don't rebuild (#4230)

src/fsharp/FSharp.Build/WriteCodeFragment.fs

@@ -113,15 +113,19 @@ open System.Reflection"
                 let sb = StringBuilder().AppendLine(boilerplate).AppendLine()
                 let code = Array.fold (fun (sb:StringBuilder) (item:ITaskItem) -> sb.AppendLine(WriteCodeFragment.GenerateAttribute item)) sb _assemblyAttributes
                 code.AppendLine().AppendLine("do()") |> ignore
+                let fileName = _outputFile.ItemSpec
                 let outputFileItem =
-                    if not (isNull _outputFile) && not (isNull _outputDirectory) && not (Path.IsPathRooted(_outputFile.ItemSpec)) then
-                        TaskItem(Path.Combine(_outputDirectory.ItemSpec, _outputFile.ItemSpec)) :> ITaskItem
+                    if not (isNull _outputFile) && not (isNull _outputDirectory) && not (Path.IsPathRooted(fileName)) then
+                        TaskItem(Path.Combine(_outputDirectory.ItemSpec, fileName)) :> ITaskItem
                     elif isNull _outputFile then
                         let tempFile = Path.Combine(Path.GetTempPath(), sprintf "tmp%s.fs" (Guid.NewGuid().ToString("N")))
                         TaskItem(tempFile) :> ITaskItem
                     else
                         _outputFile
-                File.WriteAllText(_outputFile.ItemSpec, code.ToString())
+                let codeText = code.ToString()
+                let alreadyExists = (try File.Exists fileName && File.ReadAllText(fileName) = codeText with _ -> false)
+                if not alreadyExists then
+                    File.WriteAllText(fileName, codeText)
                 _outputFile <- outputFileItem
                 true
             with e ->

tests/scripts/compiler-perf-results.txt

@@ -131,4 +131,3 @@ https://github.com/manofstick/visualfsharp.git all-your-collections-are-belong-t
 https://github.com/manofstick/visualfsharp.git all-your-collections-are-belong-to-us 87dafbc17b494c438b6db9e59e064736bd8a44e2 ba63403cb5898596c5e875a14ce22b33ef618c01 221.58 11.19                31.91                48.23                66.05                52.99               
 https://github.com/manofstick/visualfsharp.git all-your-collections-are-belong-to-us 87dafbc17b494c438b6db9e59e064736bd8a44e2 458e6c29d7e059a5a8a7b4cd7858c7d633fb5906 224.75 11.20                31.09                46.96                63.08                53.08               
 https://github.com/manofstick/visualfsharp.git all-your-collections-are-belong-to-us 87dafbc17b494c438b6db9e59e064736bd8a44e2 7e1fd6ac330f86597f3167e8067cfd805a89eec9 235.48 10.83                33.47                47.17                65.56                52.50               
-