PrefetchQuery: Implement IAsyncEnumerable to not require extra operation

- AsAsyncEnumerable() marked as Obsolete

Author: alex-kulakov

Orm/Xtensive.Orm/Orm/PrefetchQuery.cs

@@ -19,7 +19,7 @@ namespace Xtensive.Orm
   /// initial query result.
   /// </summary>
   /// <typeparam name="TElement">The type of the queried elements.</typeparam>
-  public readonly struct PrefetchQuery<TElement> : IEnumerable<TElement>
+  public readonly struct PrefetchQuery<TElement> : IEnumerable<TElement>, IAsyncEnumerable<TElement>
   {
     private readonly Session session;
     private readonly IEnumerable<TElement> source;
@@ -38,9 +38,14 @@ internal PrefetchQuery<TElement> RegisterPath<TValue>(Expression<Func<TElement,
     public IEnumerator<TElement> GetEnumerator() =>
       new PrefetchQueryEnumerable<TElement>(session, source, nodes).GetEnumerator();
 
+    /// <inheritdoc />
+    public IAsyncEnumerator<TElement> GetAsyncEnumerator(CancellationToken token = default) =>
+      new PrefetchQueryAsyncEnumerable<TElement>(session, source, nodes).GetAsyncEnumerator(token);
+
     /// <summary>
     /// Transforms <see cref="PrefetchQuery{TElement}"/> to an <see cref="IAsyncEnumerable{T}"/> sequence.
     /// </summary>
+    [Obsolete("PrefetchQuery itself is an IAsyncEnumerable implementation")]
     public IAsyncEnumerable<TElement> AsAsyncEnumerable() =>
       new PrefetchQueryAsyncEnumerable<TElement>(session, source, nodes);
 
@@ -68,8 +73,7 @@ public ExecuteAsyncResult(List<TElement> items, StrongReferenceContainer referen
     /// <para>
     /// This method internally puts all elements of the resulting sequence to a list
     /// and then it wraps mentioned list as a <see cref="QueryResult{TItem}"/>.
-    /// As a consequence it is more efficient to use asynchronous enumeration over result of
-    /// <see cref="AsAsyncEnumerable"/> method call because it can perform lazily
+    /// As a consequence it is more efficient to use asynchronous enumeration because it can perform lazily
     /// not putting everything into intermediate list.
     /// </para>
     /// <para> Multiple active operations are not supported. Use <see langword="await"/>