Avoid pre-allocating large arrays in some scenarios for Enumerable.Chunk (#67210)

* Avoid pre-allocating large arrays in some scenarios for Enumerable.Chunk().

Fix #67132

* Switch Chunk() to use List-based implementation.

See discussion here:
#67210 (comment)

* Address feedback from https://github.com/dotnet/runtime/pull/67210/files

Author: madelson

src/libraries/System.Linq/src/System/Linq/Chunk.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Linq
 {
@@ -50,26 +51,25 @@ public static IEnumerable<TSource[]> Chunk<TSource>(this IEnumerable<TSource> so
         private static IEnumerable<TSource[]> ChunkIterator<TSource>(IEnumerable<TSource> source, int size)
         {
             using IEnumerator<TSource> e = source.GetEnumerator();
-            while (e.MoveNext())
-            {
-                TSource[] chunk = new TSource[size];
-                chunk[0] = e.Current;
 
-                int i = 1;
-                for (; i < chunk.Length && e.MoveNext(); i++)
+            if (e.MoveNext())
+            {
+                List<TSource> chunkBuilder = new();
+                while (true)
                 {
-                    chunk[i] = e.Current;
-                }
+                    do
+                    {
+                        chunkBuilder.Add(e.Current);
+                    }
+                    while (chunkBuilder.Count < size && e.MoveNext());
 
-                if (i == chunk.Length)
-                {
-                    yield return chunk;
-                }
-                else
-                {
-                    Array.Resize(ref chunk, i);
-                    yield return chunk;
-                    yield break;
+                    yield return chunkBuilder.ToArray();
+
+                    if (chunkBuilder.Count < size || !e.MoveNext())
+                    {
+                        yield break;
+                    }
+                    chunkBuilder.Clear();
                 }
             }
         }

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

@@ -141,5 +141,14 @@ public void AddingToSourceBeforeIterating()
 
             Assert.Equal(new[] {new[] {9999, 0, 888}, new[] {-1, 66, -777}, new[] {1, 2, -12345}, new[] {10}}, chunks);
         }
+
+        // reproduces https://github.com/dotnet/runtime/issues/67132
+        [Fact]
+        public void DoesNotPrematurelyAllocateHugeArray()
+        {
+            int[][] chunks = Enumerable.Range(0, 10).Chunk(int.MaxValue).ToArray();
+
+            Assert.Equal(new[] { Enumerable.Range(0, 10).ToArray() }, chunks);
+        }
     }
-}
+}