Bring back old array enumerator code

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Array.NativeAot.cs

@@ -1099,7 +1099,10 @@ private Array() { }
         public new IEnumerator<T> GetEnumerator()
         {
             T[] @this = Unsafe.As<T[]>(this);
-            return @this.Length == 0 ? SZGenericArrayEnumerator<T>.Empty : new SZGenericArrayEnumerator<T>(@this);
+            // get length so we don't have to call the Length property again in ArrayEnumerator constructor
+            // and avoid more checking there too.
+            int length = @this.Length;
+            return length == 0 ? SZGenericArrayEnumerator<T>.Empty : new SZGenericArrayEnumerator<T>(@this, length);
         }
 
         public int Count
@@ -1196,6 +1199,68 @@ public void RemoveAt(int index)
         }
     }
 
+    internal class SZGenericArrayEnumeratorBase : IDisposable
+    {
+        protected int _index;
+        protected int _endIndex;
+
+        internal SZGenericArrayEnumeratorBase()
+        {
+            _index = -1;
+        }
+
+        public bool MoveNext()
+        {
+            if (_index < _endIndex)
+            {
+                _index++;
+                return (_index < _endIndex);
+            }
+            return false;
+        }
+
+        public void Dispose()
+        {
+        }
+    }
+
+    internal sealed class SZGenericArrayEnumerator<T> : SZGenericArrayEnumeratorBase, IEnumerator<T>
+    {
+        private readonly T[] _array;
+
+        // Passing -1 for endIndex so that MoveNext always returns false without mutating _index
+        internal static readonly SZGenericArrayEnumerator<T> Empty = new SZGenericArrayEnumerator<T>(null, -1);
+
+        internal SZGenericArrayEnumerator(T[] array, int endIndex)
+        {
+            _array = array;
+            _endIndex = endIndex;
+        }
+
+        public T Current
+        {
+            get
+            {
+                if ((uint)_index >= (uint)_endIndex)
+                    ThrowHelper.ThrowInvalidOperationException();
+                return _array[_index];
+            }
+        }
+
+        object IEnumerator.Current
+        {
+            get
+            {
+                return Current;
+            }
+        }
+
+        void IEnumerator.Reset()
+        {
+            _index = -1;
+        }
+    }
+
     public class MDArray
     {
         public const int MinRank = 1;

src/libraries/System.Private.CoreLib/src/System/Array.Enumerators.cs

@@ -67,6 +67,8 @@ public void Reset()
         }
     }
 
+    // Native AOT doesn't use these for size on disk reasons
+#if !NATIVEAOT
     internal abstract class SZGenericArrayEnumeratorBase : IDisposable
     {
         protected readonly Array _array;
@@ -140,6 +142,7 @@ public T Current
 
         object? IEnumerator.Current => Current;
     }
+#endif
 
     internal abstract class GenericEmptyEnumeratorBase : IDisposable, IEnumerator
     {