start to implements async linq

src/UniTask.NetCore/Linq/ToArray.cs

@@ -0,0 +1,57 @@
+using Cysharp.Threading.Tasks.Internal;
+using System;
+using System.Collections.Generic;
+using System.Threading;
+
+namespace Cysharp.Threading.Tasks.Linq
+{
+    public static partial class UniTaskAsyncEnumerable
+    {
+        public static UniTask<TSource[]> ToArrayAsync<TSource>(this IUniTaskAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default)
+        {
+            return Cysharp.Threading.Tasks.Linq.ToArray<TSource>.InvokeAsync(source, cancellationToken);
+        }
+    }
+
+    internal static class ToArray<TSource>
+    {
+        internal static async UniTask<TSource[]> InvokeAsync(IUniTaskAsyncEnumerable<TSource> source, CancellationToken cancellationToken)
+        {
+            var pool = ArrayPool<TSource>.Shared;
+            var array = pool.Rent(16);
+
+            TSource[] result = default;
+            var e = source.GetAsyncEnumerator(cancellationToken);
+            try
+            {
+                var i = 0;
+                while (await e.MoveNextAsync())
+                {
+                    ArrayPoolUtil.EnsureCapacity(ref array, i, pool);
+                    array[i++] = e.Current;
+                }
+
+                if (i == 0)
+                {
+                    result = Array.Empty<TSource>();
+                }
+                else
+                {
+                    result = new TSource[i];
+                    Array.Copy(array, result, i);
+                }
+            }
+            finally
+            {
+                pool.Return(array, clearArray: !RuntimeHelpersAbstraction.IsWellKnownNoReferenceContainsType<TSource>());
+
+                if (e != null)
+                {
+                    await e.DisposeAsync();
+                }
+            }
+
+            return result;
+        }
+    }
+}