Remove List<T>.Enumerator.MoveNextRare (#118425)

Author: stephentoub

src/libraries/Common/tests/System/Collections/IEnumerableTest.cs

@@ -20,6 +20,7 @@ public abstract class IEnumerableTest<T>
         protected T DefaultValue => default(T);
 
         protected bool MoveNextAtEndThrowsOnModifiedCollection => true;
+        protected virtual bool CurrentAfterFullEnumerationThrows => true;
 
         protected virtual CollectionOrder CollectionOrder => CollectionOrder.Sequential;
 
@@ -352,7 +353,7 @@ public void EnumeratePastEndThenModify()
             VerifyModifiedEnumerator(
                 enumerator,
                 DefaultValue,
-                false,
+                CurrentAfterFullEnumerationThrows,
                 true);
         }
 

src/libraries/System.ObjectModel/tests/ReadOnlyDictionary/ReadOnlyDictionaryTests.cs

@@ -391,6 +391,7 @@ public ReadOnlyDictionaryOverNonGenericTests()
         protected override bool IsGenericCompatibility { get { return false; } }
         protected override bool ItemsMustBeUnique { get { return true; } }
         protected override bool ItemsMustBeNonNull { get { return true; } }
+        protected override bool CurrentAfterFullEnumerationThrows { get { return false; } }
         protected override object GenerateItem()
         {
             return new KeyValuePair<string, int>(m_next_item.ToString(), m_next_item++);
@@ -429,6 +430,7 @@ public ReadOnlyDictionaryTestsStringInt()
         protected override bool IsGenericCompatibility { get { return false; } }
         protected override bool ItemsMustBeUnique { get { return true; } }
         protected override bool ItemsMustBeNonNull { get { return true; } }
+        protected override bool CurrentAfterFullEnumerationThrows { get { return false; } }
         protected override object GenerateItem()
         {
             return new KeyValuePair<string, int>(m_next_item.ToString(), m_next_item++);

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs

@@ -1185,16 +1185,15 @@ public bool TrueForAll(Predicate<T> match)
         public struct Enumerator : IEnumerator<T>, IEnumerator
         {
             private readonly List<T> _list;
-            private int _index;
             private readonly int _version;
+
+            private int _index;
             private T? _current;
 
             internal Enumerator(List<T> list)
             {
                 _list = list;
-                _index = 0;
                 _version = list._version;
-                _current = default;
             }
 
             public void Dispose()
@@ -1205,24 +1204,20 @@ public bool MoveNext()
             {
                 List<T> localList = _list;
 
-                if (_version == localList._version && ((uint)_index < (uint)localList._size))
+                if (_version != _list._version)
                 {
-                    _current = localList._items[_index];
-                    _index++;
-                    return true;
+                    ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
                 }
-                return MoveNextRare();
-            }
 
-            private bool MoveNextRare()
-            {
-                if (_version != _list._version)
+                if ((uint)_index < (uint)localList._size)
                 {
-                    ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
+                    _current = localList._items[_index];
+                    _index++;
+                    return true;
                 }
 
-                _index = _list._size + 1;
                 _current = default;
+                _index = -1;
                 return false;
             }
 
@@ -1232,11 +1227,12 @@ private bool MoveNextRare()
             {
                 get
                 {
-                    if (_index == 0 || _index == _list._size + 1)
+                    if (_index <= 0)
                     {
                         ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
                     }
-                    return Current;
+
+                    return _current;
                 }
             }