Fix a bug in the Add Open code fix

vsintegration/src/FSharp.Editor/CodeFixes/AddOpenCodeFixProvider.fs

@@ -55,21 +55,26 @@ type internal AddOpenCodeFixProvider [<ImportingConstructor>] (assemblyContentPr
 
                 // attribute, shouldn't be here
                 | line when line.StartsWith "[<" && line.EndsWith ">]" ->
-                    let moduleDeclLineNumber =
+                    let moduleDeclLineNumberOpt =
                         sourceText.Lines
                         |> Seq.skip insertionLineNumber
-                        |> Seq.findIndex (fun line -> line.ToString().Contains "module")
-                        // add back the skipped lines
-                        |> fun i -> insertionLineNumber + i
+                        |> Seq.tryFindIndex (fun line -> line.ToString().Contains "module")
 
-                    let moduleDeclLineText = sourceText.Lines[ moduleDeclLineNumber ].ToString().Trim()
+                    match moduleDeclLineNumberOpt with
+                    // implicit top level module
+                    | None -> insertionLineNumber, $"{margin}open {ns}{br}{br}"
+                    // explicit top level module
+                    | Some number ->
+                        // add back the skipped lines
+                        let moduleDeclLineNumber = insertionLineNumber + number
+                        let moduleDeclLineText = sourceText.Lines[ moduleDeclLineNumber ].ToString().Trim()
 
-                    if moduleDeclLineText.EndsWith "=" then
-                        insertionLineNumber, $"{margin}open {ns}{br}{br}"
-                    else
-                        moduleDeclLineNumber + 2, $"{margin}open {ns}{br}{br}"
+                        if moduleDeclLineText.EndsWith "=" then
+                            insertionLineNumber, $"{margin}open {ns}{br}{br}"
+                        else
+                            moduleDeclLineNumber + 2, $"{margin}open {ns}{br}{br}"
 
-                // something else, shot in the dark
+                // implicit top level module
                 | _ -> insertionLineNumber, $"{margin}open {ns}{br}{br}"
 
             | ScopeKind.Namespace -> insertionLineNumber + 3, $"{margin}open {ns}{br}{br}"

vsintegration/tests/FSharp.Editor.Tests/CodeFixes/AddOpenOnTopOffTests.fs

@@ -159,7 +159,7 @@ let ``Fixes FS0039 for missing opens - nested module`` () =
     Assert.Equal(expected, actual)
 
 [<Fact>]
-let ``Fixes FS0039 for missing opens - module has attributes`` () =
+let ``Fixes FS0039 for missing opens - explicit module has attributes`` () =
     let code =
         """
 [<AutoOpen>]
@@ -187,6 +187,33 @@ Console.WriteLine 42
 
     Assert.Equal(expected, actual)
 
+[<Fact>]
+let ``Fixes FS0039 for missing opens - implicit module has attributes`` () =
+    let code =
+        """
+[<Obsolete>]
+type MyType() =
+    let now = DateTime.Now
+"""
+
+    let expected =
+        Some
+            {
+                Message = "open System"
+                FixedCode =
+                    """
+open System
+
+[<Obsolete>]
+type MyType() =
+    let now = DateTime.Now
+"""
+            }
+
+    let actual = codeFix |> tryFix code Auto
+
+    Assert.Equal(expected, actual)
+
 // TODO: the open statement should actually be within the module
 [<Fact>]
 let ``Fixes FS0039 for missing opens - nested module has attributes`` () =

vsintegration/tests/FSharp.Editor.Tests/CodeFixes/AddOpenOnTopOnTests.fs

@@ -153,7 +153,7 @@ module Module1 =
     Assert.Equal(expected, actual)
 
 [<Fact>]
-let ``Fixes FS0039 for missing opens - module has attributes`` () =
+let ``Fixes FS0039 for missing opens - explicit module has attributes`` () =
     let code =
         """
 [<AutoOpen>]
@@ -181,6 +181,33 @@ Console.WriteLine 42
 
     Assert.Equal(expected, actual)
 
+[<Fact>]
+let ``Fixes FS0039 for missing opens - implicit module has attributes`` () =
+    let code =
+        """
+[<Obsolete>]
+type MyType() =
+    let now = DateTime.Now
+"""
+
+    let expected =
+        Some
+            {
+                Message = "open System"
+                FixedCode =
+                    """
+open System
+
+[<Obsolete>]
+type MyType() =
+    let now = DateTime.Now
+"""
+            }
+
+    let actual = codeFix |> tryFix code Auto
+
+    Assert.Equal(expected, actual)
+
 [<Fact>]
 let ``Fixes FS0039 for missing opens - nested module has attributes`` () =
     let code =