Option to allow caller to specify it does not want unchanged output files to be modified. This supports incremental build in VS. (#1373)

Check if generated code is different to existing files and only overwrite if so

Co-authored-by: Build Agent <[email protected]>

Author: IIFE

src/Generator/Driver.cs

@@ -316,14 +316,23 @@ public void SaveCode(IEnumerable<GeneratorOutput> outputs)
                     var fileRelativePath = $"{fileBase}.{template.FileExtension}";
 
                     var file = Path.Combine(outputPath, fileRelativePath);
-                    File.WriteAllText(file, template.Generate());
+                    WriteGeneratedCodeToFile(file, template.Generate());
                     output.TranslationUnit.Module.CodeFiles.Add(file);
 
                     Diagnostics.Message("Generated '{0}'", fileRelativePath);
                 }
             }
         }
 
+        private void WriteGeneratedCodeToFile(string file, string generatedCode)
+        {
+            var fi = new FileInfo(file);
+            
+            if (!fi.Exists || fi.Length != generatedCode.Length ||
+                File.ReadAllText(file) != generatedCode)
+                File.WriteAllText(file, generatedCode);
+        }
+
         private static readonly Dictionary<Module, string> libraryMappings = new Dictionary<Module, string>();
 
         public void CompileCode(Module module)