Core: add JSON converter for simple DUs

Add a JSON converter for discriminated unions that only have
cases with no fields. It maps DUs to strings, like
JsonStringEnumConverter does it for enums.

This fixes the tests for IndexerAccessorStyleConsistency rule (for
serializing its config simple DU called IndexerAccessorStyle).

Author: webwarrior-ws

src/FSharpLint.Core/Application/Configuration.fs

@@ -6,6 +6,7 @@ open System.IO
 open System.Reflection
 open System.Text.Json
 open System.Text.Json.Serialization
+open Microsoft.FSharp.Reflection
 open FSharpLint.Framework
 open FSharpLint.Framework.Rules
 open FSharpLint.Framework.HintParser
@@ -17,6 +18,40 @@ let SettingsFileName = "fsharplint.json"
 exception ConfigurationException of string
 
 module internal FSharpJsonConverter =
+    
+    type private SimpleDUConverter<'TDiscriminatedUnion>() =
+        inherit JsonConverter<'TDiscriminatedUnion>()
+
+        let allCases = FSharpType.GetUnionCases typeof<'TDiscriminatedUnion> 
+
+        override this.Read (reader: byref<Text.Json.Utf8JsonReader>, typeToConvert: Type, options: Text.Json.JsonSerializerOptions): 'TDiscriminatedUnion = 
+            let value = reader.GetString()
+            match allCases |> Array.tryFind (fun case -> case.Name = value) with
+            | Some case -> FSharpValue.MakeUnion(case, Array.empty) :?> 'TDiscriminatedUnion
+            | _ -> failwithf "Unexpected value: %A. Expected one of: %A" value allCases
+
+        override this.Write (writer: Text.Json.Utf8JsonWriter, value: 'TDiscriminatedUnion, options: Text.Json.JsonSerializerOptions): unit = 
+            writer.WriteStringValue(value.ToString())
+
+    /// JSON converter  for discriminated unions that only have cases with no fields.
+    /// Maps DUs to strings, like JsonStringEnumConverter.
+    type SimpleDiscriminatedUnionJsonConverter() =
+        inherit JsonConverterFactory()
+
+        override this.CanConvert (typeToConvert: Type): bool = 
+            FSharpType.IsUnion typeToConvert 
+            && (FSharpType.GetUnionCases typeToConvert |> Array.forall (fun typ -> typ.GetFields().Length = 0))
+
+        override this.CreateConverter (typeToConvert: Type, options: JsonSerializerOptions): JsonConverter =
+            let converterType = typedefof<SimpleDUConverter<_>>
+            Activator.CreateInstance(
+                converterType.MakeGenericType(Array.singleton typeToConvert),
+                BindingFlags.Instance ||| BindingFlags.Public,
+                binder=null,
+                args=Array.empty,
+                culture=null
+            )
+            :?> JsonConverter
 
     let jsonOptions =
         let options =
@@ -25,6 +60,7 @@ module internal FSharpJsonConverter =
                 DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
             )
         options.Converters.Add(JsonStringEnumConverter())
+        options.Converters.Add(SimpleDiscriminatedUnionJsonConverter())
         options
 
 module IgnoreFiles =