emit-module edge runs.
+add_library(L STATIC L.swift)

Tests/RunCMake/Swift/EmitModuleSeparatelyLinkDepCustomPath-check.cmake

@@ -0,0 +1,14 @@
+# Same as EmitModuleSeparatelyLinkDep but with a custom module directory.
+# The link edge must depend on the actual emit-module output path, not a
+# default that doesn't match the target's configuration.
+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)
+
+if(NOT build_ninja MATCHES "build [^\n]*(libL\\.a|L\\.lib)[^\n]*:.*\\|[^\n]*custom(/|\\\\)[^\n]*L\\.swiftmodule")
+  string(APPEND RunCMake_TEST_FAILED
+    "Link edge for L does not depend on custom/.../L.swiftmodule.\n")
+endif()

Tests/RunCMake/Swift/EmitModuleSeparatelyLinkDepCustomPath.cmake

@@ -0,0 +1,15 @@
+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)
+
+# A standalone library with a custom module output path (like swiftCore
+# in the Swift stdlib).  The link edge must depend on the actual
+# emit-module output, not GetSwiftModulePath().
+add_library(L STATIC L.swift)
+set_target_properties(L PROPERTIES
+  Swift_MODULE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/custom)

Tests/RunCMake/Swift/RunCMakeTest.cmake

@@ -170,6 +170,14 @@ if(RunCMake_GENERATOR MATCHES "Ninja")
     run_cmake_command(EmitModuleSeparatelyDirectoryStyle-build ${CMAKE_COMMAND} --build . -- -vn)
   endblock()
 
+  block()
+    run_cmake(EmitModuleSeparatelyLinkDep)
+  endblock()
+
+  block()
+    run_cmake(EmitModuleSeparatelyLinkDepCustomPath)
+  endblock()
+
   block()
     set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/EmitModuleSeparatelyExistingModulePath-build)
     run_cmake(EmitModuleSeparatelyExistingModulePath)