qmake: Introduce precompile_header_c for MSVC

MSVC requires that the C PCH file is compiled (as an object) and linked if
any C file is found, and the same for C++.

Most qmake projects are C++.

If a C++ project has a precompiled header, it is typically of C++ type, and
cannot be compiled as C (for example, it contains or includes classes).

Since there is no easy way to conditionally build the C PCH file only if C
files are found in the project (as done for g++), we need a setting that is
disabled by default.

This amends 30331afda118.

[ChangeLog][Tools][qmake] Introduced precompile_header_c CONFIG option for
MSVC to enable precompiled header for C sources.

Task-number: QTBUG-65103
Change-Id: Id9688a35ee7d9b5e4f5a846b81986cb674bc5f4e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

qmake/doc/src/qmake-manual.qdoc

@@ -939,6 +939,8 @@
             order in which they are given.
     \row    \li precompile_header \li Enables support for the use of
             \l{Using Precompiled Headers}{precompiled headers} in projects.
+    \row    \li precompile_header_c (MSVC only) \li Enables support for the use of
+            \l{Using Precompiled Headers}{precompiled headers} for C files.
     \row    \li warn_on  \li The compiler should output as many warnings as possible.
             If \c warn_off is also specified, the last one takes effect.
     \row    \li warn_off \li The compiler should output as few warnings as possible.
@@ -4623,6 +4625,11 @@
 
     \snippet code/doc_src_qmake-manual.pro 106
 
+    To use the precompiled header also for C files on MSVC nmake target, add
+    \c precompile_header_c to the \l{CONFIG} variable. If the header is
+    used also for C++ and it contains C++ keywords/includes, enclose them
+    with \c{#ifdef __cplusplus}).
+
     \section1 Notes on Possible Issues
 
     On some platforms, the file name suffix for precompiled header files is

qmake/generators/win32/msvc_nmake.cpp

@@ -51,7 +51,7 @@ static QString nmakePathList(const QStringList &list)
             .replace('#', QLatin1String("^#")).replace('$', QLatin1String("$$"));
 }
 
-NmakeMakefileGenerator::NmakeMakefileGenerator() : Win32MakefileGenerator(), usePCH(false)
+NmakeMakefileGenerator::NmakeMakefileGenerator() : usePCH(false), usePCHC(false)
 {
 
 }
@@ -324,6 +324,8 @@ void NmakeMakefileGenerator::writeNmakeParts(QTextStream &t)
           << escapeDependencyPaths(findDependencies(precompH)).join(" \\\n\t\t")
           << "\n\t$(CXX) " + precompRule +" $(CXXFLAGS) $(INCPATH) -TP "
           << escapeFilePath(precompH) << endl << endl;
+    }
+    if (usePCHC) {
         QString precompRuleC = QString("-c -Yc -Fp%1 -Fo%2")
                 .arg(escapeFilePath(precompPchC), escapeFilePath(precompObjC));
         t << escapeDependencyPath(precompObjC) << ": " << escapeDependencyPath(precompH) << ' '
@@ -335,14 +337,14 @@ void NmakeMakefileGenerator::writeNmakeParts(QTextStream &t)
 
 QString NmakeMakefileGenerator::var(const ProKey &value) const
 {
-    if (usePCH) {
+    if (usePCH || usePCHC) {
         const bool isRunC = (value == "QMAKE_RUN_CC_IMP_BATCH"
                              || value == "QMAKE_RUN_CC_IMP"
                              || value == "QMAKE_RUN_CC");
-        if (isRunC
+        const bool isRunCpp = (value == "QMAKE_RUN_CXX_IMP_BATCH"
-            || value == "QMAKE_RUN_CXX_IMP_BATCH"
+                               || value == "QMAKE_RUN_CXX_IMP"
-            || value == "QMAKE_RUN_CXX_IMP"
+                               || value == "QMAKE_RUN_CXX");
-            || value == "QMAKE_RUN_CXX") {
+        if ((isRunCpp && usePCH) || (isRunC && usePCHC)) {
             QFileInfo precompHInfo(fileInfo(precompH));
             QString precompH_f = escapeFilePath(precompHInfo.fileName());
             QString precompRule = QString("-c -FI%1 -Yu%2 -Fp%3")
@@ -410,21 +412,24 @@ void NmakeMakefileGenerator::init()
     // Setup PCH variables
     precompH = project->first("PRECOMPILED_HEADER").toQString();
     usePCH = !precompH.isEmpty() && project->isActiveConfig("precompile_header");
+    usePCHC = !precompH.isEmpty() && project->isActiveConfig("precompile_header_c");
     if (usePCH) {
         // Created files
         precompObj = var("PRECOMPILED_DIR") + project->first("TARGET") + "_pch" + Option::obj_ext;
         precompPch = var("PRECOMPILED_DIR") + project->first("TARGET") + "_pch.pch";
-        precompObjC = var("PRECOMPILED_DIR") + project->first("TARGET") + "_pch_c" + Option::obj_ext;
-        precompPchC = var("PRECOMPILED_DIR") + project->first("TARGET") + "_pch_c.pch";
         // Add linking of precompObj (required for whole precompiled classes)
-        project->values("OBJECTS")                  += precompObj;
+        project->values("OBJECTS") += precompObj;
-        project->values("OBJECTS")                  += precompObjC;
         // Add pch file to cleanup
-        project->values("QMAKE_CLEAN")          += precompPch;
+        project->values("QMAKE_CLEAN") += precompPch;
-        project->values("QMAKE_CLEAN")          += precompPchC;
         // Return to variable pool
         project->values("PRECOMPILED_OBJECT") = ProStringList(precompObj);
         project->values("PRECOMPILED_PCH")    = ProStringList(precompPch);
+    }
+    if (usePCHC) {
+        precompObjC = var("PRECOMPILED_DIR") + project->first("TARGET") + "_pch_c" + Option::obj_ext;
+        precompPchC = var("PRECOMPILED_DIR") + project->first("TARGET") + "_pch_c.pch";
+        project->values("OBJECTS") += precompObjC;
+        project->values("QMAKE_CLEAN") += precompPchC;
         project->values("PRECOMPILED_OBJECT_C") = ProStringList(precompObjC);
         project->values("PRECOMPILED_PCH_C")    = ProStringList(precompPchC);
     }

qmake/generators/win32/msvc_nmake.h

@@ -53,7 +53,7 @@ protected:
     QString var(const ProKey &value) const;
     QString precompH, precompObj, precompPch;
     QString precompObjC, precompPchC;
-    bool usePCH;
+    bool usePCH, usePCHC;
 
 public:
     NmakeMakefileGenerator();