From c5d40c9e703fd257db1b26ef4fd1399bbae73ab0 Mon Sep 17 00:00:00 2001
From: Mikel Blanchard <mblanchard@macrosssoftware.com>
Date: Tue, 22 Mar 2022 22:05:46 -0700
Subject: [PATCH] Avoid an extra copy of T inside
 System.Diagnostics.Enumerator<T>. (#67012)

---
 .../src/System/Diagnostics/DiagLinkedList.cs         | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagLinkedList.cs b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagLinkedList.cs
index 3c75f0e5e42..8ec5e93b624 100644
--- a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagLinkedList.cs
+++ b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagLinkedList.cs
@@ -156,16 +156,18 @@ public void AddFront(T value)
     // Note: Some consumers use this Enumerator dynamically to avoid allocations.
     internal struct Enumerator<T> : IEnumerator<T>
     {
+        private static readonly DiagNode<T> s_Empty = new DiagNode<T>(default!);
+
         private DiagNode<T>? _nextNode;
-        [AllowNull, MaybeNull] private T _currentItem;
+        private DiagNode<T> _currentNode;
 
         public Enumerator(DiagNode<T>? head)
         {
             _nextNode = head;
-            _currentItem = default;
+            _currentNode = s_Empty;
         }
 
-        public T Current => _currentItem!;
+        public T Current => _currentNode.Value;
 
         object? IEnumerator.Current => Current;
 
@@ -173,11 +175,11 @@ public bool MoveNext()
         {
             if (_nextNode == null)
             {
-                _currentItem = default;
+                _currentNode = s_Empty;
                 return false;
             }
 
-            _currentItem = _nextNode.Value;
+            _currentNode = _nextNode;
             _nextNode = _nextNode.Next;
             return true;
         }
-- 
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