Introduce size-optimized IListSelect iterator

src/libraries/System.Linq/src/System.Linq.csproj

@@ -62,6 +62,7 @@
     <Compile Include="System\Linq\RightJoin.cs" />
     <Compile Include="System\Linq\SegmentedArrayBuilder.cs" />
     <Compile Include="System\Linq\Select.cs" />
+    <Compile Include="System\Linq\Select.SizeOpt.cs" />
     <Compile Include="System\Linq\Select.SpeedOpt.cs" />
     <Compile Include="System\Linq\SelectMany.cs" />
     <Compile Include="System\Linq\SelectMany.SpeedOpt.cs" />
@@ -70,19 +71,18 @@
     <Compile Include="System\Linq\Single.cs" />
     <Compile Include="System\Linq\SingleLinkedNode.cs" />
     <Compile Include="System\Linq\Skip.cs" />
-    <Compile Include="System\Linq\Skip.SizeOpt.cs" />
     <Compile Include="System\Linq\Skip.SpeedOpt.cs" />
     <Compile Include="System\Linq\SkipTake.SpeedOpt.cs" />
     <Compile Include="System\Linq\Sum.cs" />
     <Compile Include="System\Linq\Take.cs" />
-    <Compile Include="System\Linq\Take.SizeOpt.cs" />
     <Compile Include="System\Linq\Take.SpeedOpt.cs" />
     <Compile Include="System\Linq\ThrowHelper.cs" />
     <Compile Include="System\Linq\ToCollection.cs" />
     <Compile Include="System\Linq\Union.cs" />
     <Compile Include="System\Linq\Union.SpeedOpt.cs" />
     <Compile Include="System\Linq\Utilities.cs" />
     <Compile Include="System\Linq\Where.cs" />
+    <Compile Include="System\Linq\Where.SizeOpt.cs" />
     <Compile Include="System\Linq\Where.SpeedOpt.cs" />
     <Compile Include="System\Linq\Zip.cs" />
   </ItemGroup>

src/libraries/System.Linq/src/System/Linq/Count.cs

@@ -20,7 +20,7 @@ public static int Count<TSource>(this IEnumerable<TSource> source)
                 return collectionoft.Count;
             }
 
-            if (!IsSizeOptimized && source is Iterator<TSource> iterator)
+            if (source is Iterator<TSource> iterator)
             {
                 return iterator.GetCount(onlyIfCheap: false);
             }
@@ -113,7 +113,7 @@ public static bool TryGetNonEnumeratedCount<TSource>(this IEnumerable<TSource> s
                 return true;
             }
 
-            if (!IsSizeOptimized && source is Iterator<TSource> iterator)
+            if (source is Iterator<TSource> iterator)
             {
                 int c = iterator.GetCount(onlyIfCheap: true);
                 if (c >= 0)

src/libraries/System.Linq/src/System/Linq/ElementAt.cs

@@ -23,7 +23,7 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, int i
 
             bool found;
             TSource? element =
-                !IsSizeOptimized && source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
+                source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
                 TryGetElementAtNonIterator(source, index, out found);
 
             if (!found)
@@ -121,7 +121,7 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, Index
             }
 
             return
-                !IsSizeOptimized && source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
+                source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
                 TryGetElementAtNonIterator(source, index, out found);
         }
 

src/libraries/System.Linq/src/System/Linq/Last.cs

@@ -69,7 +69,7 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, F
             }
 
             return
-                !IsSizeOptimized && source is Iterator<TSource> iterator ? iterator.TryGetLast(out found) :
+                source is Iterator<TSource> iterator ? iterator.TryGetLast(out found) :
                 TryGetLastNonIterator(source, out found);
         }
 

src/libraries/System.Linq/src/System/Linq/OfType.SpeedOpt.cs

@@ -167,7 +167,7 @@ public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> s
                     // they're covariant. It's not worthwhile checking for List<T> to use the ListWhereSelectIterator
                     // because List<> is not covariant.
                     Func<object, bool> isTResult = static o => o is TResult;
-                    return objectSource is object[] array ?
+                    return !IsSizeOptimized && objectSource is object[] array ?
                         new ArrayWhereSelectIterator<object, TResult2>(array, isTResult, localSelector) :
                         new IEnumerableWhereSelectIterator<object, TResult2>(objectSource, isTResult, localSelector);
                 }
@@ -177,7 +177,11 @@ public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> s
 
             public override bool Contains(TResult value)
             {
-                if (!typeof(TResult).IsValueType && // don't box TResult
+                // Avoid checking for IList when size-optimized because it keeps IList
+                // implementations which may otherwise be trimmed. Since List<T> implements
+                // IList and List<T> is popular, this could potentially be a lot of code.
+                if (!IsSizeOptimized &&
+                    !typeof(TResult).IsValueType && // don't box TResult
                     _source is IList list)
                 {
                     return list.Contains(value);

src/libraries/System.Linq/src/System/Linq/Select.SizeOpt.cs

@@ -0,0 +1,90 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace System.Linq
+{
+    public static partial class Enumerable
+    {
+        private sealed class SizeOptIListSelectIterator<TSource, TResult>(IList<TSource> _source, Func<TSource, TResult> _selector)
+            : Iterator<TResult>
+        {
+            public override int GetCount(bool onlyIfCheap)
+            {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
+                if (onlyIfCheap)
+                {
+                    return -1;
+                }
+
+                int count = 0;
+
+                foreach (TSource item in _source)
+                {
+                    _selector(item);
+                    checked
+                    {
+                        count++;
+                    }
+                }
+
+                return count;
+            }
+
+            public override Iterator<TResult> Skip(int count)
+            {
+                Debug.Assert(count > 0);
+                return new IListSkipTakeSelectIterator<TSource, TResult>(_source, _selector, count, int.MaxValue);
+            }
+
+            public override Iterator<TResult> Take(int count)
+            {
+                Debug.Assert(count > 0);
+                return new IListSkipTakeSelectIterator<TSource, TResult>(_source, _selector, 0, count - 1);
+            }
+
+            public override bool MoveNext()
+            {
+                var source = _source;
+                int index = _state - 1;
+                if ((uint)index < (uint)source.Count)
+                {
+                    _state++;
+                    _current = _selector(source[index]);
+                    return true;
+                }
+
+                Dispose();
+                return false;
+            }
+
+            public override TResult[] ToArray()
+            {
+                TResult[] array = new TResult[_source.Count];
+                for (int i = 0; i < array.Length; i++)
+                {
+                    array[i] = _selector(_source[i]);
+                }
+                return array;
+            }
+
+            public override List<TResult> ToList()
+            {
+                List<TResult> list = new List<TResult>(_source.Count);
+                for (int i = 0; i < list.Count; i++)
+                {
+                    list.Add(_selector(_source[i]));
+                }
+                return list;
+            }
+
+            private protected override Iterator<TResult> Clone()
+                => new SizeOptIListSelectIterator<TSource, TResult>(_source, _selector);
+        }
+    }
+}

src/libraries/System.Linq/src/System/Linq/Select.cs

@@ -32,7 +32,9 @@ public static IEnumerable<TResult> Select<TSource, TResult>(
                 // don't need more code, just more data structures describing the new types).
                 if (IsSizeOptimized && typeof(TResult).IsValueType)
                 {
-                    return new IEnumerableSelectIterator<TSource, TResult>(iterator, selector);
+                    return source is IList<TSource> il
+                        ? new SizeOptIListSelectIterator<TSource, TResult>(il, selector)
+                        : new IEnumerableSelectIterator<TSource, TResult>(iterator, selector);
                 }
                 else
                 {
@@ -42,6 +44,11 @@ public static IEnumerable<TResult> Select<TSource, TResult>(
 
             if (source is IList<TSource> ilist)
             {
+                if (IsSizeOptimized)
+                {
+                    return new SizeOptIListSelectIterator<TSource, TResult>(ilist, selector);
+                }
+
                 if (source is TSource[] array)
                 {
                     if (array.Length == 0)

src/libraries/System.Linq/src/System/Linq/Skip.cs

@@ -30,12 +30,12 @@ public static IEnumerable<TSource> Skip<TSource>(this IEnumerable<TSource> sourc
 
                 count = 0;
             }
-            else if (!IsSizeOptimized && source is Iterator<TSource> iterator)
+            else if (source is Iterator<TSource> iterator)
             {
                 return iterator.Skip(count) ?? Empty<TSource>();
             }
 
-            return IsSizeOptimized ? SizeOptimizedSkipIterator(source, count) : SpeedOptimizedSkipIterator(source, count);
+            return SpeedOptimizedSkipIterator(source, count);
         }
 
         public static IEnumerable<TSource> SkipWhile<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

src/libraries/System.Linq/src/System/Linq/Take.cs

@@ -20,7 +20,7 @@ public static IEnumerable<TSource> Take<TSource>(this IEnumerable<TSource> sourc
                 return [];
             }
 
-            return IsSizeOptimized ? SizeOptimizedTakeIterator(source, count) : SpeedOptimizedTakeIterator(source, count);
+            return SpeedOptimizedTakeIterator(source, count);
         }
 
         /// <summary>Returns a specified range of contiguous elements from a sequence.</summary>
@@ -68,7 +68,7 @@ public static IEnumerable<TSource> Take<TSource>(this IEnumerable<TSource> sourc
                     return [];
                 }
 
-                return IsSizeOptimized ? SizeOptimizedTakeRangeIterator(source, startIndex, endIndex) : SpeedOptimizedTakeRangeIterator(source, startIndex, endIndex);
+                return SpeedOptimizedTakeRangeIterator(source, startIndex, endIndex);
             }
 
             return TakeRangeFromEndIterator(source, isStartIndexFromEnd, startIndex, isEndIndexFromEnd, endIndex);
@@ -94,9 +94,7 @@ private static IEnumerable<TSource> TakeRangeFromEndIterator<TSource>(IEnumerabl
 
                 if (startIndex < endIndex)
                 {
-                    IEnumerable<TSource> rangeIterator = IsSizeOptimized
-                        ? SizeOptimizedTakeRangeIterator(source, startIndex, endIndex)
-                        : SpeedOptimizedTakeRangeIterator(source, startIndex, endIndex);
+                    IEnumerable<TSource> rangeIterator = SpeedOptimizedTakeRangeIterator(source, startIndex, endIndex);
                     foreach (TSource element in rangeIterator)
                     {
                         yield return element;