Improve detection for ToString case in MA0028

src/Meziantou.Analyzer/Internals/HashSetExtensions.cs

@@ -0,0 +1,13 @@
+using System.Collections.Generic;
+
+namespace Meziantou.Analyzer.Internals;
+internal static class HashSetExtensions
+{
+    public static void AddIfNotNull<T>(this HashSet<T> hashSet, T? item)
+    {
+        if (item is null)
+            return;
+
+        _ = hashSet.Add(item);
+    }
+}

src/Meziantou.Analyzer/Internals/SymbolExtensions.cs

@@ -7,7 +7,7 @@ namespace Meziantou.Analyzer;
 
 internal static class SymbolExtensions
 {
-    public static bool IsEqualTo(this ISymbol? symbol, ISymbol? expectedType)
+    public static bool IsEqualTo(this ISymbol? symbol, [NotNullWhen(true)] ISymbol? expectedType)
     {
         if (symbol is null || expectedType is null)
             return false;