Implement IList<T> on some LINQ iterators

`ICollection<T>` provides both a Count and a CopyTo, and `IList<T>` an indexer, all of which can make various consumption mechanisms more efficient. We only implement the interfaces when the underlying collection has a fixed size and all of the interface implementations are side-effect free (in particular, while appealing to do so, we don't implement them on various Select iterators).

Some of the serialization tests need to be fixed as a result. The state of Queue's array is a bit different based on how its initialized, and such private details show up in BinaryFormatter output.  Rather than special-casing the output per framework and core, I've just changed the test itself to ensure Queue can't see the size of the input collection.

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

@@ -17,7 +17,7 @@ namespace System.Linq
     /// that an operation will result in zero elements.
     /// </remarks>
     [DebuggerDisplay("Count = 0")]
-    internal sealed class EmptyPartition<TElement> : IPartition<TElement>, IEnumerator<TElement>
+    internal sealed class EmptyPartition<TElement> : IPartition<TElement>, IEnumerator<TElement>, IList<TElement>, IReadOnlyList<TElement>
     {
         /// <summary>
         /// A cached, immutable instance of an empty enumerable.
@@ -77,6 +77,32 @@ void IDisposable.Dispose()
         public List<TElement> ToList() => new List<TElement>();
 
         public int GetCount(bool onlyIfCheap) => 0;
+
+        public int Count => 0;
+
+        public bool Contains(TElement item) => false;
+
+        public int IndexOf(TElement item) => -1;
+
+        public void CopyTo(TElement[] array, int arrayIndex) { }
+
+        public TElement this[int index]
+        {
+            get
+            {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index);
+                return default!;
+            }
+            set => ThrowHelper.ThrowNotSupportedException();
+        }
+
+        public bool IsReadOnly => true;
+
+        void ICollection<TElement>.Add(TElement item) => ThrowHelper.ThrowNotSupportedException();
+        void ICollection<TElement>.Clear() => ThrowHelper.ThrowNotSupportedException();
+        void IList<TElement>.Insert(int index, TElement item) => ThrowHelper.ThrowNotSupportedException();
+        bool ICollection<TElement>.Remove(TElement item) => ThrowHelper.ThrowNotSupportedException_Boolean();
+        void IList<TElement>.RemoveAt(int index) => ThrowHelper.ThrowNotSupportedException();
     }
 
     internal sealed class OrderedPartition<TElement> : IPartition<TElement>
@@ -143,7 +169,7 @@ public static partial class Enumerable
         /// </summary>
         /// <typeparam name="TSource">The type of the source list.</typeparam>
         [DebuggerDisplay("Count = {Count}")]
-        private sealed class ListPartition<TSource> : Iterator<TSource>, IPartition<TSource>
+        private sealed class ListPartition<TSource> : Iterator<TSource>, IPartition<TSource>, IList<TSource>, IReadOnlyList<TSource>
         {
             private readonly IList<TSource> _source;
             private readonly int _minIndexInclusive;
@@ -231,7 +257,7 @@ public IPartition<TSource> Take(int count)
                 return default;
             }
 
-            private int Count
+            public int Count
             {
                 get
                 {
@@ -245,6 +271,8 @@ private int Count
                 }
             }
 
+            public int GetCount(bool onlyIfCheap) => Count;
+
             public TSource[] ToArray()
             {
                 int count = Count;
@@ -271,6 +299,9 @@ public List<TSource> ToList()
                 return list;
             }
 
+            public void CopyTo(TSource[] array, int arrayIndex) =>
+                Fill(_source, array.AsSpan(arrayIndex, Count), _minIndexInclusive);
+
             private static void Fill(IList<TSource> source, Span<TSource> destination, int sourceIndex)
             {
                 for (int i = 0; i < destination.Length; i++, sourceIndex++)
@@ -279,7 +310,45 @@ private static void Fill(IList<TSource> source, Span<TSource> destination, int s
                 }
             }
 
-            public int GetCount(bool onlyIfCheap) => Count;
+            public bool Contains(TSource item) => IndexOf(item) >= 0;
+
+            public int IndexOf(TSource item)
+            {
+                IList<TSource> source = _source;
+
+                int end = _minIndexInclusive + Count;
+                for (int i = _minIndexInclusive; i < end; i++)
+                {
+                    if (EqualityComparer<TSource>.Default.Equals(source[i], item))
+                    {
+                        return i - _minIndexInclusive;
+                    }
+                }
+
+                return -1;
+            }
+
+            public TSource this[int index]
+            {
+                get
+                {
+                    if ((uint)index >= (uint)Count)
+                    {
+                        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index);
+                    }
+
+                    return _source[_minIndexInclusive + index];
+                }
+                set => ThrowHelper.ThrowNotSupportedException();
+            }
+
+            public bool IsReadOnly => true;
+
+            void ICollection<TSource>.Add(TSource item) => ThrowHelper.ThrowNotSupportedException();
+            void ICollection<TSource>.Clear() => ThrowHelper.ThrowNotSupportedException();
+            void IList<TSource>.Insert(int index, TSource item) => ThrowHelper.ThrowNotSupportedException();
+            bool ICollection<TSource>.Remove(TSource item) => ThrowHelper.ThrowNotSupportedException_Boolean();
+            void IList<TSource>.RemoveAt(int index) => ThrowHelper.ThrowNotSupportedException();
         }
 
         /// <summary>

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

@@ -7,7 +7,7 @@ namespace System.Linq
 {
     public static partial class Enumerable
     {
-        private sealed partial class RangeIterator : IPartition<int>
+        private sealed partial class RangeIterator : IPartition<int>, IList<int>, IReadOnlyList<int>
         {
             public override IEnumerable<TResult> Select<TResult>(Func<int, TResult> selector)
             {
@@ -28,6 +28,9 @@ public List<int> ToList()
                 return list;
             }
 
+            public void CopyTo(int[] array, int arrayIndex) =>
+                Fill(array.AsSpan(arrayIndex, _end - _start), _start);
+
             private static void Fill(Span<int> destination, int value)
             {
                 for (int i = 0; i < destination.Length; i++, value++)
@@ -38,6 +41,8 @@ private static void Fill(Span<int> destination, int value)
 
             public int GetCount(bool onlyIfCheap) => unchecked(_end - _start);
 
+            public int Count => _end - _start;
+
             public IPartition<int> Skip(int count)
             {
                 if (count >= _end - _start)
@@ -82,6 +87,34 @@ public int TryGetLast(out bool found)
                 found = true;
                 return _end - 1;
             }
+
+            public bool Contains(int item) =>
+                (uint)(item - _start) < (uint)(_end - _start);
+
+            public int IndexOf(int item) =>
+                Contains(item) ? item - _start : -1;
+
+            public int this[int index]
+            {
+                get
+                {
+                    if ((uint)index >= (uint)(_end - _start))
+                    {
+                        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index);
+                    }
+
+                    return _start + index;
+                }
+                set => ThrowHelper.ThrowNotSupportedException();
+            }
+
+            public bool IsReadOnly => true;
+
+            void ICollection<int>.Add(int item) => ThrowHelper.ThrowNotSupportedException();
+            void ICollection<int>.Clear() => ThrowHelper.ThrowNotSupportedException();
+            void IList<int>.Insert(int index, int item) => ThrowHelper.ThrowNotSupportedException();
+            bool ICollection<int>.Remove(int item) => ThrowHelper.ThrowNotSupportedException_Boolean();
+            void IList<int>.RemoveAt(int index) => ThrowHelper.ThrowNotSupportedException();
         }
     }
 }

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

@@ -3,13 +3,12 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Diagnostics.CodeAnalysis;
 
 namespace System.Linq
 {
     public static partial class Enumerable
     {
-        private sealed partial class RepeatIterator<TResult> : IPartition<TResult>
+        private sealed partial class RepeatIterator<TResult> : IPartition<TResult>, IList<TResult>, IReadOnlyList<TResult>
         {
             public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> selector) =>
                 new SelectIPartitionIterator<TResult, TResult2>(this, selector);
@@ -35,6 +34,8 @@ public List<TResult> ToList()
 
             public int GetCount(bool onlyIfCheap) => _count;
 
+            public int Count => _count;
+
             public IPartition<TResult> Skip(int count)
             {
                 Debug.Assert(count > 0);
@@ -82,6 +83,39 @@ public TResult TryGetLast(out bool found)
                 found = true;
                 return _current;
             }
+
+            public bool Contains(TResult item)
+            {
+                Debug.Assert(_count > 0);
+                return EqualityComparer<TResult>.Default.Equals(_current, item);
+            }
+
+            public int IndexOf(TResult item) => Contains(item) ? 0 : -1;
+
+            public void CopyTo(TResult[] array, int arrayIndex) =>
+                array.AsSpan(arrayIndex, _count).Fill(_current);
+
+            public TResult this[int index]
+            {
+                get
+                {
+                    if ((uint)index >= (uint)_count)
+                    {
+                        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index);
+                    }
+
+                    return _current;
+                }
+                set => ThrowHelper.ThrowNotSupportedException();
+            }
+
+            public bool IsReadOnly => true;
+
+            void ICollection<TResult>.Add(TResult item) => ThrowHelper.ThrowNotSupportedException();
+            void ICollection<TResult>.Clear() => ThrowHelper.ThrowNotSupportedException();
+            void IList<TResult>.Insert(int index, TResult item) => ThrowHelper.ThrowNotSupportedException();
+            bool ICollection<TResult>.Remove(TResult item) => ThrowHelper.ThrowNotSupportedException_Boolean();
+            void IList<TResult>.RemoveAt(int index) => ThrowHelper.ThrowNotSupportedException();
         }
     }
 }

src/libraries/System.Linq/src/System/Linq/ThrowHelper.cs

@@ -29,6 +29,9 @@ internal static class ThrowHelper
         [DoesNotReturn]
         internal static void ThrowNotSupportedException() => throw new NotSupportedException();
 
+        [DoesNotReturn]
+        internal static bool ThrowNotSupportedException_Boolean() => throw new NotSupportedException();
+
         [DoesNotReturn]
         internal static void ThrowOverflowException() => throw new OverflowException();
 

src/libraries/System.Linq/tests/EmptyEnumerable.cs

@@ -1,6 +1,7 @@
 // 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.Generic;
 using Xunit;
 
 namespace System.Linq.Tests
@@ -12,7 +13,7 @@ private void TestEmptyCached<T>()
             var enumerable1 = Enumerable.Empty<T>();
             var enumerable2 = Enumerable.Empty<T>();
 
-            Assert.Same(enumerable1, enumerable2); // Enumerable.Empty is not cached if not the same.
+            Assert.Same(enumerable1, enumerable2);
         }
 
         [Fact]
@@ -39,5 +40,34 @@ public void EmptyEnumerableIsIndeedEmpty()
             TestEmptyEmpty<object>();
             TestEmptyEmpty<EmptyEnumerableTest>();
         }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
+        public void IListImplementationIsValid()
+        {
+            IList<int> list = Assert.IsAssignableFrom<IList<int>>(Enumerable.Empty<int>());
+            IReadOnlyList<int> roList = Assert.IsAssignableFrom<IReadOnlyList<int>>(Enumerable.Empty<int>());
+
+            Assert.Throws<NotSupportedException>(() => list.Add(42));
+            Assert.Throws<NotSupportedException>(() => list.Insert(0, 42));
+            Assert.Throws<NotSupportedException>(() => list.Clear());
+            Assert.Throws<NotSupportedException>(() => list.Remove(42));
+            Assert.Throws<NotSupportedException>(() => list.RemoveAt(0));
+            Assert.Throws<NotSupportedException>(() => list[0] = 42);
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => list[0]);
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => roList[0]);
+
+            Assert.True(list.IsReadOnly);
+            Assert.Equal(0, list.Count);
+            Assert.Equal(0, roList.Count);
+
+            Assert.False(list.Contains(42));
+            Assert.Equal(-1, list.IndexOf(42));
+
+            list.CopyTo(Array.Empty<int>(), 0);
+            list.CopyTo(Array.Empty<int>(), 1);
+            int[] array = new int[1] { 42 };
+            list.CopyTo(array, 0);
+            Assert.Equal(42, array[0]);
+        }
     }
 }

src/libraries/System.Linq/tests/EmptyPartitionTests.cs

@@ -102,5 +102,34 @@ public void ResetIsNop()
             en.Reset();
             en.Reset();
         }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
+        public void IListImplementationIsValid()
+        {
+            IList<int> list = Assert.IsAssignableFrom<IList<int>>(Enumerable.Empty<int>());
+            IReadOnlyList<int> roList = Assert.IsAssignableFrom<IReadOnlyList<int>>(Enumerable.Empty<int>());
+
+            Assert.Throws<NotSupportedException>(() => list.Add(42));
+            Assert.Throws<NotSupportedException>(() => list.Insert(0, 42));
+            Assert.Throws<NotSupportedException>(() => list.Clear());
+            Assert.Throws<NotSupportedException>(() => list.Remove(42));
+            Assert.Throws<NotSupportedException>(() => list.RemoveAt(0));
+            Assert.Throws<NotSupportedException>(() => list[0] = 42);
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => list[0]);
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => roList[0]);
+
+            Assert.True(list.IsReadOnly);
+            Assert.Equal(0, list.Count);
+            Assert.Equal(0, roList.Count);
+
+            Assert.False(list.Contains(42));
+            Assert.Equal(-1, list.IndexOf(42));
+
+            list.CopyTo(Array.Empty<int>(), 0);
+            list.CopyTo(Array.Empty<int>(), 1);
+            int[] array = new int[1] { 42 };
+            list.CopyTo(array, 0);
+            Assert.Equal(42, array[0]);
+        }
     }
 }

src/libraries/System.Linq/tests/RangeTests.cs

@@ -222,5 +222,48 @@ public void LastOrDefault()
         {
             Assert.Equal(int.MaxValue - 101, Enumerable.Range(-100, int.MaxValue).LastOrDefault());
         }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
+        public void IListImplementationIsValid()
+        {
+            Validate(Enumerable.Range(42, 10), new[] { 42, 43, 44, 45, 46, 47, 48, 49, 50, 51 });
+            Validate(Enumerable.Range(42, 10).Skip(3).Take(4), new[] { 45, 46, 47, 48 });
+
+            static void Validate(IEnumerable<int> e, int[] expected)
+            {
+                IList<int> list = Assert.IsAssignableFrom<IList<int>>(e);
+                IReadOnlyList<int> roList = Assert.IsAssignableFrom<IReadOnlyList<int>>(e);
+
+                Assert.Throws<NotSupportedException>(() => list.Add(42));
+                Assert.Throws<NotSupportedException>(() => list.Insert(0, 42));
+                Assert.Throws<NotSupportedException>(() => list.Clear());
+                Assert.Throws<NotSupportedException>(() => list.Remove(42));
+                Assert.Throws<NotSupportedException>(() => list[0] = 42);
+                AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => list[-1]);
+                AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => list[expected.Length]);
+                AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => roList[-1]);
+                AssertExtensions.Throws<ArgumentOutOfRangeException>("index", () => roList[expected.Length]);
+
+                Assert.True(list.IsReadOnly);
+                Assert.Equal(expected.Length, list.Count);
+                Assert.Equal(expected.Length, roList.Count);
+
+                Assert.False(list.Contains(expected[0] - 1));
+                Assert.False(list.Contains(expected[^1] + 1));
+                Assert.All(expected, i => Assert.True(list.Contains(i)));
+                Assert.All(expected, i => Assert.Equal(Array.IndexOf(expected, i), list.IndexOf(i)));
+                for (int i = 0; i < expected.Length; i++)
+                {
+                    Assert.Equal(expected[i], list[i]);
+                    Assert.Equal(expected[i], roList[i]);
+                }
+
+                int[] actual = new int[expected.Length + 2];
+                list.CopyTo(actual, 1);
+                Assert.Equal(0, actual[0]);
+                Assert.Equal(0, actual[^1]);
+                AssertExtensions.SequenceEqual(expected, actual.AsSpan(1, expected.Length));
+            }
+        }
     }
 }