IWYU: Add `--driver-mode=cl` when applicable

We already do this for `clang-tidy`.

Fixes: #16554

2 files changed, 39 insertions, 3 deletions

diff --git a/Source/cmMakefileTargetGenerator.cxx b/Source/cmMakefileTargetGenerator.cxx
index c83a7ab..70b6d27 100644
--- a/Source/cmMakefileTargetGenerator.cxx
+++ b/Source/cmMakefileTargetGenerator.cxx
@@ -999,7 +999,25 @@ void cmMakefileTargetGenerator::WriteObjectRuleFiles(
         }
         if (cmNonempty(iwyu)) {
           run_iwyu += " --iwyu=";
-          run_iwyu += this->LocalGenerator->EscapeForShell(*iwyu);
+
+          // Only add --driver-mode if it is not already specified, as adding
+          // it unconditionally might override a user-specified driver-mode
+          if (iwyu.Get()->find("--driver-mode=") == std::string::npos) {
+            cmProp p = this->Makefile->GetDefinition(
+              cmStrCat("CMAKE_", lang, "_INCLUDE_WHAT_YOU_USE_DRIVER_MODE"));
+            std::string driverMode;
+
+            if (cmNonempty(p)) {
+              driverMode = *p;
+            } else {
+              driverMode = lang == "C" ? "gcc" : "g++";
+            }
+
+            run_iwyu += this->LocalGenerator->EscapeForShell(
+              cmStrCat(*iwyu, ";--driver-mode=", driverMode));
+          } else {
+            run_iwyu += this->LocalGenerator->EscapeForShell(*iwyu);
+          }
         }
         if (cmNonempty(tidy)) {
           run_iwyu += " --tidy=";
diff --git a/Source/cmNinjaTargetGenerator.cxx b/Source/cmNinjaTargetGenerator.cxx
index 1b6b834..f1ea229 100644
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -874,8 +874,26 @@ void cmNinjaTargetGenerator::WriteCompileRule(const std::string& lang,
         compilerLauncher.clear();
       }
       if (cmNonempty(iwyu)) {
-        run_iwyu += cmStrCat(" --iwyu=",
-                             this->GetLocalGenerator()->EscapeForShell(*iwyu));
+        run_iwyu += " --iwyu=";
+
+        // Only add --driver-mode if it is not already specified, as adding
+        // it unconditionally might override a user-specified driver-mode
+        if (iwyu.Get()->find("--driver-mode=") == std::string::npos) {
+          cmProp p = this->Makefile->GetDefinition(
+            cmStrCat("CMAKE_", lang, "_INCLUDE_WHAT_YOU_USE_DRIVER_MODE"));
+          std::string driverMode;
+
+          if (cmNonempty(p)) {
+            driverMode = *p;
+          } else {
+            driverMode = lang == "C" ? "gcc" : "g++";
+          }
+
+          run_iwyu += this->LocalGenerator->EscapeForShell(
+            cmStrCat(*iwyu, ";--driver-mode=", driverMode));
+        } else {
+          run_iwyu += this->LocalGenerator->EscapeForShell(*iwyu);
+        }
       }
       if (cmNonempty(tidy)) {
         run_iwyu += " --tidy=";