Reuse some default values in FCS (#13035)

src/fsharp/service/FSharpCheckerResults.fs

@@ -1246,7 +1246,7 @@ type internal TypeCheckInfo
                         loadClosure.PackageReferences
                         |> Array.tryFind (fun (m, _) -> rangeContainsPos m pos)
                 match matches with
-                | None -> ToolTipText.ToolTipText []
+                | None -> emptyToolTip
                 | Some (_, lines) ->
                     let lines = lines |> List.filter (fun line -> not (line.StartsWith("//")) && not (String.IsNullOrEmpty line))
                     ToolTipText.ToolTipText
@@ -1283,7 +1283,7 @@ type internal TypeCheckInfo
                             ResolveOverloads.Yes, None, (fun() -> []))
 
                     match declItemsOpt with
-                    | None -> ToolTipText []
+                    | None -> emptyToolTip
                     | Some(items, denv, _, m) ->
                          ToolTipText(items |> List.map (fun x -> FormatStructuredDescriptionOfItem false infoReader tcAccessRights m denv x.ItemWithInst)))
 
@@ -1976,6 +1976,8 @@ type FSharpCheckFileResults
         | None -> dflt()
         | Some (scope, _builderOpt) -> f scope
 
+    static let emptyFindDeclResult = FindDeclResult.DeclNotFound (FindDeclFailureReason.Unknown "")
+
     member _.Diagnostics = errors
 
     member _.HasFullTypeCheckInfo = details.IsSome
@@ -1998,19 +2000,18 @@ type FSharpCheckFileResults
 
     /// Resolve the names at the given location to give a data tip
     member _.GetToolTip(line, colAtEndOfNames, lineText, names, tokenTag) =
-        let dflt = ToolTipText []
         match tokenTagToTokenId tokenTag with
         | TOKEN_IDENT ->
-            threadSafeOp (fun () -> dflt) (fun scope ->
+            threadSafeOp (fun () -> emptyToolTip) (fun scope ->
                 scope.GetStructuredToolTipText(line, lineText, colAtEndOfNames, names))
         | TOKEN_STRING | TOKEN_STRING_TEXT ->
-            threadSafeOp (fun () -> dflt) (fun scope ->
+            threadSafeOp (fun () -> emptyToolTip) (fun scope ->
                 scope.GetReferenceResolutionStructuredToolTipText(line, colAtEndOfNames) )
         | _ ->
-            dflt
+            emptyToolTip
 
     member _.GetDescription(symbol: FSharpSymbol, inst: (FSharpGenericParameter * FSharpType) list, displayFullName, range: range) =
-        threadSafeOp (fun () -> ToolTipText []) (fun scope ->
+        threadSafeOp (fun () -> emptyToolTip) (fun scope ->
             scope.GetDescription(symbol, inst, displayFullName, range))
 
     member _.GetF1Keyword (line, colAtEndOfNames, lineText, names) =
@@ -2019,13 +2020,11 @@ type FSharpCheckFileResults
 
     // Resolve the names at the given location to a set of methods
     member _.GetMethods(line, colAtEndOfNames, lineText, names) =
-        let dflt = MethodGroup("",[| |])
-        threadSafeOp (fun () -> dflt) (fun scope ->
+        threadSafeOp (fun () -> MethodGroup.Empty) (fun scope ->
             scope.GetMethods (line, lineText, colAtEndOfNames, names))
 
     member _.GetDeclarationLocation (line, colAtEndOfNames, lineText, names, ?preferFlag) =
-        let dflt = FindDeclResult.DeclNotFound (FindDeclFailureReason.Unknown "")
-        threadSafeOp (fun () -> dflt) (fun scope ->
+        threadSafeOp (fun () -> emptyFindDeclResult) (fun scope ->
             scope.GetDeclarationLocation (line, lineText, colAtEndOfNames, names, preferFlag))
 
     member _.GetSymbolUseAtLocation (line, colAtEndOfNames, lineText, names) =

src/fsharp/service/ServiceDeclarationLists.fs

@@ -59,7 +59,7 @@ type ToolTipElement =
 /// Information for building a data tip box.
 type ToolTipText = 
     /// A list of data tip elements to display.
-    | ToolTipText of ToolTipElement list  
+    | ToolTipText of ToolTipElement list
 
 [<RequireQualifiedAccess>]
 type CompletionItemKind =
@@ -87,7 +87,9 @@ type CompletionItem =
 
 [<AutoOpen>]
 module DeclarationListHelpers =
-    let mutable ToolTipFault  = None
+    let mutable ToolTipFault = None
+
+    let emptyToolTip = ToolTipText []
 
     /// Generate the structured tooltip for a method info
     let FormatOverloadsToList (infoReader: InfoReader) m denv (item: ItemWithInst) minfos : ToolTipElement = 
@@ -942,6 +944,8 @@ type DeclarationListItem(textInDeclList: string, textInCode: string, fullName: s
 type DeclarationListInfo(declarations: DeclarationListItem[], isForType: bool, isError: bool) = 
     static let fsharpNamespace = [|"Microsoft"; "FSharp"|]
 
+    static let empty = DeclarationListInfo ([| |], false, false)
+
     // Check whether this item looks like an operator.
     static let isOperatorItem name (items: CompletionItem list) =
         match items with
@@ -1088,7 +1092,7 @@ type DeclarationListInfo(declarations: DeclarationListItem[], isForType: bool, i
                 [| DeclarationListItem("<Note>", "<Note>", "<Note>", FSharpGlyph.Error, Choice2Of2 (ToolTipText [ToolTipElement.CompositionError message]),
                                              FSharpAccessibility(taccessPublic), CompletionItemKind.Other, false, 0, false, None) |], false, true)
     
-    static member Empty = DeclarationListInfo([| |], false, false)
+    static member Empty = empty
 
 
 
@@ -1134,6 +1138,8 @@ type MethodGroup( name: string, unsortedMethods: MethodGroupItem[] ) =
     static let methodOverloadsCache = System.Runtime.CompilerServices.ConditionalWeakTable<ItemWithInst, MethodGroupItem[]>()
 #endif
 
+    static let empty = MethodGroup ("", [| |])
+
     let methods = 
         unsortedMethods 
         // Methods with zero arguments show up here as taking a single argument of type 'unit'.  Patch them now to appear as having zero arguments.
@@ -1154,7 +1160,7 @@ type MethodGroup( name: string, unsortedMethods: MethodGroupItem[] ) =
 
     static member Create (infoReader: InfoReader, ad, m, denv, items:ItemWithInst list) = 
         let g = infoReader.g
-        if isNil items then MethodGroup("", [| |]) else
+        if isNil items then empty else
         let name = items.Head.Item.DisplayName 
 
         let methods = 
@@ -1210,5 +1216,5 @@ type MethodGroup( name: string, unsortedMethods: MethodGroupItem[] ) =
 
         MethodGroup(name, methods)
 
-
+    static member internal Empty = empty
 

src/fsharp/service/ServiceDeclarationLists.fsi

@@ -51,7 +51,7 @@ type public ToolTipElement =
 type public ToolTipText = 
 
     /// A list of data tip elements to display.
-    | ToolTipText of ToolTipElement list  
+    | ToolTipText of ToolTipElement list
 
 [<RequireQualifiedAccess>]
 type public CompletionItemKind =
@@ -209,6 +209,8 @@ type public MethodGroup =
 
     static member internal Create: InfoReader * AccessorDomain * range * DisplayEnv * ItemWithInst list -> MethodGroup
 
+    static member internal Empty: MethodGroup
+
 module internal DeclarationListHelpers =
     val FormatStructuredDescriptionOfItem: isDecl:bool -> InfoReader -> AccessorDomain -> range -> DisplayEnv -> ItemWithInst -> ToolTipElement
 
@@ -218,3 +220,5 @@ module internal DeclarationListHelpers =
 
     val mutable ToolTipFault: string option
 
+    val emptyToolTip: ToolTipText
+