Autogen: Pass explicit predefines header to moc if possible

Qt is relying on whoever calls moc to include a file with the predefined
values that will be used by the compiler, otherwise moc takes wrong
paths and weird things happen.
Instead, generate an include file and feed it to all mocs to make sure
it's generating correct code.

Co-Author: Sebastian Holtermann <sebholt@xwmw.org>
Fixes: #16640

Source/cmQtAutoGenerators.cxx

@@ -360,6 +360,8 @@ bool cmQtAutoGenerators::ReadAutogenInfoFile(
   InfoGet(makefile, "AM_QT_MOC_EXECUTABLE", this->MocExecutable);
   InfoGet(makefile, "AM_QT_UIC_EXECUTABLE", this->UicExecutable);
   InfoGet(makefile, "AM_QT_RCC_EXECUTABLE", this->RccExecutable);
+
+  InfoGet(makefile, "AM_MOC_PREDEFS_CMD", this->MocPredefsCmd);
   // Check Qt version
   if ((this->QtMajorVersion != "4") && (this->QtMajorVersion != "5")) {
     this->LogError("AutoGen: Error: Unsupported Qt version: " +
@@ -579,6 +581,12 @@ void cmQtAutoGenerators::Init(cmMakefile* makefile)
 
   this->MocCppFilenameAbs = this->CurrentBinaryDir + this->MocCppFilenameRel;
 
+  // Moc predefs file
+  if (!this->MocPredefsCmd.empty()) {
+    this->MocPredefsFileRel = this->AutogenBuildSubDir + "moc_predefs.h";
+    this->MocPredefsFileAbs = this->CurrentBinaryDir + this->MocPredefsFileRel;
+  }
+
   // Init file path checksum generator
   fpathCheckSum.setupParentDirs(this->CurrentSourceDir, this->CurrentBinaryDir,
                                 this->ProjectSourceDir,
@@ -1142,6 +1150,50 @@ bool cmQtAutoGenerators::MocGenerateAll(
     return true;
   }
 
+  // Generate moc_predefs
+  if (!this->MocPredefsCmd.empty()) {
+    if (!this->MakeParentDirectory(this->MocPredefsFileAbs)) {
+      this->LogError("AutoMoc: Error creating directory for " +
+                     this->MocPredefsFileRel);
+      return false;
+    }
+    this->LogBold("Generating MOC predefs " + this->MocPredefsFileRel);
+
+    std::vector<std::string> cmd = this->MocPredefsCmd;
+    cmd.insert(cmd.end(), this->MocIncludes.begin(), this->MocIncludes.end());
+    for (std::vector<std::string>::const_iterator it =
+           this->MocDefinitions.begin();
+         it != this->MocDefinitions.end(); ++it) {
+      cmd.push_back("-D" + (*it));
+    }
+    cmd.insert(cmd.end(), this->MocOptions.begin(), this->MocOptions.end());
+
+    std::string output;
+    bool moc_predefsGenerated = this->RunCommand(cmd, output, false);
+    if (!moc_predefsGenerated) {
+      return false;
+    }
+
+    // actually write the file
+    cmsys::ofstream outfile;
+    outfile.open(this->MocPredefsFileAbs.c_str(), std::ios::trunc);
+    if (!outfile) {
+      moc_predefsGenerated = false;
+      this->LogError("AutoMoc: Error opening " + this->MocPredefsFileRel);
+    } else {
+      outfile << output;
+      // Check for write errors
+      if (!outfile.good()) {
+        moc_predefsGenerated = false;
+        this->LogError("AutoMoc: Error writing " + this->MocPredefsFileRel);
+      }
+    }
+
+    if (!moc_predefsGenerated) {
+      return false;
+    }
+  }
+
   bool mocCompFileGenerated = false;
   bool mocCompChanged = false;
 
@@ -1305,6 +1357,10 @@ bool cmQtAutoGenerators::MocGenerateFile(
         cmd.push_back("-D" + (*it));
       }
       cmd.insert(cmd.end(), this->MocOptions.begin(), this->MocOptions.end());
+      if (!this->MocPredefsFileAbs.empty()) {
+        cmd.push_back("--include");
+        cmd.push_back(this->MocPredefsFileAbs);
+      }
 #ifdef _WIN32
       cmd.push_back("-DWIN32");
 #endif
@@ -1805,7 +1861,7 @@ bool cmQtAutoGenerators::MakeParentDirectory(const std::string& filename) const
  * @return True on success
  */
 bool cmQtAutoGenerators::RunCommand(const std::vector<std::string>& command,
-                                    std::string& output) const
+                                    std::string& output, bool verbose) const
 {
   // Log command
   if (this->Verbose) {
@@ -1813,8 +1869,9 @@ bool cmQtAutoGenerators::RunCommand(const std::vector<std::string>& command,
   }
   // Execute command
   int retVal = 0;
-  bool res =
-    cmSystemTools::RunSingleCommand(command, &output, &output, &retVal);
+  bool res = cmSystemTools::RunSingleCommand(
+    command, &output, &output, &retVal, CM_NULLPTR,
+    verbose ? cmSystemTools::OUTPUT_MERGE : cmSystemTools::OUTPUT_NONE);
   return (res && (retVal == 0));
 }