Optimize tag helper change detection logic in source generator

Refactors the tag helper change detection logic to be more efficient and avoid allocations by directly comparing collections rather than using LINQ's Except method.

Author: DustinCampbell

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;