Fix null handling and use Set for disabledLanguageFeatures

- Added null check in TryParseFeature using Option.bind before downcasting
- Changed disabledLanguageFeatures from array to Set in CompilerConfig (better semantics)
- Updated CompilerOptions to use Set.add and convert to array when calling WithDisabledFeatures
- Applied code formatting

Addresses comments from @T-Gro about null handling and using Set in the builder.

Co-authored-by: T-Gro <[email protected]>

Author: Copilot

src/Compiler/Driver/CompilerConfig.fs

@@ -644,7 +644,7 @@ type TcConfigBuilder =
 
         mutable langVersion: LanguageVersion
 
-        mutable disabledLanguageFeatures: LanguageFeature array
+        mutable disabledLanguageFeatures: Set<LanguageFeature>
 
         mutable xmlDocInfoLoader: IXmlDocumentationInfoLoader option
 
@@ -838,7 +838,7 @@ type TcConfigBuilder =
             pathMap = PathMap.empty
             applyLineDirectives = true
             langVersion = LanguageVersion.Default
-            disabledLanguageFeatures = [||]
+            disabledLanguageFeatures = Set.empty
             implicitIncludeDir = implicitIncludeDir
             defaultFSharpBinariesDir = defaultFSharpBinariesDir
             reduceMemoryUsage = reduceMemoryUsage

src/Compiler/Driver/CompilerConfig.fsi

@@ -512,7 +512,7 @@ type TcConfigBuilder =
 
         mutable langVersion: LanguageVersion
 
-        mutable disabledLanguageFeatures: LanguageFeature array
+        mutable disabledLanguageFeatures: Set<LanguageFeature>
 
         mutable xmlDocInfoLoader: IXmlDocumentationInfoLoader option
 

src/Compiler/Driver/CompilerOptions.fs

@@ -1180,7 +1180,7 @@ let languageFlags tcConfigB =
             OptionString(fun switch ->
                 let newVersion = setLanguageVersion switch
                 // Preserve disabled features when updating version
-                tcConfigB.langVersion <- newVersion.WithDisabledFeatures(tcConfigB.disabledLanguageFeatures)),
+                tcConfigB.langVersion <- newVersion.WithDisabledFeatures(Set.toArray tcConfigB.disabledLanguageFeatures)),
             None,
             Some(FSComp.SR.optsSetLangVersion ())
         )
@@ -1192,8 +1192,8 @@ let languageFlags tcConfigB =
             OptionStringList(fun featureName ->
                 match LanguageVersion.TryParseFeature(featureName) with
                 | Some feature ->
-                    tcConfigB.disabledLanguageFeatures <- Array.append tcConfigB.disabledLanguageFeatures [| feature |]
-                    tcConfigB.langVersion <- tcConfigB.langVersion.WithDisabledFeatures(tcConfigB.disabledLanguageFeatures)
+                    tcConfigB.disabledLanguageFeatures <- Set.add feature tcConfigB.disabledLanguageFeatures
+                    tcConfigB.langVersion <- tcConfigB.langVersion.WithDisabledFeatures(Set.toArray tcConfigB.disabledLanguageFeatures)
                 | None -> error (Error(FSComp.SR.optsUnrecognizedLanguageFeature featureName, rangeCmdArgs))),
             None,
             Some(FSComp.SR.optsDisableLanguageFeature ())

src/Compiler/Facilities/LanguageFeatures.fs

@@ -440,7 +440,13 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
             ||| System.Reflection.BindingFlags.NonPublic
         )
         |> Array.tryFind (fun case -> System.String.Equals(case.Name, normalized, System.StringComparison.OrdinalIgnoreCase))
-        |> Option.map (fun case -> Microsoft.FSharp.Reflection.FSharpValue.MakeUnion(case, [||]) :?> LanguageFeature)
+        |> Option.bind (fun case ->
+            let union = Microsoft.FSharp.Reflection.FSharpValue.MakeUnion(case, [||])
+
+            if isNull union then
+                None
+            else
+                Some(union :?> LanguageFeature))
 
     override x.Equals(yobj: obj) =
         match yobj with