Ninja: Add order-only dependencies to link commands (#14728)

A library or executable target that consists only of a reference to an
object library may have no sources to compile or custom commands to run.
The command in the target is the link (or archive) command.  Add missing
order-only dependencies to link commands so that target ordering
dependencies are satisfied for it without depending on an intermediate
compilation rule.

Extend the ObjectLibrary test to cover this case.

Co-Author: Brad King <brad.king@kitware.com>

Source/cmNinjaNormalTargetGenerator.cxx

@@ -608,6 +608,11 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement()
       std::string(cmake::GetCMakeFilesDirectoryPostSlash())
       + target.GetName() + ".rsp";
 
+  // Gather order-only dependencies.
+  cmNinjaDeps orderOnlyDeps;
+  this->GetLocalGenerator()->AppendTargetDepends(this->GetTarget(),
+    orderOnlyDeps);
+
   // Write the build statement for this target.
   globalGen.WriteBuild(this->GetBuildFileStream(),
                         comment.str(),
@@ -615,7 +620,7 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement()
                         outputs,
                         explicitDeps,
                         implicitDeps,
-                        emptyDeps,
+                        orderOnlyDeps,
                         vars,
                         rspfile,
                         commandLineLengthLimit);

Tests/ObjectLibrary/CMakeLists.txt

@@ -59,4 +59,10 @@ add_executable(UseABinternal ${dummy}
   $<TARGET_OBJECTS:ABmain> $<TARGET_OBJECTS:A> $<TARGET_OBJECTS:B>
   )
 
+# Test target-level dependencies of executable without sources.
+file(REMOVE ${CMAKE_CURRENT_BINARY_DIR}/UseABinternalDep.cmake)
+add_custom_target(UseABinternalDep COMMAND ${CMAKE_COMMAND} -E touch UseABinternalDep.cmake)
+add_custom_command(TARGET UseABinternal POST_BUILD COMMAND ${CMAKE_COMMAND} -P UseABinternalDep.cmake)
+add_dependencies(UseABinternal UseABinternalDep)
+
 add_subdirectory(ExportLanguages)