ENH: In cmSourceFile::GetLanguage use the file extension (if not ambiguous) to determine the language without requiring the source file to exist.

Source/cmSourceFile.cxx

@@ -46,10 +46,28 @@ std::string const& cmSourceFile::GetExtension() const
 //----------------------------------------------------------------------------
 const char* cmSourceFile::GetLanguage()
 {
-  // Compute the final location of the file if necessary.
+  // If the language was set explicitly by the user then use it.
-  if(this->FullPath.empty())
+  if(const char* lang = this->GetProperty("LANGUAGE"))
     {
-    this->GetFullPath();
+    return lang;
+    }
+
+  // Perform computation needed to get the language if necessary.
+  if(this->FullPath.empty() && this->Language.empty())
+    {
+    if(this->Location.ExtensionIsAmbiguous())
+      {
+      // Finalize the file location to get the extension and set the
+      // language.
+      this->GetFullPath();
+      }
+    else
+      {
+      // Use the known extension to get the language if possible.
+      std::string ext =
+        cmSystemTools::GetFilenameLastExtension(this->Location.GetName());
+      this->CheckLanguage(ext);
+      }
     }
 
   // Now try to determine the language.
@@ -252,8 +270,19 @@ void cmSourceFile::CheckExtension()
     }
 
   // Try to identify the source file language from the extension.
+  if(this->Language.empty())
+    {
+    this->CheckLanguage(this->Extension);
+    }
+}
+
+//----------------------------------------------------------------------------
+void cmSourceFile::CheckLanguage(std::string const& ext)
+{
+  // Try to identify the source file language from the extension.
+  cmMakefile* mf = this->Location.GetMakefile();
   cmGlobalGenerator* gg = mf->GetLocalGenerator()->GetGlobalGenerator();
-  if(const char* l = gg->GetLanguageFromExtension(this->Extension.c_str()))
+  if(const char* l = gg->GetLanguageFromExtension(ext.c_str()))
     {
     this->Language = l;
     }

Source/cmSourceFile.h

@@ -116,6 +116,7 @@ private:
   bool FindFullPath();
   bool TryFullPath(const char* tryPath, const char* ext);
   void CheckExtension();
+  void CheckLanguage(std::string const& ext);
 
   std::vector<std::string> Depends;
 };