From 0d74d1d515926055ca4b4871e8002210755a7369 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Mon, 4 Mar 2024 10:45:14 -0500
Subject: [PATCH] Use Iterator<T>.TryGetFirst in Enumerable.Any() (#99218)

Enumerable.Any() currently uses TryGetNonEnumeratedCount, comparing the result to 0, and TryGetNonEnumeratedCount uses `Iterator<T>.GetCount(onlyIfCheap: true)`. But this leaves out iterators for which it's not cheap to compute the count; for these, however, we can now benefit from every `Iterator<T>` providing a `TryGetFirst`; its `found` result can be used as the result of `Any`.  This PR inlines TryGetNonEnumeratedCount into Any and then updates its use of `Iterator<T>` to first use GetCount and then fall back to using TryGetFirst if GetCount is unsuccessful.
---
 .../System.Linq/src/System/Linq/AnyAll.cs     | 35 +++++++++++++++----
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/src/libraries/System.Linq/src/System/Linq/AnyAll.cs b/src/libraries/System.Linq/src/System/Linq/AnyAll.cs
index 1fd5e5b4ae447..4fe6ae0b1b9bd 100644
--- a/src/libraries/System.Linq/src/System/Linq/AnyAll.cs
+++ b/src/libraries/System.Linq/src/System/Linq/AnyAll.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;
 using System.Collections.Generic;
 
 namespace System.Linq
@@ -9,15 +10,37 @@ public static partial class Enumerable
     {
         public static bool Any<TSource>(this IEnumerable<TSource> source)
         {
-            return
-                TryGetNonEnumeratedCount(source, out int count) ? count != 0 :
-                WithEnumerator(source);
+            if (source is null)
+            {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
+            }
 
-            static bool WithEnumerator(IEnumerable<TSource> source)
+            if (source is ICollection<TSource> gc)
             {
-                using IEnumerator<TSource> e = source.GetEnumerator();
-                return e.MoveNext();
+                return gc.Count != 0;
             }
+
+#if !OPTIMIZE_FOR_SIZE
+            if (source is Iterator<TSource> iterator)
+            {
+                int count = iterator.GetCount(onlyIfCheap: true);
+                if (count >= 0)
+                {
+                    return count != 0;
+                }
+
+                iterator.TryGetFirst(out bool found);
+                return found;
+            }
+#endif
+
+            if (source is ICollection ngc)
+            {
+                return ngc.Count != 0;
+            }
+
+            using IEnumerator<TSource> e = source.GetEnumerator();
+            return e.MoveNext();
         }
 
         public static bool Any<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)