TagHelperCollection Part 3: Optimize tag helper change detection logic in source generator

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/SourceGeneratorProjectEngine.cs

@@ -1,12 +1,10 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Microsoft.AspNetCore.Razor.Language;
 using System;
-using System.Collections.Generic;
 using System.Diagnostics;
-using System.Linq;
 using System.Threading;
-using Microsoft.AspNetCore.Razor.Language;
 
 namespace Microsoft.NET.Sdk.Razor.SourceGenerators;
 
@@ -65,7 +63,11 @@ public SourceGeneratorRazorCodeDocument ProcessInitialParse(RazorProjectItem pro
         return new SourceGeneratorRazorCodeDocument(codeDocument);
     }
 
-    public SourceGeneratorRazorCodeDocument ProcessTagHelpers(SourceGeneratorRazorCodeDocument sgDocument, TagHelperCollection tagHelpers, bool checkForIdempotency, CancellationToken cancellationToken)
+    public SourceGeneratorRazorCodeDocument ProcessTagHelpers(
+        SourceGeneratorRazorCodeDocument sgDocument, 
+        TagHelperCollection tagHelpers, 
+        bool checkForIdempotency, 
+        CancellationToken cancellationToken)
     {
         Debug.Assert(sgDocument.CodeDocument.GetPreTagHelperSyntaxTree() is not null);
 
@@ -88,12 +90,11 @@ public SourceGeneratorRazorCodeDocument ProcessTagHelpers(SourceGeneratorRazorCo
             // re-run discovery to figure out which tag helpers are now in scope for this document
             codeDocument.SetTagHelpers(tagHelpers);
             _discoveryPhase.Execute(codeDocument, cancellationToken);
-            var tagHelpersInScope = codeDocument.GetRequiredTagHelperContext().TagHelpers;
+
+            var newTagHelpersInScope = codeDocument.GetRequiredTagHelperContext().TagHelpers;
 
             // Check if any new tag helpers were added or ones we previously used were removed
-            var newVisibleTagHelpers = tagHelpersInScope.Except(previousTagHelpersInScope);
-            var newUnusedTagHelpers = previousUsedTagHelpers.Except(tagHelpersInScope);
-            if (!newVisibleTagHelpers.Any() && !newUnusedTagHelpers.Any())
+            if (!RequiresRewrite(newTagHelpersInScope, previousTagHelpersInScope, previousUsedTagHelpers))
             {
                 // No newly visible tag helpers, and any that got removed weren't used by this document anyway
                 return sgDocument;
@@ -112,6 +113,56 @@ public SourceGeneratorRazorCodeDocument ProcessTagHelpers(SourceGeneratorRazorCo
         return new SourceGeneratorRazorCodeDocument(codeDocument);
     }
 
+    private static bool RequiresRewrite(
+        TagHelperCollection newTagHelpers,
+        TagHelperCollection previousTagHelpers,
+        TagHelperCollection previousUsedTagHelpers)
+    {
+        // Check if any new tag helpers were added (that weren't in scope before)
+        // Check if any previously used tag helpers were removed (no longer in scope)
+        return HasAnyNotIn(newTagHelpers, previousTagHelpers) ||
+               HasAnyNotIn(previousUsedTagHelpers, newTagHelpers);
+    }
+
+    /// <summary>
+    ///  Determines whether the first collection contains any tag helper descriptors that are not present
+    ///  in the second collection.
+    /// </summary>
+    /// <param name="first">The collection to check for unique items.</param>
+    /// <param name="second">The collection to compare against.</param>
+    /// <returns>
+    ///  <see langword="true"/> if <paramref name="first"/> contains any descriptors not present in 
+    ///  <paramref name="second"/>; otherwise, <see langword="false"/>.
+    /// </returns>
+    private static bool HasAnyNotIn(TagHelperCollection first, TagHelperCollection second)
+    {
+        if (first.IsEmpty)
+        {
+            return false;
+        }
+
+        if (second.IsEmpty)
+        {
+            return true;
+        }
+
+        if (first.Equals(second))
+        {
+            return false;
+        }
+
+        // For each item in the first collection, check if it exists in the second collection
+        foreach (var item in first)
+        {
+            if (!second.Contains(item))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     public SourceGeneratorRazorCodeDocument ProcessRemaining(SourceGeneratorRazorCodeDocument sgDocument, CancellationToken cancellationToken)
     {
         var codeDocument = sgDocument.CodeDocument;