Provide guidance when trying to use non-enabled language

Fixes #23463

Source/cmCommonTargetGenerator.cxx

@@ -2,7 +2,7 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #include "cmCommonTargetGenerator.h"
 
-#include <set>
+#include <algorithm>
 #include <sstream>
 #include <utility>
 
@@ -13,9 +13,11 @@
 #include "cmLocalCommonGenerator.h"
 #include "cmLocalGenerator.h"
 #include "cmMakefile.h"
+#include "cmMessageType.h"
 #include "cmOutputConverter.h"
 #include "cmRange.h"
 #include "cmSourceFile.h"
+#include "cmState.h"
 #include "cmStateTypes.h"
 #include "cmStringAlgorithms.h"
 #include "cmTarget.h"
@@ -321,3 +323,29 @@ std::string cmCommonTargetGenerator::GetLinkerLauncher(
   }
   return std::string();
 }
+
+bool cmCommonTargetGenerator::HaveRequiredLanguages(
+  const std::vector<cmSourceFile const*>& sources,
+  std::set<std::string>& languagesNeeded) const
+{
+  for (cmSourceFile const* sf : sources) {
+    languagesNeeded.insert(sf->GetLanguage());
+  }
+
+  auto* makefile = this->Makefile;
+  auto* state = makefile->GetState();
+  auto unary = [&state, &makefile](const std::string& lang) -> bool {
+    const bool valid = state->GetLanguageEnabled(lang);
+    if (!valid) {
+      makefile->IssueMessage(
+        MessageType::FATAL_ERROR,
+        cmStrCat("The language ", lang,
+                 " was requested for compilation but was not enabled."
+                 " To enable a language it needs to be specified in a"
+                 " 'project' or 'enable_language' command in the root"
+                 " CMakeLists.txt"));
+    }
+    return valid;
+  };
+  return std::all_of(languagesNeeded.cbegin(), languagesNeeded.cend(), unary);
+}

Source/cmCommonTargetGenerator.h

@@ -5,6 +5,7 @@
 #include "cmConfigure.h" // IWYU pragma: keep
 
 #include <map>
+#include <set>
 #include <string>
 #include <vector>
 
@@ -74,6 +75,9 @@ protected:
 
   std::string GetLinkerLauncher(const std::string& config);
 
+  bool HaveRequiredLanguages(const std::vector<cmSourceFile const*>& sources,
+                             std::set<std::string>& languagesNeeded) const;
+
 private:
   using ByLanguageMap = std::map<std::string, std::string>;
   struct ByConfig

Source/cmMakefileTargetGenerator.cxx

@@ -305,9 +305,14 @@ void cmMakefileTargetGenerator::WriteTargetBuildRules()
   std::vector<cmSourceFile const*> objectSources;
   this->GeneratorTarget->GetObjectSources(objectSources,
                                           this->GetConfigName());
-  for (cmSourceFile const* sf : objectSources) {
-    // Generate this object file's rule file.
-    this->WriteObjectRuleFiles(*sf);
+
+  // validate that all languages requested are enabled.
+  std::set<std::string> requiredLangs;
+  if (this->HaveRequiredLanguages(objectSources, requiredLangs)) {
+    for (cmSourceFile const* sf : objectSources) {
+      // Generate this object file's rule file.
+      this->WriteObjectRuleFiles(*sf);
+    }
   }
 }
 
@@ -532,8 +537,7 @@ void cmMakefileTargetGenerator::WriteObjectRuleFiles(
   cmSourceFile const& source)
 {
   // Identify the language of the source file.
-  const std::string& lang =
-    this->LocalGenerator->GetSourceFileLanguage(source);
+  const std::string& lang = source.GetLanguage();
   if (lang.empty()) {
     // don't know anything about this file so skip it
     return;

Source/cmNinjaNormalTargetGenerator.cxx

@@ -126,15 +126,11 @@ void cmNinjaNormalTargetGenerator::WriteLanguagesRules(
   std::set<std::string> languages;
   std::vector<cmSourceFile const*> sourceFiles;
   this->GetGeneratorTarget()->GetObjectSources(sourceFiles, config);
-  for (cmSourceFile const* sf : sourceFiles) {
-    std::string const lang = sf->GetLanguage();
-    if (!lang.empty()) {
-      languages.insert(lang);
+  if (this->HaveRequiredLanguages(sourceFiles, languages)) {
+    for (std::string const& language : languages) {
+      this->WriteLanguageRules(language, config);
     }
   }
-  for (std::string const& language : languages) {
-    this->WriteLanguageRules(language, config);
-  }
 }
 
 const char* cmNinjaNormalTargetGenerator::GetVisibleTypeName() const

Tests/RunCMake/project/LanguagesUsedButNotEnabled-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/project/LanguagesUsedButNotEnabled-stderr.txt

@@ -0,0 +1,4 @@
+CMake Error in CMakeLists.txt:
+  The language CXX was requested for compilation but was not enabled.  To
+  enable a language it needs to be specified in a 'project' or
+  'enable_language' command in the root CMakeLists.txt

Tests/RunCMake/project/LanguagesUsedButNotEnabled.cmake

@@ -0,0 +1,3 @@
+
+add_executable(UsesCXXLang empty.cxx)
+set_source_files_properties(empty.cxx PROPERTIES GENERATED TRUE LANGUAGE CXX )

Tests/RunCMake/project/RunCMakeTest.cmake

@@ -8,6 +8,9 @@ run_cmake(LanguagesEmpty)
 run_cmake(LanguagesNONE)
 run_cmake(LanguagesTwice)
 run_cmake(LanguagesUnordered)
+if(RunCMake_GENERATOR MATCHES "Make|Ninja")
+  run_cmake(LanguagesUsedButNotEnabled)
+endif()
 run_cmake(ProjectDescription)
 run_cmake(ProjectDescription2)
 run_cmake(ProjectDescriptionNoArg)