[Perfathon] Stop resizing a list

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/DefaultRazorTagHelperContextDiscoveryPhase.cs

@@ -5,11 +5,12 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Diagnostics;
-using System.Linq;
 using Microsoft.AspNetCore.Razor.Language.Components;
 using Microsoft.AspNetCore.Razor.Language.Legacy;
 using Microsoft.AspNetCore.Razor.Language.Syntax;
+using Microsoft.AspNetCore.Razor.PooledObjects;
 
 namespace Microsoft.AspNetCore.Razor.Language;
 
@@ -216,14 +217,16 @@ private bool AssemblyContainsTagHelpers(string assemblyName, IReadOnlyList<TagHe
 
     internal sealed class ComponentDirectiveVisitor : DirectiveVisitor
     {
-        private readonly List<TagHelperDescriptor> _notFullyQualifiedComponents;
+        private readonly ImmutableArray<TagHelperDescriptor> _notFullyQualifiedComponents;
         private readonly string _filePath;
         private RazorSourceDocument _source;
 
         public ComponentDirectiveVisitor(string filePath, IReadOnlyList<TagHelperDescriptor> tagHelpers, string currentNamespace)
         {
             _filePath = filePath;
 
+            using var builder = new PooledArrayBuilder<TagHelperDescriptor>(capacity: tagHelpers.Count);
+
             for (var i = 0; i < tagHelpers.Count; i++)
             {
                 var tagHelper = tagHelpers[i];
@@ -240,8 +243,7 @@ public ComponentDirectiveVisitor(string filePath, IReadOnlyList<TagHelperDescrip
                     continue;
                 }
 
-                _notFullyQualifiedComponents ??= new();
-                _notFullyQualifiedComponents.Add(tagHelper);
+                builder.Add(tagHelper);
 
                 if (currentNamespace is null)
                 {
@@ -263,6 +265,8 @@ public ComponentDirectiveVisitor(string filePath, IReadOnlyList<TagHelperDescrip
                     Matches.Add(tagHelper);
                 }
             }
+
+            _notFullyQualifiedComponents = builder.DrainToImmutable();
         }
 
         public override HashSet<TagHelperDescriptor> Matches { get; } = new HashSet<TagHelperDescriptor>();
@@ -324,9 +328,8 @@ public override void VisitRazorDirective(RazorDirectiveSyntax node)
                             continue;
                         }
 
-                        for (var i = 0; _notFullyQualifiedComponents is not null && i < _notFullyQualifiedComponents.Count; i++)
+                        foreach (var tagHelper in _notFullyQualifiedComponents)
                         {
-                            var tagHelper = _notFullyQualifiedComponents[i];
                             Debug.Assert(!tagHelper.IsComponentFullyQualifiedNameMatch(), "We've already processed these.");
 
                             if (tagHelper.IsChildContentTagHelper())

src/Shared/Microsoft.AspNetCore.Razor.Utilities.Shared/PooledObjects/PooledArrayBuilder`1.cs

@@ -16,16 +16,18 @@ namespace Microsoft.AspNetCore.Razor.PooledObjects;
 internal ref struct PooledArrayBuilder<T>
 {
     private readonly ObjectPool<ImmutableArray<T>.Builder> _pool;
+    private readonly int? _capacity;
     private ImmutableArray<T>.Builder? _builder;
 
     public PooledArrayBuilder()
-        : this(ArrayBuilderPool<T>.Default)
+        : this(null, null)
     {
     }
 
-    public PooledArrayBuilder(ObjectPool<ImmutableArray<T>.Builder> pool)
+    public PooledArrayBuilder(ObjectPool<ImmutableArray<T>.Builder>? pool = null, int? capacity = null)
     {
-        _pool = pool;
+        _pool = pool ?? ArrayBuilderPool<T>.Default;
+        _capacity = capacity;
     }
 
     public void Dispose()
@@ -38,7 +40,7 @@ public readonly int Count
 
     public void Add(T item)
     {
-        _builder ??= _pool.Get();
+        _builder ??= GetBuilder();
         _builder.Add(item);
     }
 
@@ -49,13 +51,13 @@ public void AddRange(IReadOnlyList<T> items)
             return;
         }
 
-        _builder ??= _pool.Get();
+        _builder ??= GetBuilder();
         _builder.AddRange(items);
     }
 
     public void AddRange(IEnumerable<T> items)
     {
-        _builder ??= _pool.Get();
+        _builder ??= GetBuilder();
         _builder.AddRange(items);
     }
 
@@ -68,6 +70,17 @@ public void ClearAndFree()
         }
     }
 
+    private readonly ImmutableArray<T>.Builder GetBuilder()
+    {
+        var result = _pool.Get();
+        if (_capacity is int capacity)
+        {
+            result.SetCapacityIfNeeded(capacity);
+        }
+
+        return result;
+    }
+
     /// <summary>
     ///  Returns the current contents as an <see cref="ImmutableArray{T}"/> and sets
     ///  the collection to a zero length array.