Fix completion with backticks, underscores, numbers

vsintegration/src/FSharp.Editor/Completion/CompletionService.fs

@@ -10,9 +10,12 @@ open Microsoft.CodeAnalysis.Completion
 open Microsoft.CodeAnalysis.Host
 open Microsoft.CodeAnalysis.Host.Mef
 open Microsoft.CodeAnalysis.ExternalAccess.FSharp.Completion
+open Microsoft.CodeAnalysis.Text
 
 open Microsoft.VisualStudio.Shell
 
+open FSharp.Compiler.PrettyNaming
+
 type internal FSharpCompletionService
     (
         workspace: Workspace,
@@ -43,6 +46,41 @@ type internal FSharpCompletionService
             .WithDismissIfLastCharacterDeleted(true)
             .WithDefaultEnterKeyRule(enterKeyRule)
 
+    override _.GetDefaultCompletionListSpan( sourceText , caretIndex ) =
+
+        let mutable startIndex = 0
+        let mutable endIndex = 0
+
+        // Get single line text and index
+        let textLines = sourceText.Lines
+        let lineText = textLines.GetLineFromPosition(caretIndex).ToString()
+        let lineCaretIndex = textLines.GetLinePosition(caretIndex).Character
+
+        // Check for enclosing backticks or leading backticks, else capture valid identifier characters
+        // TODO Replace naive backtick check with safer alternative
+        match lineText.IndexOf "``", lineText.LastIndexOf "``" with
+        | startTickIndex, endTickIndex when startTickIndex > -1 && endTickIndex > -1 && startTickIndex <> endTickIndex && lineCaretIndex >= startTickIndex && lineCaretIndex <= endTickIndex + 2 ->
+            // Cursor is at or between a pair of double ticks, select enclosed range including ticks as identifier
+            startIndex <- startTickIndex
+            endIndex <- endTickIndex + 2
+        | startTickIndex, endTickIndex when startTickIndex > -1 && endTickIndex > -1 && startTickIndex = endTickIndex && lineCaretIndex >= startTickIndex ->
+            // Cursor is at or after double ticks with none following, select ticks and all following text as identifier
+            startIndex <- startTickIndex
+            endIndex <- lineText.Length
+        | _ ->
+            // No ticks, capture identifier-part chars backward and forward from cursor as identifier
+            startIndex <- lineCaretIndex
+            while startIndex > 0 && IsIdentifierPartCharacter lineText.[startIndex - 1] do startIndex <- startIndex - 1
+            endIndex <- lineCaretIndex
+            if startIndex <> lineCaretIndex then
+                while endIndex < lineText.Length && IsIdentifierPartCharacter lineText.[endIndex] do endIndex <- endIndex + 1
+
+        // Translate line index back to document index
+        startIndex <- caretIndex - (lineCaretIndex - startIndex)
+        endIndex <- caretIndex + (endIndex - lineCaretIndex)
+
+        TextSpan.FromBounds(startIndex, endIndex)
+
 [<Shared>]
 [<ExportLanguageServiceFactory(typeof<CompletionService>, FSharpConstants.FSharpLanguageName)>]
 type internal FSharpCompletionServiceFactory