fsxc: use external file for fsiStub

This way we will not add 3 extra lines to the beginning
of the file, which would make line numbers to be off in
case of errors.

fsxc/Fsxc.fs

@@ -45,6 +45,8 @@ type ProgramInvocationType =
     | FsxLauncherScript
     | FsxcPureInvocation
 
+type ExtraExtension = string
+
 exception NoScriptProvided
 
 module Program =
@@ -210,7 +212,7 @@ module Program =
     let GetBinFolderForAScript(script: FileInfo) =
         DirectoryInfo(Path.Combine(script.Directory.FullName, "bin"))
 
-    let GetAutoGenerationTargets (orig: FileInfo) (extension: string) =
+    let GetAutoGenerationTargets (orig: FileInfo) (extension: ExtraExtension) =
         let binDir = GetBinFolderForAScript(orig)
 
         let autogeneratedFileName =
@@ -477,7 +479,7 @@ module Program =
         let preprocessScriptContents
             (origScript: FileInfo)
             (contents: List<LineAction>)
-            : List<CompilerInput> * FileInfo =
+            : List<CompilerInput> * List<ExtraExtension> * FileInfo =
 
 #if LEGACY_FRAMEWORK
             // from "Microsoft.Build", "16.11.0" to .../packages/Microsoft.Build.16.11.0/lib/net472/Microsoft.Build.dll
@@ -580,13 +582,17 @@ module Program =
                 | Some location -> location
 #endif
 
-            let initialInjectedContents =
-                """module FsxScript
+            let initialFileContent =
+                """[<AutoOpen>]
+module FsxScriptInit
 type FsiStub = { CommandLineArgs: array<string> }
 let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
 """
 
-            let autogeneratedFile = GetAutoGenerationTargets origScript "fs"
+            let extraExtensionForMain: ExtraExtension = "fs"
+
+            let autogeneratedFile =
+                GetAutoGenerationTargets origScript extraExtensionForMain
 
             let binFolder = autogeneratedFile.Directory
 
@@ -597,11 +603,28 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
 
             File.WriteAllText(
                 autogeneratedFile.FullName,
-                initialInjectedContents
+                """module FsxScript
+"""
             )
 
-            seq {
+            let extraExtensionForInitFile: ExtraExtension = "init.fs"
+
+            let initFile =
+                GetAutoGenerationTargets origScript extraExtensionForInitFile
 
+            File.WriteAllText(initFile.FullName, initialFileContent)
+
+            let extraExtensions =
+                [
+                    extraExtensionForInitFile
+                    extraExtensionForMain
+                ]
+
+            seq {
+                // for !LEGACY_FRAMEWORK, this init file will be added to the fsproj file
+#if LEGACY_FRAMEWORK
+                yield CompilerInput.SourceFile initFile
+#endif
                 let startCommentInFSharp = "// "
 
                 for maybeDep in contents do
@@ -682,6 +705,7 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
                     )
             }
             |> List.ofSeq,
+            extraExtensions,
             autogeneratedFile
 
 #if LEGACY_FRAMEWORK
@@ -707,6 +731,7 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
         let generateProjectFile
             (origScript: FileInfo)
             (contents: List<LineAction>)
+            (extraExtensions: seq<ExtraExtension>)
             : FileInfo =
             let projectFile = GetAutoGenerationTargets origScript "fsproj"
 
@@ -805,7 +830,8 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
 
             iterate contents
 
-            addFile(sprintf "%s.fs" origScript.Name)
+            for ext in extraExtensions do
+                addFile(sprintf "%s.%s" origScript.Name ext)
 
             File.AppendAllText(projectFile.FullName, "</Project>")
 
@@ -817,7 +843,7 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
 
         let binFolder = GetBinFolderForAScript script
 
-        let compilerInputs, autoGenFile =
+        let compilerInputs, extraExtensions, autoGenFile =
             preprocessScriptContents script contents
 
         Console.WriteLine(
@@ -829,7 +855,10 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
 
         Console.WriteLine("_________________END AUTOGEN FILE_________________")
 #if !LEGACY_FRAMEWORK
-        let projectFile = generateProjectFile script contents
+        let projectFile = generateProjectFile script contents extraExtensions
+#else // dummy for to just prevent "unused" warning, meh
+        for _ext in extraExtensions do
+            ()
 #endif
 
         let exitCode, exeTarget =