Add parameter name hints for discriminated unions

src/Compiler/Service/FSharpCheckerResults.fs

@@ -242,6 +242,8 @@ type FSharpSymbolUse(denv: DisplayEnv, symbol: FSharpSymbol, inst: TyparInstanti
 
     member _.IsFromDispatchSlotImplementation = itemOcc = ItemOccurence.Implemented
 
+    member _.IsFromUse = itemOcc = ItemOccurence.Use
+
     member _.IsFromComputationExpression =
         match symbol.Item, itemOcc with
         // 'seq' in 'seq { ... }' gets colored as keywords

src/Compiler/Service/FSharpCheckerResults.fsi

@@ -171,6 +171,9 @@ type public FSharpSymbolUse =
     /// Indicates if the reference is in open statement
     member IsFromOpenStatement: bool
 
+    /// Indicates if the reference is used for example at a call site
+    member IsFromUse: bool
+
     /// The file name the reference occurs in
     member FileName: string
 

tests/FSharp.Compiler.Service.Tests/FSharp.CompilerService.SurfaceArea.netstandard.expected

@@ -2163,6 +2163,7 @@ FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean IsFromOpenStatement
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean IsFromPattern
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean IsFromType
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean IsPrivateToFile
+FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean IsFromUse
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsFromAttribute()
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsFromComputationExpression()
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsFromDefinition()
@@ -2171,6 +2172,7 @@ FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsFromOpenStatement()
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsFromPattern()
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsFromType()
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsPrivateToFile()
+FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: Boolean get_IsFromUse()
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: FSharp.Compiler.Symbols.FSharpDisplayContext DisplayContext
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: FSharp.Compiler.Symbols.FSharpDisplayContext get_DisplayContext()
 FSharp.Compiler.CodeAnalysis.FSharpSymbolUse: FSharp.Compiler.Symbols.FSharpSymbol Symbol

vsintegration/src/FSharp.Editor/Hints/HintService.fs

@@ -19,9 +19,15 @@ module HintService =
 
         | :? FSharpMemberOrFunctionOrValue as symbol
           when hintKinds |> Set.contains HintKind.ParameterNameHint 
-            && InlineParameterNameHints.isValidForHint symbol ->
+            && InlineParameterNameHints.isMemberOrFunctionOrValueValidForHint symbol ->
 
-            InlineParameterNameHints.getHints parseResults symbol symbolUse
+            InlineParameterNameHints.getHintsForMemberOrFunctionOrValue parseResults symbol symbolUse
+
+        | :? FSharpUnionCase as symbol
+          when hintKinds |> Set.contains HintKind.ParameterNameHint
+            && InlineParameterNameHints.isUnionCaseValidForHint symbol symbolUse ->
+
+          InlineParameterNameHints.getHintsForUnionCase parseResults symbol symbolUse
 
         // we'll be adding other stuff gradually here
         | _ -> 

vsintegration/src/FSharp.Editor/Hints/InlineParameterNameHints.fs

@@ -10,17 +10,27 @@ open Hints
 
 module InlineParameterNameHints =
 
-    let private getHint (range: range, parameter: FSharpParameter) =
+    let private getParameterHint (range: range, parameter: FSharpParameter) =
         {
             Kind = HintKind.ParameterNameHint
             Range = range.StartRange
             Parts = [ TaggedText(TextTag.Text, $"{parameter.DisplayName} = ") ]
         }
 
+    let private getFieldHint (range: range, field: FSharpField) =
+        {
+            Kind = HintKind.ParameterNameHint
+            Range = range.StartRange
+            Parts = [ TaggedText(TextTag.Text, $"{field.Name} = ") ]
+        }
+
     let private doesParameterNameExist (parameter: FSharpParameter) = 
         parameter.DisplayName <> ""
 
-    let isValidForHint (symbol: FSharpMemberOrFunctionOrValue) =
+    let private doesFieldNameExist (field: FSharpField) = 
+        not field.IsNameGenerated
+
+    let isMemberOrFunctionOrValueValidForHint (symbol: FSharpMemberOrFunctionOrValue) =
         // is there a better way?
         let isNotBuiltInOperator = 
             symbol.DeclaringEntity 
@@ -29,7 +39,12 @@ module InlineParameterNameHints =
         symbol.IsFunction
         && isNotBuiltInOperator // arguably, hints for those would be rather useless
 
-    let getHints 
+    let isUnionCaseValidForHint (symbol: FSharpUnionCase) (symbolUse: FSharpSymbolUse) =
+        // is the union case being used as a constructor and is it not Cons
+        symbolUse.IsFromUse
+        && symbol.DisplayName <> "(::)"
+
+    let getHintsForMemberOrFunctionOrValue
         (parseResults: FSharpParseFileResults) 
         (symbol: FSharpMemberOrFunctionOrValue) 
         (symbolUse: FSharpSymbolUse) =
@@ -42,8 +57,31 @@ module InlineParameterNameHints =
             parameters
             |> Seq.zip ranges
             |> Seq.where (snd >> doesParameterNameExist)
-            |> Seq.map getHint
+            |> Seq.map getParameterHint
             |> Seq.toList
 
         // this is the case at least for custom operators
         | None -> []
+
+    let getHintsForUnionCase
+        (parseResults: FSharpParseFileResults) 
+        (symbol: FSharpUnionCase) 
+        (symbolUse: FSharpSymbolUse) =
+
+        let fields = Seq.toList symbol.Fields
+
+        // If a case does not use field names, don't even bother getting applied argument ranges
+        if fields |> List.exists doesFieldNameExist |> not then
+            []
+        else
+            let ranges = parseResults.GetAllArgumentsForFunctionApplicationAtPosition symbolUse.Range.Start
+
+            // When not all field values are provided (as the user is typing), don't show anything yet
+            match ranges with
+            | Some ranges when ranges.Length = fields.Length -> 
+                fields
+                |> List.zip ranges
+                |> List.where (snd >> doesFieldNameExist)
+                |> List.map getFieldHint
+
+            | _ -> []

vsintegration/src/FSharp.Editor/Hints/RoslynAdapter.fs

@@ -22,19 +22,22 @@ type internal RoslynAdapter
     interface IFSharpInlineHintsService with
         member _.GetInlineHintsAsync(document, _, cancellationToken) =
             async {
-                let! sourceText = document.GetTextAsync cancellationToken |> Async.AwaitTask
                 let hintKinds = OptionParser.getHintKinds settings.Advanced
 
-                let! nativeHints =
-                    HintService.getHintsForDocument 
-                        document 
-                        hintKinds
-                        userOpName 
-                        cancellationToken
-
-                let roslynHints = 
-                    nativeHints 
-                    |> Seq.map (NativeToRoslynHintConverter.convert sourceText)
-
-                return roslynHints.ToImmutableArray()
+                if hintKinds.IsEmpty then
+                    return ImmutableArray.Empty
+                else
+                    let! sourceText = document.GetTextAsync cancellationToken |> Async.AwaitTask
+                    let! nativeHints =
+                        HintService.getHintsForDocument 
+                            document 
+                            hintKinds
+                            userOpName 
+                            cancellationToken
+
+                    let roslynHints = 
+                        nativeHints 
+                        |> Seq.map (NativeToRoslynHintConverter.convert sourceText)
+
+                    return roslynHints.ToImmutableArray()
             } |> RoslynHelpers.StartAsyncAsTask cancellationToken

vsintegration/tests/UnitTests/InlineParameterNameHintTests.fs

@@ -159,17 +159,72 @@ let wrapped = WrappedThing 42
     Assert.IsEmpty(result)
 
 [<Test>]
-let ``Hints are not (yet) shown for dicrimanted unions`` () =
+let ``Hints are shown for discriminated union case fields with explicit names`` () =
     let code = """
 type Shape =
-    | Square of side : float
-    | Circle of radius : float
+    | Square of side: int
+    | Rectangle of width: int * height: int
+
+let a = Square 1
+let b = Rectangle (1, 2)
+"""
+    let document = getFsDocument code
+
+    let expected = [
+        { Content = "side = "; Location = (5, 16) }
+        { Content = "width = "; Location = (6, 20) }
+        { Content = "height = "; Location = (6, 23) }
+    ]
+
+    let actual = getParameterNameHints document
+
+    Assert.AreEqual(expected, actual)
 
-let circle = Circle 42
+[<Test>]
+let ``Hints for discriminated union case fields are not shown when names are generated`` () =
+    let code = """
+type Shape =
+    | Triangle of side1: int * int * side3: int
+    | Circle of int
+
+let c = Triangle (1, 2, 3)
+let d = Circle 1
 """
     let document = getFsDocument code
 
-    let result = getParameterNameHints document
+    let expected = [
+        { Content = "side1 = "; Location = (5, 19) }
+        { Content = "side3 = "; Location = (5, 25) }
+    ]
 
-    Assert.IsEmpty(result)
+    let actual = getParameterNameHints document
+
+    Assert.AreEqual(expected, actual)
+
+[<Test>]
+let ``Hints for discriminated union case fields are not shown when provided arguments don't match the expected count`` () =
+    let code = """
+type Shape =
+    | Triangle of side1: int * side2: int * side3: int
+    | Circle of int
+
+let c = Triangle (1, 2)
+"""
+    let document = getFsDocument code
+
+    let actual = getParameterNameHints document
+
+    Assert.IsEmpty(actual)
+
+[<Test>]
+let ``Hints for discriminated union case fields are not shown for Cons`` () =
+    let code = """
+type X =
+    member _.Test() = 42 :: [42; 42]
+"""
+    let document = getFsDocument code
+
+    let actual = getParameterNameHints document
+
+    Assert.IsEmpty(actual)
 

vsintegration/tests/UnitTests/OverallHintExperienceTests.fs

@@ -15,12 +15,24 @@ type Song = { Artist: string; Title: string }
 let whoSings song = song.Artist
 
 let artist = whoSings { Artist = "Květy"; Title = "Je podzim" }
+
+type Shape =
+    | Square of side: int
+    | Rectangle of width: int * height: int
+
+let a = Square 1
+let b = Rectangle (1, 2)
 """
     let document = getFsDocument code
     let expected = [
         { Content = ": Song"; Location = (2, 18) }
-        { Content = ": string"; Location = (4, 11) }
         { Content = "song = "; Location = (4, 23) }
+        { Content = ": string"; Location = (4, 11) }
+        { Content = "side = "; Location = (10, 16) }
+        { Content = ": Shape"; Location = (10, 6) }
+        { Content = "width = "; Location = (11, 20) }
+        { Content = "height = "; Location = (11, 23) }
+        { Content = ": Shape"; Location = (11, 6) }
     ]
 
     let actual = getAllHints document