Don't trim end of line inside a String. Fixes fsprojects#1941.

.fantomasignore

@@ -0,0 +1,2 @@
+**/src/Fantomas.Tests/StringTests.fs
+**/src/Fantomas.Tests/CommentTests.fs

src/Fantomas.Tests/CommentTests.fs

@@ -1685,3 +1685,22 @@ with
     with
     | d -> ()
 """
+
+[<Test>]
+let ``trailing spaces in comments should be removed`` () =
+    formatSourceString
+        false
+        """
+// foo       
+// bar          
+let a = 9
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+// foo
+// bar
+let a = 9
+"""

src/Fantomas.Tests/ContextTests.fs

@@ -66,6 +66,7 @@ let ``don't add space before block comment`` () =
 Long comment
 
 *)"""
+        |> String.normalizeNewLine
 
     let expr =
         sepNone +> sepSpace -- comment

src/Fantomas.Tests/StringTests.fs

@@ -323,3 +323,21 @@ let ``single string with compiler define`` () =
         """
 "#if FOO"
 """
+
+[<Test>]
+let ``trailing spaces in string should be preserved, 1941`` () =
+    formatSourceString
+        false
+        "
+let s = \"\"\"aaaa   
+bbb\"\"\"
+"
+        config
+    |> prepend newline
+    |> should
+        equal
+        "
+let s =
+    \"\"\"aaaa   
+bbb\"\"\"
+"

src/Fantomas/Context.fs

@@ -83,22 +83,35 @@ module WriterModel =
                 { m with
                       Indent = max m.Indent m.AtColumn }
 
+            let nextLine = String.replicate m.Indent " "
+
+            let currentLine =
+                String
+                    .Concat(List.head m.Lines, m.WriteBeforeNewline)
+                    .TrimEnd()
+
+            let otherLines = List.tail m.Lines
+
             { m with
-                  Lines =
-                      String.replicate m.Indent " "
-                      :: (List.head m.Lines + m.WriteBeforeNewline)
-                         :: (List.tail m.Lines)
+                  Lines = nextLine :: currentLine :: otherLines
                   WriteBeforeNewline = ""
                   Column = m.Indent }
 
         let updateCmd cmd =
             match cmd with
             | WriteLine
             | WriteLineBecauseOfTrivia -> doNewline m
-            | WriteLineInsideStringConst
+            | WriteLineInsideStringConst ->
+                { m with
+                      Lines = String.empty :: m.Lines
+                      Column = 0 }
             | WriteLineInsideTrivia ->
+                let lines =
+                    match m.Lines with
+                    | [] -> [ String.empty ]
+                    | h::tail -> String.empty :: (h.TrimEnd()) :: tail
                 { m with
-                      Lines = "" :: m.Lines
+                      Lines = lines
                       Column = 0 }
             | Write s ->
                 { m with
@@ -319,10 +332,13 @@ let internal finalizeWriterModel (ctx: Context) =
 let internal dump (ctx: Context) =
     let ctx = finalizeWriterModel ctx
 
-    ctx.WriterModel.Lines
+    match ctx.WriterModel.Lines with
+    | [] -> []
+    | h :: tail ->
+        // Always trim the last line
+        h.TrimEnd() :: tail
     |> List.rev
     |> List.skipWhile ((=) "")
-    |> List.map (fun line -> line.TrimEnd())
     |> String.concat ctx.Config.EndOfLine.NewLineString
 
 let internal dumpAndContinue (ctx: Context) =

src/Fantomas/Utils.fs

@@ -80,8 +80,6 @@ module String =
                 else
                     b')
 
-
-
     let empty = String.Empty
 
     let isNotNullOrEmpty = String.IsNullOrEmpty >> not