Fix #2649 Backslash not escaped in interpolated

src/Fable.Transforms/BabelPrinter.fs

@@ -489,17 +489,21 @@ module PrinterExtensions =
                 let segmentLength = segmentEnd - segmentStart
                 if segmentLength > 0 then
                     let segment = value.Substring(segmentStart, segmentLength)
-                    let subSegments = System.Text.RegularExpressions.Regex.Split(segment, @"\r?\n")
-                    for i = 1 to subSegments.Length do
-                        let subSegment =
-                            // Remove whitespace in front of new lines,
-                            // indent will be automatically applied
-                            if printer.Column = 0 then subSegments.[i - 1].TrimStart()
-                            else subSegments.[i - 1]
-                        if subSegment.Length > 0 then
-                            printer.Print(subSegment)
-                            if i < subSegments.Length then
-                                printer.PrintNewLine()
+
+                    // TODO: In Fable 3 we're using emit for js template strings, if we give them
+                    // their own AST entry in Fable 4 we can activate the below code to clean up printed code
+                    printer.Print(segment)
+                    // let subSegments = System.Text.RegularExpressions.Regex.Split(segment, @"\r?\n")
+                    // for i = 1 to subSegments.Length do
+                    //     let subSegment =
+                    //         // Remove whitespace in front of new lines,
+                    //         // indent will be automatically applied
+                    //         if printer.Column = 0 then subSegments.[i - 1].TrimStart()
+                    //         else subSegments.[i - 1]
+                    //     if subSegment.Length > 0 then
+                    //         printer.Print(subSegment)
+                    //         if i < subSegments.Length then
+                    //             printer.PrintNewLine()
 
             // Macro transformations
             // https://fable.io/docs/communicate/js-from-fable.html#Emit-when-F-is-not-enough

src/Fable.Transforms/Replacements.fs

@@ -1403,7 +1403,7 @@ let printJsTaggedTemplate (str: string) (holes: {| Index: int; Length: int |}[])
     // Escape ` quotations for JS. Note F# escapes for {, } and % are already replaced by the compiler
     // TODO: Do we need to escape other sequences? See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#tagged_templates_and_escape_sequences
     let escape (str: string) =
-        str.Replace("`", "\\`") //.Replace("{{", "{").Replace("}}", "}").Replace("%%", "%")
+        Regex.Replace(str, @"(?<!\\)\\", @"\\").Replace("`", @"\`") //.Replace("{{", "{").Replace("}}", "}").Replace("%%", "%")
 
     let sb = System.Text.StringBuilder("`")
     let mutable prevIndex = 0

tests/Main/StringTests.fs

@@ -213,8 +213,8 @@ let tests =
             let o = obj()
             o?self <- o
             sprintf "%A" o |> ignore
+#endif
 
-// TODO!!! Enable these in .NET when CI supports net5.0
       testCase "F# nameof works" <| fun () ->
           M.f 12 |> equal "x"
           nameof M |> equal "M"
@@ -238,7 +238,44 @@ let tests =
                  Country = "The United Kingdom" |}
           $"Hi! My name is %s{person.Name} %s{person.Surname.ToUpper()}. I'm %i{person.Age} years old and I'm from %s{person.Country}!"
           |> equal "Hi! My name is John DOE. I'm 32 years old and I'm from The United Kingdom!"
-#endif
+
+      testCase "Interpolated strings keep empty lines" <| fun () ->
+        let s1 = $"""1
+
+
+    {1+1}
+
+
+3"""
+        let s2 = """1
+
+
+    2
+
+
+3"""
+        equal s1 s2
+        equal s1.Length s2.Length
+        equal 13 s1.Length
+
+      testCase "Can use backslash is interpolated strings" <| fun () ->
+        $"\n{1+1}\n" |> equal """
+2
+"""
+
+      testCase "Backslash is escaped in interpolated strings" <| fun () -> // See #2649
+            $"\\" |> equal @"\"
+            $"\\".Length |> equal 1
+            $@"\" |> equal @"\"
+            $@"\".Length |> equal 1
+            @$"\" |> equal @"\"
+            @$"\".Length |> equal 1
+            $"\\{4}" |> equal @"\4"
+            $"\\{4}".Length |> equal 2
+            $@"\{4}" |> equal @"\4"
+            $@"\{4}".Length |> equal 2
+            @$"\{4}" |> equal @"\4"
+            @$"\{4}".Length |> equal 2
 
       testCase "sprintf \"%A\" with lists works" <| fun () ->
             let xs = ["Hi"; "Hello"; "Hola"]